B65813J0160A041 - EPCOS - Datasheet.Live

Features

•High Performance, Low Power AVR® 8-Bit Microcontroller

•Advanced RISC Architecture

– 131 Powerful Instructions – Most Single Clock Cycle Execution

– 32 x 8 General Purpose Working Registers

– Fully Static Operation

– Up to 16 MIPS Throughput at 16 MHz

– On-chip 2-cycle Multiplier

•Non-volatile Program and Data Memories

– 4/8/16K Bytes of In-System Self-Programmable Flash (ATmega48/88/168)

• Endurance: 75,000 Write/Erase Cycles

– Optional Boot Code Section with Independent Lock Bits

• In-System Programming by On-chip Boot Program

• True Read-While-Write Operation

– 256/512/512 Bytes EEPROM (ATmega48/88/168)

• Endurance: 100,000 Write/Erase Cycles

– 512/1K/1K Byte Internal SRAM (ATmega48/88/168)

– Programming Lock for Software Security

•Peripheral Features

– Two 8-bit Timer/Counters with Separate Prescaler and Compare Mode

– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture

Mode

– Real Time Counter with Separate Oscillator

– Six PWM Channels

– 8-channel 10-bit ADC

– Programmable Serial USART

– Master/Slave SPI Serial Interface

– Byte-oriented 2-wire Serial Interface

– Programmable Watchdog Timer with Separate On-chip Oscillator

– On-chip Analog Comparator

– Interrupt and Wake-up on Pin Change

•Special Microcontroller Features

– Power-on Reset and Programmable Brown-out Detection

– Internal Calibrated Oscillator

– External and Internal Interrupt Sources

– Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and

Standby

•I/O and Packages

– 23 Programmable I/O Lines

– Green/ROHS 32-lead TQFP and 32-pad QFN

•Operating Voltage:

– 2.7 - 5.5V for ATmega48/88/168

•Temperature Range:

–-40

°C to 125°C

•Speed Grade:

– ATmega48/88/168: 0 - 8 MHz @ 2.7 - 5.5V, 0 - 16 MHz @ 4.5 - 5.5V

•Low Power Consumption

– Active Mode:

• 4 MHz, 3.0V: 1.8mA

– Power-down Mode:

• 5µA at 3.0V

8-bit

Microcontroller

with 8K Bytes

In-System

Programmable

Flash

ATmega48

ATmega88

ATmega168

Automotive

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1. Pin Configurations

Figure 1-1. Pinout ATmega48/88/168

1.1 Disclaimer

Typical values contained in this datasheet are based on simulations and characterization of

other AVR microcontrollers manufactured on the same process technology. Min and Max values

will be available after the device is characterized.

2. Overview

The ATmega48/88/168 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced

RISC architecture. By executing powerful instructions in a single clock cycle, the

ATmega48/88/168 achieves throughputs approaching 1 MIPS per MHz allowing the system

designer to optimize power consumption versus processing speed.

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND

VCC

GND

VCC

(PCINT6/XTAL1/TOSC1) PB6

(PCINT7/XTAL2/TOSC2) PB7

PC1 (ADC1/PCINT9)

PC0 (ADC0/PCINT8)

ADC7

GND

AREF

ADC6

AVCC

PB5 (SCK/PCINT5)

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT3/OC2A/MOSI) PB3

(PCINT4/MISO) PB4

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

TQFP Top View

MLF Top View

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND

VCC

GND

VCC

(PCINT6/XTAL1/TOSC1) PB6

(PCINT7/XTAL2/TOSC2) PB7

PC1 (ADC1/PCINT9)

PC0 (ADC0/PCINT8)

ADC7

GND

AREF

ADC6

AVCC

PB5 (SCK/PCINT5)

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT3/OC2A/MOSI) PB3

(PCINT4/MISO) PB4

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

NOTE: Bottom pad should be soldered to ground.

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2.1 Block Diagram

Figure 2-1. Block Diagram

PORT C (7)PORT B (8)PORT D (8)

USART 0

8bit T/C 2

16bit T/C 18bit T/C 0 A/D Conv.

Internal

Bandgap

Analog

Comp.

SPI TWI

SRAMFlash

EEPROM

Watchdog

Oscillator

Watchdog

Timer

Oscillator

Circuits /

Clock

Generation

Power

Supervision

POR / BOD &

RESET

VCC

GND

PROGRAM

LOGIC

debugWIRE

GND

AREF

AVCC

DATABU S

ADC[6..7]PC[0..6]PB[0..7]PD[0..7]

RESET

XTAL[1..2]

CPU

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The AVR core combines a rich instruction set with 32 general purpose working registers. All the

32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent

registers to be accessed in one single instruction executed in one clock cycle. The resulting

architecture is more code efficient while achieving throughputs up to ten times faster than con-

ventional CISC microcontrollers.

The ATmega48/88/168 provides the following features: 4K/8K/16K bytes of In-System Program-

mable Flash with Read-While-Write capabilities, 256/512/512 bytes EEPROM, 512/1K/1K bytes

SRAM, 23 general purpose I/O lines, 32 general purpose working registers, three flexible

Timer/Counters with compare modes, internal and external interrupts, a serial programmable

USART, a byte-oriented 2-wire Serial Interface, an SPI serial port, a 6-channel 10-bit ADC (8

channels in TQFP and QFN packages), a programmable Watchdog Timer with internal Oscilla-

tor, and five software selectable power saving modes. The Idle mode stops the CPU while

allowing the SRAM, Timer/Counters, USART, 2-wire Serial Interface, SPI port, and interrupt sys-

tem to continue functioning. The Power-down mode saves the register contents but freezes the

Oscillator, disabling all other chip functions until the next interrupt or hardware reset. In

Power-save mode, the asynchronous timer continues to run, allowing the user to maintain a

timer base while the rest of the device is sleeping. The ADC Noise Reduction mode stops the

CPU and all I/O modules except asynchronous timer and ADC, to minimize switching noise dur-

ing ADC conversions. In Standby mode, the crystal/resonator Oscillator is running while the rest

of the device is sleeping. This allows very fast start-up combined with low power consumption.

The device is manufactured using Atmel’s high density non-volatile memory technology. The

On-chip ISP Flash allows the program memory to be reprogrammed In-System through an SPI

serial interface, by a conventional non-volatile memory programmer, or by an On-chip Boot pro-

gram running on the AVR core. The Boot program can use any interface to download the

application program in the Application Flash memory. Software in the Boot Flash section will

continue to run while the Application Flash section is updated, providing true Read-While-Write

operation. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a

monolithic chip, the Atmel ATmega48/88/168 is a powerful microcontroller that provides a highly

flexible and cost effective solution to many embedded control applications.

The ATmega48/88/168 AVR is supported with a full suite of program and system development

tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators, In-Circuit Emu-

lators, and Evaluation kits.

2.2 Automotive Quality Grade

The ATmega48-15AZ, ATmega88-15AZ and ATmega168-15AZ have been developed and man-

ufactured according to the most stringent requirements of the international standard

ISO-TS-16949 grade 1. This data sheet contains limit values extracted from the results of exten-

sive characterization (Temperature and Voltage). The quality and reliability of the

ATmega48-15AZ, ATmega88-15AZ and ATmega168-15AZ have been verified during regular

product qualification as per AEC-Q100.

As indicated in the ordering information paragraph (see “Ordering Information” on page 325), the

products are available in three different temperature grades, but with equivalent quality and reli-

ability objectives. Different temperature identifiers have been defined as listed in Table 2-1.

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2.3 Comparison Between ATmega48, ATmega88, and ATmega168

The ATmega48, ATmega88 and ATmega168 differ only in memory sizes, boot loader support,

and interrupt vector sizes. Table 2-2 summarizes the different memory and interrupt vector sizes

for the three devices.

ATmega88 and ATmega168 support a real Read-While-Write Self-Programming mechanism.

There is a separate Boot Loader Section, and the SPM instruction can only execute from there.

In ATmega48, there is no Read-While-Write support and no separate Boot Loader Section. The

SPM instruction can execute from the entire Flash.

2.4 Pin Descriptions

2.4.1 VCC

Digital supply voltage.

2.4.2 GND

Ground.

2.4.3 Port B (PB7..0) XTAL1/

XTAL2/TOSC1/TOSC2

Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port B output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port B pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscil-

lator amplifier and input to the internal clock operating circuit.

Table 2-1. Temperature Grade Identification for Automotive Products

Temperature

Identifier Comments

-40 ; +85 T Similar to Industrial Temperature Grade but with Automotive

Quality

-40 ; +105 T1 Reduced Automotive Temperature Range

-40 ; +125 Z Full AutomotiveTemperature Range

Table 2-2. Memory Size Summary

Device Flash EEPROM RAM Interrupt Vector Size

ATmega48 4K Bytes 256 Bytes 512 Bytes 1 instruction word/vector

ATmega88 8K Bytes 512 Bytes 1K Bytes 1 instruction word/vector

ATmega168 16K Bytes 512 Bytes 1K Bytes 2 instruction words/vector

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Depending on the clock selection fuse settings, PB7 can be used as output from the inverting

Oscillator amplifier.

If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as TOSC2..1

input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.

The various special features of Port B are elaborated in “Alternate Functions of Port B” on page

70 and “System Clock and Clock Options” on page 24.

2.4.4 Port C (PC5..0)

Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

PC5..0 output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port C pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

2.4.5 PC6/RESET

If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical char-

acteristics of PC6 differ from those of the other pins of Port C.

If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin

for longer than the minimum pulse length will generate a Reset, even if the clock is not running.

The minimum pulse length is given in Table 8-1 on page 43. Shorter pulses are not guaranteed

to generate a Reset.

The various special features of Port C are elaborated in “Alternate Functions of Port C” on page

74.

2.4.6 Port D (PD7..0)

Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port D output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port D pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

The various special features of Port D are elaborated in “Alternate Functions of Port D” on page

77.

2.4.7 AVCC

AVCC is the supply voltage pin for the A/D Converter, PC3..0, and ADC7..6. It should be exter-

nally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected

to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC.

2.4.8 AREF

AREF is the analog reference pin for the A/D Converter.

2.4.9 ADC7..6 (TQFP and QFN Package Only)

In the TQFP and QFN package, ADC7..6 serve as analog inputs to the A/D converter. These

pins are powered from the analog supply and serve as 10-bit ADC channels.

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3. About Code Examples

This documentation contains simple code examples that briefly show how to use various parts of

the device. These code examples assume that the part specific header file is included before

compilation. Be aware that not all C compiler vendors include bit definitions in the header files

and interrupt handling in C is compiler dependent. Please confirm with the C compiler documen-

tation for more details.

4. AVR CPU Core

4.1 Introduction

This section discusses the AVR core architecture in general. The main function of the CPU core

is to ensure correct program execution. The CPU must therefore be able to access memories,

perform calculations, control peripherals, and handle interrupts.

4.2 Architectural Overview

Figure 4-1. Block Diagram of the AVR Architecture

Flash

Program

Memory

Instruction

Decoder

Program

Counter

Control Lines

32 x 8

General

Purpose

Registrers

ALU

Status

and Control

I/O Lines

EEPROM

Data Bus 8-bit

Data

SRAM

Direct Addressing

Indirect Addressing

Interrupt

Unit

SPI

Unit

Watchdog

Timer

Analog

Comparator

I/O Module 2

I/O Module1

I/O Module n

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In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with

separate memories and buses for program and data. Instructions in the program memory are

executed with a single level pipelining. While one instruction is being executed, the next instruc-

tion is pre-fetched from the program memory. This concept enables instructions to be executed

in every clock cycle. The program memory is In-System Reprogrammable Flash memory.

The fast-access Register File contains 32 x 8-bit general purpose working registers with a single

clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typ-

ical ALU operation, two operands are output from the Register File, the operation is executed,

and the result is stored back in the Register File – in one clock cycle.

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data

Space addressing – enabling efficient address calculations. One of the these address pointers

can also be used as an address pointer for look up tables in Flash program memory. These

added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.

The ALU supports arithmetic and logic operations between registers or between a constant and

a register. Single register operations can also be executed in the ALU. After an arithmetic opera-

tion, the Status Register is updated to reflect information about the result of the operation.

Program flow is provided by conditional and unconditional jump and call instructions, able to

directly address the whole address space. Most AVR instructions have a single 16-bit word for-

mat. Every program memory address contains a 16- or 32-bit instruction.

Program Flash memory space is divided in two sections, the Boot Program section and the

Application Program section. Both sections have dedicated Lock bits for write and read/write

protection. The SPM instruction that writes into the Application Flash memory section must

reside in the Boot Program section.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the

Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack

size is only limited by the total SRAM size and the usage of the SRAM. All user programs must

initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack

Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed

through the five different addressing modes supported in the AVR architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps.

A flexible interrupt module has its control registers in the I/O space with an additional Global

Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the

Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector posi-

tion. The lower the Interrupt Vector address, the higher the priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-

ters, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data

Space locations following those of the Register File, 0x20 - 0x5F. In addition, the

ATmega48/88/168 has Extended I/O space from 0x60 - 0xFF in SRAM where only the

ST/STS/STD and LD/LDS/LDD instructions can be used.

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4.3 ALU – Arithmetic Logic Unit

The high-performance AVR ALU operates in direct connection with all the 32 general purpose

working registers. Within a single clock cycle, arithmetic operations between general purpose

registers or between a register and an immediate are executed. The ALU operations are divided

into three main categories – arithmetic, logical, and bit-functions. Some implementations of the

architecture also provide a powerful multiplier supporting both signed/unsigned multiplication

and fractional format. See the “Instruction Set” section for a detailed description.

4.4 Status Register

The Status Register contains information about the result of the most recently executed arithme-

tic instruction. This information can be used for altering program flow in order to perform

conditional operations. Note that the Status Register is updated after all ALU operations, as

specified in the Instruction Set Reference. This will in many cases remove the need for using the

dedicated compare instructions, resulting in faster and more compact code.

The Status Register is not automatically stored when entering an interrupt routine and restored

when returning from an interrupt. This must be handled by software.

The AVR Status Register – SREG – is defined as:

• Bit 7 – I: Global Interrupt Enable

The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-

rupt enable control is then performed in separate control registers. If the Global Interrupt Enable

enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by

the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by

the application with the SEI and CLI instructions, as described in the instruction set reference.

• Bit 6 – T: Bit Copy Storage

The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or desti-

nation for the operated bit. A bit from a register in the Register File can be copied into T by the

BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the

BLD instruction.

• Bit 5 – H: Half Carry Flag

The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful

in BCD arithmetic. See the “Instruction Set Description” for detailed information.

• Bit 4 – S: Sign Bit, S = N ⊕ V

The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement

Overflow Flag V. See the “Instruction Set Description” for detailed information.

• Bit 3 – V: Two’s Complement Overflow Flag

The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the

“Instruction Set Description” for detailed information.

Bit 76543210

ITHSVNZCSREG

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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• Bit 2 – N: Negative Flag

The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the

“Instruction Set Description” for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction

Set Description” for detailed information.

• Bit 0 – C: Carry Flag

The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set

Description” for detailed information.

4.5 General Purpose Register File

The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve

the required performance and flexibility, the following input/output schemes are supported by the

• One 8-bit output operand and one 8-bit result input

• Two 8-bit output operands and one 8-bit result input

• Two 8-bit output operands and one 16-bit result input

• One 16-bit output operand and one 16-bit result input

Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.

Figure 4-2. AVR CPU General Purpose Working Registers

Most of the instructions operating on the Register File have direct access to all registers, and

most of them are single cycle instructions.

As shown in Figure 4-2, each register is also assigned a data memory address, mapping them

directly into the first 32 locations of the user Data Space. Although not being physically imple-

mented as SRAM locations, this memory organization provides great flexibility in access of the

registers, as the X-, Y- and Z-pointer registers can be set to index any register in the file.

70Addr.

R0 0x00

R1 0x01

R2 0x02

…

R13 0x0D

General R14 0x0E

Purpose R15 0x0F

Working R16 0x10

Registers R17 0x11

…

R26 0x1A X-register Low Byte

R27 0x1B X-register High Byte

R28 0x1C Y-register Low Byte

R29 0x1D Y-register High Byte

R30 0x1E Z-register Low Byte

R31 0x1F Z-register High Byte

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4.5.1 The X-register, Y-register, and Z-register

The registers R26..R31 have some added functions to their general purpose usage. These reg-

isters are 16-bit address pointers for indirect addressing of the data space. The three indirect

address registers X, Y, and Z are defined as described in Figure 4-3.

Figure 4-3. The X-, Y-, and Z-registers

In the different addressing modes these address registers have functions as fixed displacement,

automatic increment, and automatic decrement (see the instruction set reference for details).

4.6 Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for storing

return addresses after interrupts and subroutine calls. The Stack Pointer Register always points

to the top of the Stack. Note that the Stack is implemented as growing from higher memory loca-

tions to lower memory locations. This implies that a Stack PUSH command decreases the Stack

Pointer.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt

Stacks are located. This Stack space in the data SRAM must be defined by the program before

any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to

point above 0x0100, preferably RAMEND. The Stack Pointer is decremented by one when data

is pushed onto the Stack with the PUSH instruction, and it is decremented by two when the

return address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is

incremented by one when data is popped from the Stack with the POP instruction, and it is incre-

mented by two when data is popped from the Stack with return from subroutine RET or return

from interrupt RETI.

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of

bits actually used is implementation dependent. Note that the data space in some implementa-

tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register

will not be present.

15 XH XL 0

X-register 707 0

R27 (0x1B) R26 (0x1A)

15 YH YL 0

Y-register 707 0

R29 (0x1D) R28 (0x1C)

15 ZH ZL 0

Z-register 7070

R31 (0x1F) R30 (0x1E)

Bit 151413121110 9 8

SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND

RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND

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4.7 Instruction Execution Timing

This section describes the general access timing concepts for instruction execution. The AVR

CPU is driven by the CPU clock clkCPU, directly generated from the selected clock source for the

chip. No internal clock division is used.

Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Har-

vard architecture and the fast-access Register File concept. This is the basic pipelining concept

to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,

functions per clocks, and functions per power-unit.

Figure 4-4. The Parallel Instruction Fetches and Instruction Executions

Figure 4-5 shows the internal timing concept for the Register File. In a single clock cycle an ALU

operation using two register operands is executed, and the result is stored back to the destina-

tion register.

Figure 4-5. Single Cycle ALU Operation

4.8 Reset and Interrupt Handling

The AVR provides several different interrupt sources. These interrupts and the separate Reset

Vector each have a separate program vector in the program memory space. All interrupts are

assigned individual enable bits which must be written logic one together with the Global Interrupt

Enable bit in the Status Register in order to enable the interrupt. Depending on the Program

Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12

are programmed. This feature improves software security. See the section “Memory Program-

ming” on page 278 for details.

The lowest addresses in the program memory space are by default defined as the Reset and

Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page 52. The list also

determines the priority levels of the different interrupts. The lower the address the higher is the

clk

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

T1 T2 T3 T4

CPU

Total Execution Time

ALU Operation Execute

Result Write Back

T1 T2 T3 T4

clkCPU

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priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request

0. The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL

bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 52 for more information.

The Reset Vector can also be moved to the start of the Boot Flash section by programming the

BOOTRST Fuse, see “Boot Loader Support – Read-While-Write Self-Programming, ATmega88

and ATmega168” on page 263.

When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are dis-

abled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled

interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a

Return from Interrupt instruction – RETI – is executed.

There are basically two types of interrupts. The first type is triggered by an event that sets the

Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vec-

tor in order to execute the interrupt handling routine, and hardware clears the corresponding

Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s)

to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is

cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is

cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt

Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the

Global Interrupt Enable bit is set, and will then be executed by order of priority.

The second type of interrupts will trigger as long as the interrupt condition is present. These

interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the

interrupt is enabled, the interrupt will not be triggered.

When the AVR exits from an interrupt, it will always return to the main program and execute one

more instruction before any pending interrupt is served.

Note that the Status Register is not automatically stored when entering an interrupt routine, nor

restored when returning from an interrupt routine. This must be handled by software.

When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled.

No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the

CLI instruction. The following example shows how this can be used to avoid interrupts during the

timed EEPROM write sequence.

Assembly Code Example

in r16, SREG ; store SREG value

cli ; disable interrupts during timed sequence

sbi EECR, EEMPE ; start EEPROM write

sbi EECR, EEPE

out SREG, r16 ; restore SREG value (I-bit)

C Code Example

char cSREG;

cSREG = SREG; /* store SREG value */

/* disable interrupts during timed sequence */

_CLI();

EECR |= (1<<EEMPE); /* start EEPROM write */

EECR |= (1<<EEPE);

SREG = cSREG; /* restore SREG value (I-bit) */

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When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-

cuted before any pending interrupts, as shown in this example.

4.8.1 Interrupt Response Time

The interrupt execution response for all the enabled AVR interrupts is four clock cycles mini-

mum. After four clock cycles the program vector address for the actual interrupt handling routine

is executed. During this four clock cycle period, the Program Counter is pushed onto the Stack.

The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. If

an interrupt occurs during execution of a multi-cycle instruction, this instruction is completed

before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt

execution response time is increased by four clock cycles. This increase comes in addition to the

start-up time from the selected sleep mode.

A return from an interrupt handling routine takes four clock cycles. During these four clock

cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is

incremented by two, and the I-bit in SREG is set.

5. AVR ATmega48/88/168 Memories

This section describes the different memories in the ATmega48/88/168. The AVR architecture

has two main memory spaces, the Data Memory and the Program Memory space. In addition,

the ATmega48/88/168 features an EEPROM Memory for data storage. All three memory spaces

are linear and regular.

5.1 In-System Reprogrammable Flash Program Memory

The ATmega48/88/168 contains 4/8/16K bytes On-chip In-System Reprogrammable Flash

memory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is orga-

nized as 2/4/8K x 16. For software security, the Flash Program memory space is divided into two

sections, Boot Loader Section and Application Program Section in ATmega88 and ATmega168.

ATmega48 does not have separate Boot Loader and Application Program sections, and the

SPM instruction can be executed from the entire Flash. See SELFPRGEN description in section

“Store Program Memory Control and Status Register – SPMCSR” on page 258 and page 268for

more details.

The Flash memory has an endurance of at least 75,000 write/erase cycles. The

ATmega48/88/168 Program Counter (PC) is 11/12/13 bits wide, thus addressing the 2/4/8K pro-

gram memory locations. The operation of Boot Program section and associated Boot Lock bits

for software protection are described in detail in “Self-Programming the Flash, ATmega48” on

page 256 and “Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and

ATmega168” on page 263. “Memory Programming” on page 278 contains a detailed description

on Flash Programming in SPI- or Parallel Programming mode.

Assembly Code Example

sei ; set Global Interrupt Enable

sleep; enter sleep, waiting for interrupt

; note: will enter sleep before any pending interrupt(s)

C Code Example

__enable_interrupt(); /* set Global Interrupt Enable */

__sleep(); /* enter sleep, waiting for interrupt */

/* note: will enter sleep before any pending interrupt(s) */

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Constant tables can be allocated within the entire program memory address space (see the LPM

– Load Program Memory instruction description).

Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Tim-

ing” on page 12.

Figure 5-1. Program Memory Map, ATmega48

0x0000

0x7FF

Program Memory

Application Flash Section

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Figure 5-2. Program Memory Map, ATmega88 and ATmega168

5.2 SRAM Data Memory

Figure 5-3 shows how the ATmega48/88/168 SRAM Memory is organized.

The ATmega48/88/168 is a complex microcontroller with more peripheral units than can be sup-

ported within the 64 locations reserved in the Opcode for the IN and OUT instructions. For the

Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instruc-

tions can be used.

The lower 768/1280/1280 data memory locations address both the Register File, the I/O mem-

ory, Extended I/O memory, and the internal data SRAM. The first 32 locations address the

memory, and the next 512/1024/1024 locations address the internal data SRAM.

The five different addressing modes for the data memory cover: Direct, Indirect with Displace-

ment, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register

File, registers R26 to R31 feature the indirect addressing pointer registers.

The direct addressing reaches the entire data space.

The Indirect with Displacement mode reaches 63 address locations from the base address given

by the Y- or Z-register.

When using register indirect addressing modes with automatic pre-decrement and post-incre-

ment, the address registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O Registers, 160 Extended I/O Registers, and

the 512/1024/1024 bytes of internal data SRAM in the ATmega48/88/168 are all accessible

through all these addressing modes. The Register File is described in “General Purpose Regis-

ter File” on page 10.

0x0000

0x0FFF/0x1FFF

Program Memory

Application Flash Section

Boot Flash Section

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Figure 5-3. Data Memory Map

5.2.1 Data Memory Access Times

This section describes the general access timing concepts for internal memory access. The

internal data SRAM access is performed in two clkCPU cycles as described in Figure 5-4.

Figure 5-4. On-chip Data SRAM Access Cycles

5.3 EEPROM Data Memory

The ATmega48/88/168 contains 256/512/512 bytes of data EEPROM memory. It is organized

as a separate data space, in which single bytes can be read and written. The EEPROM has an

endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the

CPU is described in the following, specifying the EEPROM Address Registers, the EEPROM

Data Register, and the EEPROM Control Register.

“Memory Programming” on page 278 contains a detailed description on EEPROM Programming

in SPI or Parallel Programming mode.

32 Registers

64 I/O Registers

Internal SRAM

(512/1024/1024 x 8)

0x0000 - 0x001F

0x0020 - 0x005F

0x02FF/0x04FF/0x04F

0x0060 - 0x00FF

Data Memory

160 Ext I/O Reg.

0x0100

clk

Data

Address Address valid

T1 T2 T3

Compute Address

Read Write

CPU

Memory Access Instruction Next Instruction

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5.3.1 EEPROM Read/Write Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time for the EEPROM is given in Table 5-2. A self-timing function, however,

lets the user software detect when the next byte can be written. If the user code contains instruc-

tions that write the EEPROM, some precautions must be taken. In heavily filtered power

supplies, VCC is likely to rise or fall slowly on power-up/down. This causes the device for some

period of time to run at a voltage lower than specified as minimum for the clock frequency used.

See “Preventing EEPROM Corruption” on page 22 for details on how to avoid problems in these

situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.

Refer to the description of the EEPROM Control Register for details on this.

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is

executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next

instruction is executed.

5.3.2 The EEPROM Address Register – EEARH and EEARL

• Bits 15..9 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bits 8..0 – EEAR8..0: EEPROM Address

The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the

256/512/512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0

and 255/511/511. The initial value of EEAR is undefined. A proper value must be written before

the EEPROM may be accessed.

EEAR8 is an unused bit in ATmega48 and must always be written to zero.

5.3.3 The EEPROM Data Register – EEDR

• Bits 7..0 – EEDR7.0: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to the

EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the

EEDR contains the data read out from the EEPROM at the address given by EEAR.

Bit 151413121110 9 8

–––––––EEAR8EEARH

EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL

76543210

Read/Write RRRRRRRR/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0000000X

XXXXXXXX

Bit 76543210

MSB LSB EEDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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5.3.4 The EEPROM Control Register – EECR

• Bits 7..6 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits

The EEPROM Programming mode bit setting defines which programming action that will be trig-

gered when writing EEPE. It is possible to program data in one atomic operation (erase the old

value and program the new value) or to split the Erase and Write operations in two different

operations. The Programming times for the different modes are shown in Table 5-1. While EEPE

is set, any write to EEPMn will be ignored. During reset, the EEPMn bits will be reset to 0b00

unless the EEPROM is busy programming.

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing

EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant inter-

rupt when EEPE is cleared.

• Bit 2 – EEMPE: EEPROM Master Write Enable

The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written.

When EEMPE is set, setting EEPE within four clock cycles will write data to the EEPROM at the

selected address If EEMPE is zero, setting EEPE will have no effect. When EEMPE has been

written to one by software, hardware clears the bit to zero after four clock cycles. See the

description of the EEPE bit for an EEPROM write procedure.

• Bit 1 – EEPE: EEPROM Write Enable

The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address

and data are correctly set up, the EEPE bit must be written to one to write the value into the

EEPROM. The EEMPE bit must be written to one before a logical one is written to EEPE, other-

wise no EEPROM write takes place. The following procedure should be followed when writing

the EEPROM (the order of steps 3 and 4 is not essential):

1. Wait until EEPE becomes zero.

2. Wait until SELFPRGEN in SPMCSR becomes zero.

3. Write new EEPROM address to EEAR (optional).

4. Write new EEPROM data to EEDR (optional).

Bit 76543210

– – EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial Value 0 0 X X 0 0 X 0

Table 5-1. EEPROM Mode Bits

EEPM1 EEPM0

Programming

Time Operation

0 0 3.4 ms Erase and Write in one operation (Atomic Operation)

0 1 1.8 ms Erase Only

1 0 1.8 ms Write Only

1 1 – Reserved for future use

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5. Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR.

6. Within four clock cycles after setting EEMPE, write a logical one to EEPE.

The EEPROM can not be programmed during a CPU write to the Flash memory. The software

must check that the Flash programming is completed before initiating a new EEPROM write.

Step 2 is only relevant if the software contains a Boot Loader allowing the CPU to program the

Flash. If the Flash is never being updated by the CPU, step 2 can be omitted. See “Boot Loader

Support – Read-While-Write Self-Programming, ATmega88 and ATmega168” on page 263 for

details about Boot programming.

Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the

EEPROM Master Write Enable will time-out. If an interrupt routine accessing the EEPROM is

interrupting another EEPROM access, the EEAR or EEDR Register will be modified, causing the

interrupted EEPROM access to fail. It is recommended to have the Global Interrupt Flag cleared

during all the steps to avoid these problems.

When the write access time has elapsed, the EEPE bit is cleared by hardware. The user soft-

ware can poll this bit and wait for a zero before writing the next byte. When EEPE has been set,

the CPU is halted for two cycles before the next instruction is executed.

• Bit 0 – EERE: EEPROM Read Enable

The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct

address is set up in the EEAR Register, the EERE bit must be written to a logic one to trigger the

EEPROM read. The EEPROM read access takes one instruction, and the requested data is

available immediately. When the EEPROM is read, the CPU is halted for four cycles before the

next instruction is executed.

The user should poll the EEPE bit before starting the read operation. If a write operation is in

progress, it is neither possible to read the EEPROM, nor to change the EEAR Register.

The calibrated Oscillator is used to time the EEPROM accesses. Table 5-2 lists the typical pro-

gramming time for EEPROM access from the CPU.

The following code examples show one assembly and one C function for writing to the

EEPROM. The examples assume that interrupts are controlled (e.g. by disabling interrupts glob-

ally) so that no interrupts will occur during execution of these functions. The examples also

assume that no Flash Boot Loader is present in the software. If such code is present, the

EEPROM write function must also wait for any ongoing SPM command to finish.

Table 5-2. EEPROM Programming Time

Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time

EEPROM write

(from CPU) 26,368 3.3 ms

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The next code examples show assembly and C functions for reading the EEPROM. The exam-

ples assume that interrupts are controlled so that no interrupts will occur during execution of

these functions.

Assembly Code Example

EEPROM_write:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_write

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Write data (r16) to Data Register

out EEDR,r16

; Write logical one to EEMPE

sbi EECR,EEMPE

; Start eeprom write by setting EEPE

sbi EECR,EEPE

ret

C Code Example

void EEPROM_write(unsigned int uiAddress, unsigned char ucData)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address and Data Registers */

EEAR = uiAddress;

EEDR = ucData;

/* Write logical one to EEMPE */

EECR |= (1<<EEMPE);

/* Start eeprom write by setting EEPE */

EECR |= (1<<EEPE);

}

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5.3.5 Preventing EEPROM Corruption

During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is

too low for the CPU and the EEPROM to operate properly. These issues are the same as for

board level systems using EEPROM, and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First,

a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Sec-

ondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can

be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal

BOD does not match the needed detection level, an external low VCC reset Protection circuit can

be used. If a reset occurs while a write operation is in progress, the write operation will be com-

pleted provided that the power supply voltage is sufficient.

Assembly Code Example

EEPROM_read:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_read

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Start eeprom read by writing EERE

sbi EECR,EERE

; Read data from Data Register

in r16,EEDR

ret

C Code Example

unsigned char EEPROM_read(unsigned int uiAddress)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from Data Register */

return EEDR;

}

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5.4 I/O Memory

The I/O space definition of the ATmega48/88/168 is shown in “Register Summary” on page 318.

All ATmega48/88/168 I/Os and peripherals are placed in the I/O space. All I/O locations may be

accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32

general purpose working registers and the I/O space. I/O Registers within the address range

0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the

value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the

instruction set section for more details. When using the I/O specific commands IN and OUT, the

I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using

LD and ST instructions, 0x20 must be added to these addresses. The ATmega48/88/168 is a

complex microcontroller with more peripheral units than can be supported within the 64 location

reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60 -

0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.

For compatibility with future devices, reserved bits should be written to zero if accessed.

Reserved I/O memory addresses should never be written.

Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most

other AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore

be used on registers containing such Status Flags. The CBI and SBI instructions work with reg-

isters 0x00 to 0x1F only.

The I/O and peripherals control registers are explained in later sections.

5.4.1 General Purpose I/O Registers

The ATmega48/88/168 contains three General Purpose I/O Registers. These registers can be

used for storing any information, and they are particularly useful for storing global variables and

Status Flags. General Purpose I/O Registers within the address range 0x00 - 0x1F are directly

bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.

5.4.2 General Purpose I/O Register 2 – GPIOR2

5.4.3 General Purpose I/O Register 1 – GPIOR1

5.4.4 General Purpose I/O Register 0 – GPIOR0

Bit 76543210

MSB LSB GPIOR2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

MSB LSB GPIOR1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

MSB LSB GPIOR0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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6. System Clock and Clock Options

6.1 Clock Systems and their Distribution

Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks

need not be active at a given time. In order to reduce power consumption, the clocks to modules

not being used can be halted by using different sleep modes, as described in “Power Manage-

ment and Sleep Modes” on page 36. The clock systems are detailed below.

Figure 6-1. Clock Distribution

6.1.1 CPU Clock – clkCPU

The CPU clock is routed to parts of the system concerned with operation of the AVR core.

Examples of such modules are the General Purpose Register File, the Status Register and the

data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing

general operations and calculations.

6.1.2 I/O Clock – clkI/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART.

The I/O clock is also used by the External Interrupt module, but note that some external inter-

rupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O

clock is halted. Also note that start condition detection in the USI module is carried out asynchro-

nously when clkI/O is halted, TWI address recognition in all sleep modes.

6.1.3 Flash Clock – clkFLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-

taneously with the CPU clock.

General I/O

Modules

Asynchronous

Timer/Counter CPU Core RAM

clk

I/O

clk

ASY

AVR Clock

Control Unit

clk

CPU

Flash and

EEPROM

clk

FLASH

Source clock

Watchdog Timer

Watchdog

Oscillator

Reset Logic

Clock

Multiplexer

Watchdog clock

Calibrated RC

Oscillator

Timer/Counter

Oscillator

Crystal

Oscillator

Low-frequency

Crystal Oscillator

External Clock

ADC

clkADC

System Clock

Prescaler

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6.1.4 Asynchronous Timer Clock – clkASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly

from an external clock or an external 32 kHz clock crystal. The dedicated clock domain allows

using this Timer/Counter as a real-time counter even when the device is in sleep mode.

6.1.5 ADC Clock – clkADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks

in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion

results.

6.2 Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown

below. The clock from the selected source is input to the AVR clock generator, and routed to the

appropriate modules.

Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.

6.2.1 Default Clock Source

The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 pro-

grammed, resulting in 1.0MHz system clock. The startup time is set to maximum and time-out

period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that

all users can make their desired clock source setting using any available programming interface.

6.2.2 Clock Startup Sequence

Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating

cycles before it can be considered stable.

To ensure sufficient VCC, the device issues an internal reset with a time-out delay (tTOUT) after

the device reset is released by all other reset sources. “System Control and Reset” on page 41

describes the start conditions for the internal reset. The delay (tTOUT) is timed from the Watchdog

Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. Th e

selectable delays are shown in Table 6-2. The frequency of the Watchdog Oscillator is voltage

dependent as shown in “Register Summary” on page 318.

Table 6-1. Device Clocking Options Select(1)

Device Clocking Option CKSEL3..0

Low Power Crystal Oscillator 1111 - 1000

Full Swing Crystal Oscillator 0111 - 0110

Low Frequency Crystal Oscillator 0101 - 0100

Internal 128 kHz RC Oscillator 0011

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

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Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum VCC. The

delay will not monitor the actual voltage and it will be required to select a delay longer than the

VCC rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be

used. A BOD circuit will ensure sufficient VCC before it releases the reset, and the time-out delay

can be disabled. Disabling the time-out delay without utilizing a Brown-Out Detection circuit is

not recommended.

The oscillator is required to oscillate for a minimum number of cycles before the clock is consid-

ered stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal

reset active for a given number of clock cycles. The reset is then released and the device will

start to execute. The recommended oscillator start-up time is dependent on the clock type, and

varies from 6 cycles for an externally applied clock to 32K cycles for a low frequency crystal.

The start-up sequence for the clock includes both the time-out delay and the start-up time when

the device starts up from reset. When starting up from Power-save or Power-down mode, VCC is

assumed to be at a sufficient level and only the start-up time is included.

6.3 Low Power Crystal Oscillator

Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be

configured for use as an On-chip Oscillator, as shown in Figure 6-2. Either a quartz crystal or a

ceramic resonator may be used.

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 out-

put. It gives the lowest power consumption, but is not capable of driving other clock inputs, and

may be more susceptible to noise in noisy environments. In these cases, refer to the “Full Swing

Crystal Oscillator” on page 28.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the

capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the

electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for

use with crystals are given in Table 6-3. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

Table 6-2. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles

0 ms 0 ms 0

4.1 ms 4.3 ms 4K (4,096)

65 ms 69 ms 8K (8,192)

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Figure 6-2. Crystal Oscillator Connections

The Low Power Oscillator can operate in three different modes, each optimized for a specific fre-

quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-3

on page 27.

Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.

2. This option should not be used with crystals, only with ceramic resonators.

3. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8

Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table

6-4.

Table 6-3. Low Power Crystal Oscillator Operating Modes(3)

Frequency Range(1) (MHz) CKSEL3..1

Recommended Range for Capacitors C1

and C2 (pF)

0.4 - 0.9 100(2) –

0.9 - 3.0 101 12 - 22

3.0 - 8.0 110 12 - 22

8.0 - 16.0 111 12 - 22

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Ceramic resonator, fast

rising power 258 CK 14CK + 4.1 ms(1) 000

Ceramic resonator, slowly

rising power 258 CK 14CK + 65 ms(1) 001

Ceramic resonator, BOD

enabled 1K CK 14CK(2) 010

Ceramic resonator, fast

rising power 1K CK 14CK + 4.1 ms(2) 011

Ceramic resonator, slowly

rising power 1K CK 14CK + 65 ms(2) 100

XTAL2

XTAL1

GND

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Atmel ATmega48/88/168 Automotive

Notes: 1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

6.4 Full Swing Crystal Oscillator

Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be

configured for use as an On-chip Oscillator, as shown in Figure 6-2. Either a quartz crystal or a

ceramic resonator may be used.

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is

useful for driving other clock inputs and in noisy environments. The current consumption is

higher than the “Low Power Crystal Oscillator” on page 26. Note that the Full Swing Crystal

Oscillator will only operate for VCC = 2.7 - 5.5 volts.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the

capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the

electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for

use with crystals are given in Table 6-6. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-5.

Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8

Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

Crystal Oscillator, BOD

enabled 16K CK 14CK 1 01

Crystal Oscillator, fast

rising power 16K CK 14CK + 4.1 ms 1 10

Crystal Oscillator, slowly

rising power 16K CK 14CK + 65 ms 1 11

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection (Continued)

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Table 6-5. Full Swing Crystal Oscillator operating modes(2)

Frequency Range(1) (MHz) CKSEL3..1

Recommended Range for Capacitors C1

and C2 (pF)

0.4 - 20 011 12 - 22

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Figure 6-3. Crystal Oscillator Connections

Notes: 1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

Table 6-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Ceramic resonator, fast

rising power 258 CK 14CK + 4.1 ms(1) 000

Ceramic resonator, slowly

rising power 258 CK 14CK + 65 ms(1) 001

Ceramic resonator, BOD

enabled 1K CK 14CK(2) 010

Ceramic resonator, fast

rising power 1K CK 14CK + 4.1 ms(2) 011

Ceramic resonator, slowly

rising power 1K CK 14CK + 65 ms(2) 100

Crystal Oscillator, BOD

enabled 16K CK 14CK 1 01

Crystal Oscillator, fast

rising power 16K CK 14CK + 4.1 ms 1 10

Crystal Oscillator, slowly

rising power 16K CK 14CK + 65 ms 1 11

XTAL2

XTAL1

GND

30
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Atmel ATmega48/88/168 Automotive
6.5 Low Frequency Crystal Oscillator
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-
quency Crystal Oscillator. The crystal should be connected as shown in Figure 6-2. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 6-7.
Note: 1. These options should only be used if frequency stability at start-up is not important for the 
application.
6.6 Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-
inal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
“System Clock Prescaler” on page 33 for more details. This clock may be selected as the system
clock by programming the CKSEL Fuses as shown in Table 6-8. If selected, it will operate with
no external components. During reset, hardware loads the calibration byte into the OSCCAL
Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration
gives a frequency of 8 MHz ± 1%. The tolerance of the internal RC oscillator remains better than
±10% within the whole automotive temperature and voltage ranges (2.7V to 5.5V, -40°C to
+125°C). The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1%
accuracy, by changing the OSCCAL register. When this Oscillator is used as the chip clock, the
Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For
more information on the pre-programmed calibration value, see the section “Calibration Byte” on
page 282.
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 
Fuse can be programmed in order to divide the internal frequency by 8.
Table 6-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from 
Power-down and 
Power-save
Additional Delay 
from Reset 
(VCC = 5.0V) CKSEL0 SUT1..0
BOD enabled 1K CK 14CK(1) 000
Fast rising power 1K CK 14CK + 4.1 ms(1) 001
Slowly rising power 1K CK 14CK + 65 ms(1) 010
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4.1 ms 1 01
Slowly rising power 32K CK 14CK + 65 ms 1 10
Reserved 1 11
Table 6-8. Internal Calibrated RC Oscillator Operating Modes(1)(3)
Frequency Range(2) (MHz)  CKSEL3..0
7.3 - 8.1 0010

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When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-9 on page 31.

Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

2. The device is shipped with this option selected.

6.6.1 Oscillator Calibration Register – OSCCAL

• Bits 7..0 – CAL7..0: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to

remove process variations from the oscillator frequency. The factory-calibrated value is automat-

ically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C.

The application software can write this register to change the oscillator frequency. The oscillator

can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1% accuracy. Calibration

outside that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write

times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more

than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.

The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the

lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-

quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher

frequency than OSCCAL = 0x80.

The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00

gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the

range. Incrementing CAL6..0 by 1 will give a frequency increment of less than 2% in the fre-

quency range 7.3 - 8.1 MHz.

6.7 128 kHz Internal Oscillator

The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-

quency is nominal at 3V and 25°C. This clock may be select as the system clock by

Table 6-9. Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from

Power-down and Power-save

Additional Delay from

Reset (VCC = 5.0V) SUT1..0

BOD enabled 6 CK 14CK(1) 00

Fast rising power 6 CK 14CK + 4.1 ms 01

Slowly rising power 6 CK 14CK + 65 ms(2) 10

Reserved 11

Bit 76543210

CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value Device Specific Calibration Value

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Atmel ATmega48/88/168 Automotive

programming the CKSEL Fuses to “11” as shown in Table 6-10.

Note: 1. The frequency is preliminary value. Actual value is TBD.

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-11.

Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

6.8 External Clock

The device can utilize a external clock source as shown in Figure 6-4. To run the device on an

external clock, the CKSEL Fuses must be programmed as shown in Table 6-12.

Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8

Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

Figure 6-4. External Clock Drive Configuration

Table 6-10. 128 kHz Internal Oscillator Operating Modes

Nominal Frequency CKSEL3..0

128 kHz 0011

Table 6-11. Start-up Times for the 128 kHz Internal Oscillator

Power Conditions

Start-up Time from

Power-down and Power-save

Additional Delay from

Reset SUT1..0

BOD enabled 6 CK 14CK(1) 00

Fast rising power 6 CK 14CK + 4 ms 01

Slowly rising power 6 CK 14CK + 64 ms 10

Reserved 11

Table 6-12. Full Swing Crystal Oscillator operating modes(2)

Frequency Range(1) (MHz) CKSEL3..0

Recommended Range for Capacitors C1

and C2 (pF)

0 - 100 0000 12 - 22

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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Atmel ATmega48/88/168 Automotive

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-13.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is

required, ensure that the MCU is kept in Reset during the changes.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal

clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page

33 for details.

6.9 Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT

Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other cir-

cuits on the system. The clock also will be output during reset, and the normal operation of I/O

pin will be overridden when the fuse is programmed. Any clock source, including the internal RC

Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is

used, it is the divided system clock that is output.

6.10 Timer/Counter Oscillator

The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a exter-

nal clock source. The Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with

XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only be used when an

internal RC Oscillator is selected as system clock source. See Figure 6-2 on page 27 for crystal

connection.

Applying an external clock source to TOSC1 requires EXTCLK in the ASSR Register written to

logic one. See “Asynchronous operation of the Timer/Counter” on page 152 for further descrip-

tion on selecting external clock as input instead of a 32 kHz crystal.

6.11 System Clock Prescaler

The ATmega48/88/168 has a system clock prescaler, and the system clock can be divided by

setting the “Clock Prescale Register – CLKPR” on page 357. This feature can be used to

decrease the system clock frequency and the power consumption when the requirement for pro-

cessing power is low. This can be used with all clock source options, and it will affect the clock

frequency of the CPU and all synchronous peripherals. clkI/O, clkADC, clkCPU, and clkFLASH are

divided by a factor as shown in Table 8-1 on page 43.

When switching between prescaler settings, the System Clock Prescaler ensures that no

glitches occurs in the clock system. It also ensures that no intermediate frequency is higher than

Table 6-13. Start-up Times for the External Clock Selection

Power Conditions

Start-up Time from

Power-down and Power-save

Additional Delay from

Reset (VCC = 5.0V) SUT1..0

BOD enabled 6 CK 14CK 00

Fast rising power 6 CK 14CK + 4.1 ms 01

Slowly rising power 6 CK 14CK + 65 ms 10

Reserved 11

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Atmel ATmega48/88/168 Automotive

neither the clock frequency corresponding to the previous setting, nor the clock frequency corre-

sponding to the new setting. The ripple counter that implements the prescaler runs at the

frequency of the undivided clock, which may be faster than the CPU's clock frequency. Hence, it

is not possible to determine the state of the prescaler - even if it were readable, and the exact

time it takes to switch from one clock division to the other cannot be exactly predicted. From the

time the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new

clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is the pre-

vious clock period, and T2 is the period corresponding to the new prescaler setting.

To avoid unintentional changes of clock frequency, a special write procedure must befollowed to

change the CLKPS bits:

1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bitsin

CLKPR to zero.

2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.

Interrupts must be disabled when changing prescaler setting to make sure the write procedure is

not interrupted.

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Atmel ATmega48/88/168 Automotive

6.11.1 Clock Prescale Register – CLKPR

• Bit 7 – CLKPCE: Clock Prescaler Change Enable

The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE

bit is only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is

cleared by hardware four cycles after it is written or when CLKPS bits are written. Rewriting the

CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the

CLKPCE bit.

• Bits 3..0 – CLKPS3..0: Clock Prescaler Select Bits 3 - 0

These bits define the division factor between the selected clock source and the internal system

clock. These bits can be written run-time to vary the clock frequency to suit the application

requirements. As the divider divides the master clock input to the MCU, the speed of all synchro-

nous peripherals is reduced when a division factor is used. The division factors are given in

Table 6-14.

The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed,

the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to

“0011”, giving a division factor of 8 at start up. This feature should be used if the selected clock

source has a higher frequency than the maximum frequency of the device at the present operat-

ing conditions. Note that any value can be written to the CLKPS bits regardless of the CKDIV8

Fuse setting. The Application software must ensure that a sufficient division factor is chosen if

the selected clock source has a higher frequency than the maximum frequency of the device at

the present operating conditions. The device is shipped with the CKDIV8 Fuse programmed.

Bit 76543210

CLKPCE – – – CLKPS3 CLKPS2 CLKPS1 CLKPS0 CLKPR

Read/Write R/W R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 See Bit Description

Table 6-14. Clock Prescaler Select

CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor

0000 1

0001 2

0010 4

0011 8

0100 16

0101 32

0110 64

0111 128

1000 256

1001 Reserved

1010 Reserved

1011 Reserved

1100 Reserved

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Atmel ATmega48/88/168 Automotive

7. Power Management and Sleep Modes

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving

power. The AVR provides various sleep modes allowing the user to tailor the power consump-

tion to the application’s requirements.

To enter any of the five sleep modes, the SE bit in SMCR must be written to logic one and a

SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select

which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, or Standby) will be

activated by the SLEEP instruction. See Table 7-1 for a summary. If an enabled interrupt occurs

while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for four cycles in

addition to the start-up time, executes the interrupt routine, and resumes execution from the

instruction following SLEEP. The contents of the Register File and SRAM are unaltered when

the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes up and

executes from the Reset Vector.

Figure 6-1 on page 24 presents the different clock systems in the ATmega48/88/168, and their

distribution. The figure is helpful in selecting an appropriate sleep mode.

7.0.1 Sleep Mode Control Register – SMCR

The Sleep Mode Control Register contains control bits for power management.

• Bits 7..4 Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bits 3..1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the five available sleep modes as shown in Table 7-1.

1101 Reserved

1110 Reserved

1111 Reserved

Table 6-14. Clock Prescaler Select

CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor

Bit 76543210

––––SM2SM1SM0SESMCR

Read/Write RRRRR/WR/WR/WR/W

Initial Value00000000

Table 7-1. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

000Idle

0 0 1 ADC Noise Reduction

010Power-down

011Power-save

100Reserved

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Atmel ATmega48/88/168 Automotive

Note: 1. Standby mode is only recommended for use with external crystals or resonators.

• Bit 0 – SE: Sleep Enable

The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP

instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s

purpose, it is recommended to write the Sleep Enable (SE) bit to one just before the execution of

the SLEEP instruction and to clear it immediately after waking up.

7.1 Idle Mode

When the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle

mode, stopping the CPU but allowing the SPI, USART, Analog Comparator, ADC, 2-wire Serial

Interface, Timer/Counters, Watchdog, and the interrupt system to continue operating. This sleep

mode basically halts clkCPU and clkFLASH, while allowing the other clocks to run.

Idle mode enables the MCU to wake up from external triggered interrupts as well as internal

ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the

Analog Comparator interrupt is not required, the Analog Comparator can be powered down by

setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will

reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automati-

cally when this mode is entered.

7.2 ADC Noise Reduction Mode

When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC

Noise Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the

2-wire Serial Interface address watch, Timer/Counter2, and the Watchdog to continue operating

(if enabled). This sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other

clocks to run.

This improves the noise environment for the ADC, enabling higher resolution measurements. If

the ADC is enabled, a conversion starts automatically when this mode is entered. Apart from the

ADC Conversion Complete interrupt, only an External Reset, a Watchdog System Reset, a

Watchdog Interrupt, a Brown-out Reset, a 2-wire Serial Interface address match, a

Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, an external level interrupt on INT0

or INT1 or a pin change interrupt can wake up the MCU from ADC Noise Reduction mode.

7.3 Power-down Mode

When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter

Power-down mode. In this mode, the external Oscillator is stopped, while the external interrupts,

the 2-wire Serial Interface address watch, and the Watchdog continue operating (if enabled).

Only an External Reset, a Watchdog System Reset, a Watchdog Interrupt, a Brown-out Reset, a

2-wire Serial Interface address match, an external level interrupt on INT0 or INT1, or a pin

change interrupt can wake up the MCU. This sleep mode basically halts all generated clocks,

allowing operation of asynchronous modules only.

101Reserved

110Standby

(1)

111Reserved

Table 7-1. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

38
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Atmel ATmega48/88/168 Automotive
Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed
level must be held for some time to wake up the MCU. Refer to “External Interrupts” on page 81
for details.
When waking up from Power-down mode, there is a delay from the wake-up condition occurs
until the wake-up becomes effective. This allows the clock to restart and become stable after
having been stopped. The wake-up period is defined by the same CKSEL Fuses that define the
Reset Time-out period, as described in “Clock Sources” on page 25.
7.4 Power-save Mode
When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter
Power-save mode. This mode is identical to Power-down, with one exception:
If Timer/Counter2 is enabled, it will keep running during sleep. The device can wake up from
either Timer Overflow or Output Compare event from Timer/Counter2 if the corresponding
Timer/Counter2 interrupt enable bits are set in TIMSK2, and the Global Interrupt Enable bit in
SREG is set.
If Timer/Counter2 is not running, Power-down mode is recommended instead of Power-save
mode.
The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-save
mode. If Timer/Counter2 is not using the asynchronous clock, the Timer/Counter Oscillator is
stopped during sleep. If Timer/Counter2 is not using the synchronous clock, the clock source is
stopped during sleep. Note that even if the synchronous clock is running in Power-save, this
clock is only available for Timer/Counter2.
7.5 Standby Mode
When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down
with the exception that the Oscillator is kept running. From Standby mode, the device wakes up
in six clock cycles. 
Notes: 1. Only recommended with external crystal or resonator selected as clock source.
2. If Timer/Counter2 is running in asynchronous mode.
3. For INT1 and INT0, only level interrupt.
Table 7-2. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Active Clock Domains Oscillators Wake-up Sources
Sleep Mode
clkCPU
clkFLASH
clkIO
clkADC
clkASY
Main Clock 
Source Enabled
Timer Oscillator
Enabled
INT1, INT0 and 
Pin Change
TWI Address 
Match
Timer2
SPM/EEPROM
Ready
ADC
WDT
OtherI/O
Idle XXX X X
(2) X XXXXXX
ADC Noise
Reduction XX X X
(2) X(3) XXXXX
Power-down X(3) XX
Power-save X X X(3) XX X
Standby(1) XX
(3) XX

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Atmel ATmega48/88/168 Automotive

7.6 Power Reduction Register

The Power Reduction Register, PRR, provides a method to stop the clock to individual peripher-

als to reduce power consumption. The current state of the peripheral is frozen and the I/O

registers can not be read or written. Resources used by the peripheral when stopping the clock

will remain occupied, hence the peripheral should in most cases be disabled before stopping the

clock. Waking up a module, which is done by clearing the bit in PRR, puts the module in the

same state as before shutdown.

Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall

power consumption. See Section 28.1.1 “Power-Down Supply Current” on page 308 for exam-

ples. In all other sleep modes, the clock is already stopped.

7.6.1 Power Reduction Register - PRR

• Bit 7 - PRTWI: Power Reduction TWI

Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When

waking up the TWI again, the TWI should be re-initialized to ensure proper operation.

Bit 6 - PRTIM2: Power Reduction Timer/Counter2

Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous mode (AS2

is 0). When the Timer/Counter2 is enabled, operation will continue like before the shutdown.

• Bit 5 - PRTIM0: Power Reduction Timer/Counter0

Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0

is enabled, operation will continue like before the shutdown.

• Bit 4 - Res: Reserved bit

This bit is reserved in ATmega48/88/168 and will always read as zero.

• Bit 3 - PRTIM1: Power Reduction Timer/Counter1

Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1

is enabled, operation will continue like before the shutdown.

• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface

If using debugWIRE On-chip Debug System, this bit should not be written to one. Writing a logic

one to this bit shuts down the Serial Peripheral Interface by stopping the clock to the module.

When waking up the SPI again, the SPI should be re-initialized to ensure proper operation.

• Bit 1 - PRUSART0: Power Reduction USART0

Writing a logic one to this bit shuts down the USART by stopping the clock to the module. When

waking up the USART again, the USART should be re-initialized to ensure proper operation.

• Bit 0 - PRADC: Power Reduction ADC

Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down.

The analog comparator cannot use the ADC input MUX when the ADC is shut down.

Bit 765432 1 0

PRTWI PRTIM2 PRTIM0 – PRTIM1 PRSPI PRUSART0 PRADC PRR

Read/Write R/W R/W R/W R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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Atmel ATmega48/88/168 Automotive

7.7 Minimizing Power Consumption

There are several possibilities to consider when trying to minimize the power consumption in an

AVR controlled system. In general, sleep modes should be used as much as possible, and the

sleep mode should be selected so that as few as possible of the device’s functions are operat-

ing. All functions not needed should be disabled. In particular, the following modules may need

special consideration when trying to achieve the lowest possible power consumption.

7.7.1 Analog to Digital Converter

If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be dis-

abled before entering any sleep mode. When the ADC is turned off and on again, the next

conversion will be an extended conversion. Refer to “Analog-to-Digital Converter” on page 237

for details on ADC operation.

7.7.2 Analog Comparator

When entering Idle mode, the Analog Comparator should be disabled if not used. When entering

ADC Noise Reduction mode, the Analog Comparator should be disabled. In other sleep modes,

the Analog Comparator is automatically disabled. However, if the Analog Comparator is set up

to use the Internal Voltage Reference as input, the Analog Comparator should be disabled in all

sleep modes. Otherwise, the Internal Voltage Reference will be enabled, independent of sleep

mode. Refer to “Analog Comparator” on page 233 for details on how to configure the Analog

Comparator.

7.7.3 Brown-out Detector

If the Brown-out Detector is not needed by the application, this module should be turned off. If

the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep

modes, and hence, always consume power. In the deeper sleep modes, this will contribute sig-

nificantly to the total current consumption. Refer to “Brown-out Detection” on page 44 for details

on how to configure the Brown-out Detector.

7.7.4 Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the

Analog Comparator or the ADC. If these modules are disabled as described in the sections

above, the internal voltage reference will be disabled and it will not be consuming power. When

turned on again, the user must allow the reference to start up before the output is used. If the

reference is kept on in sleep mode, the output can be used immediately. Refer to “Internal Volt-

age Reference” on page 46 for details on the start-up time.

7.7.5 Watchdog Timer

If the Watchdog Timer is not needed in the application, the module should be turned off. If the

Watchdog Timer is enabled, it will be enabled in all sleep modes and hence always consume

power. In the deeper sleep modes, this will contribute significantly to the total current consump-

tion. Refer to “Watchdog Timer” on page 47 for details on how to configure the Watchdog Timer.

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7.7.6 Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power. The

most important is then to ensure that no pins drive resistive loads. In sleep modes where both

the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will

be disabled. This ensures that no power is consumed by the input logic when not needed. In

some cases, the input logic is needed for detecting wake-up conditions, and it will then be

enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 67 for details on

which pins are enabled. If the input buffer is enabled and the input signal is left floating or have

an analog signal level close to VCC/2, the input buffer will use excessive power.

For analog input pins, the digital input buffer should be disabled at all times. An analog signal

level close to VCC/2 on an input pin can cause significant current even in active mode. Digital

input buffers can be disabled by writing to the Digital Input Disable Registers (DIDR1 and

DIDR0). Refer to “Digital Input Disable Register 1 – DIDR1” on page 236 and “Digital Input Dis-

able Register 0 – DIDR0” on page 253 for details.

7.7.7 On-chip Debug System

If the On-chip debug system is enabled by the DWEN Fuse and the chip enters sleep mode, the

main clock source is enabled and hence always consumes power. In the deeper sleep modes,

this will contribute significantly to the total current consumption.

8. System Control and Reset

8.0.1 Resetting the AVR

During reset, all I/O Registers are set to their initial values, and the program starts execution

from the Reset Vector. For the ATmega168, the instruction placed at the Reset Vector must be a

JMP – Absolute Jump – instruction to the reset handling routine. For the ATmega48 and

ATmega88, the instruction placed at the Reset Vector must be an RJMP – Relative Jump –

instruction to the reset handling routine. If the program never enables an interrupt source, the

Interrupt Vectors are not used, and regular program code can be placed at these locations. This

is also the case if the Reset Vector is in the Application section while the Interrupt Vectors are in

the Boot section or vice versa (ATmega88/168 only). The circuit diagram in Figure 8-1 shows the

reset logic. Table 8-1 on page 43 defines the electrical parameters of the reset circuitry.

The I/O ports of the AVR are immediately reset to their initial state when a reset source goes

active. This does not require any clock source to be running.

After all reset sources have gone inactive, a delay counter is invoked, stretching the internal

reset. This allows the power to reach a stable level before normal operation starts. The time-out

period of the delay counter is defined by the user through the SUT and CKSEL Fuses. The dif-

ferent selections for the delay period are presented in “Clock Sources” on page 25.

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8.0.2 Reset Sources

The ATmega48/88/168 has four sources of reset:

• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset

threshold (VPOT).

• External Reset. The MCU is reset when a low level is present on the RESET pin for longer

than the minimum pulse length.

• Watchdog System Reset. The MCU is reset when the Watchdog Timer period expires and

the Watchdog System Reset mode is enabled.

• Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out

Reset threshold (VBOT) and the Brown-out Detector is enabled.

Figure 8-1. Reset Logic

8.0.3 Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level

is defined in Table 8-1 on page 43. The POR is activated whenever VCC is below the detection

level. The POR circuit can be used to trigger the start-up Reset, as well as to detect a failure in

supply voltage.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the

Power-on Reset threshold voltage invokes the delay counter, which determines how long the

device is kept in RESET after VCC rise. The RESET signal is activated again, without any delay,

when VCC decreases below the detection level.

MCU Status

Brown-out

Reset Circuit

BODLEVEL [2..0]

Delay Counters

CKSEL[3:0]

TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA BUS

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog

Oscillator

SUT[1:0]

Power-on Reset

Circuit

RSTDISBL

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Figure 8-2. MCU Start-up, RESET Tied to VCC

Figure 8-3. MCU Start-up, RESET Extended Externally

Note: 1. Before rising, the supply has to be between VPORMIN and VPORMAX to ensure a Reset.

Table 8-1. Power On Reset Specifications

Symbol Parameter Min Typ Max Units

VPOT

Power-on Reset Threshold Voltage (rising) 1.1 1.4 1.7 V

Power-on Reset Threshold Voltage (falling)(1) 0.8 1.3 1.6 V

VPORMAX VCC Max. start voltage to ensure internal

Power-on Reset signal 0.4 V

VPORMIN VCC Min. start voltage to ensure internal

Power-on Reset signal –0.1 V

VCCRR VCC Rise Rate to ensure Power-on Reset 0.01 V/ms

VRST RESET Pin Threshold Voltage 0.1VCC 0.9VCC V

tRST Minimum pulse width on RESET pin 2.5 µs

RESET

TIME-OUT

INTERNAL

RESET

TOUT

RST

PORMAX

CCRR

PORMIN

RESET

TIME-OUT

INTERNAL

RESET

TOUT

RST

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8.0.4 External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the

minimum pulse width (see Table 8-1 on page 43) will generate a reset, even if the clock is not

running. Shorter pulses are not guaranteed to generate a reset. When the applied signal

reaches the Reset Threshold Voltage – VRST – on its positive edge, the delay counter starts the

MCU after the Time-out period – tTOUT – has expired. The External Reset can be disabled by the

RSTDISBL fuse, see Table 25-6 on page 281.

Figure 8-4. External Reset During Operation

8.0.5 Brown-out Detection

ATmega48/88/168 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC

level during operation by comparing it to a fixed trigger level. The trigger level for the BOD can

be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free

Brown-out Detection. The hysteresis on the detection level should be interpreted as VBOT+ =

VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.

Notes: 1. VBOT may be below nominal minimum operating voltage for some devices. For devices where

this is the case, the device is tested down to VCC = VBOT during the production test. This guar-

antees that a Brown-Out Reset will occur before VCC drops to a voltage where correct

operation of the microcontroller is no longer guaranteed. The test is performed using

BODLEVEL = 110 and BODLEVEL = 101 for ATmega48V/88V/168V, and BODLEVEL = 101

and BODLEVEL = 101 for ATmega48/88/168.

2. Min/Max values applicable for ATmega48.

Table 8-2. BODLEVEL Fuse Coding(1)

BODLEVEL 2..0 Fuses Min VBOT Typ VBOT Max VBOT Units

111 BOD Disabled

110 1.7(2) 1.8 2.0(2)

V101 2.5(2) 2.7 2.9(2)

100 4.1(2) 4.3 4.5(2)

011

Reserved

010

001

000

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When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in Figure

8-5), the Brown-out Reset is immediately activated. When VCC increases above the trigger level

(VBOT+ in Figure 8-5), the delay counter starts the MCU after the Time-out period tTOUT has

expired.

The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for lon-

ger than tBOD given in Table 8-1 on page 43.

Figure 8-5. Brown-out Reset During Operation

8.0.6 Watchdog System Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On

the falling edge of this pulse, the delay timer starts counting the Time-out period tTOUT. Refer to

page 47 for details on operation of the Watchdog Timer.

Figure 8-6. Watchdog System Reset During Operation

Table 8-3. Brown-out Characteristics

Symbol Parameter Min Typ Max Units

VHYST Brown-out Detector Hysteresis 50 mV

tBOD Min Pulse Width on Brown-out Reset ns

VCC

RESET

TIME-OUT

INTERNAL

RESET

VBOT-

VBOT+

tTOUT

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8.0.7 MCU Status Register – MCUSR

The MCU Status Register provides information on which reset source caused an MCU reset.

• Bit 7..4: Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 3 – WDRF: Watchdog System Reset Flag

This bit is set if a Watchdog System Reset occurs. The bit is reset by a Power-on Reset, or by

writing a logic zero to the flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and then

Reset the MCUSR as early as possible in the program. If the register is cleared before another

reset occurs, the source of the reset can be found by examining the Reset Flags.

8.1 Internal Voltage Reference

ATmega48/88/168 features an internal bandgap reference. This reference is used for Brown-out

Detection, and it can be used as an input to the Analog Comparator or the ADC.

8.1.1 Voltage Reference Enable Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The

start-up time is given in Table 8-4. To save power, the reference is not always turned on. The

reference is on during the following situations:

1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuses).

2. When the bandgap reference is connected to the Analog Comparator (by setting the

ACBG bit in ACSR).

3. When the ADC is enabled.

Bit 76543210

– – – – WDRF BORF EXTRF PORF MCUSR

Read/Write R R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 See Bit Description

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Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user

must always allow the reference to start up before the output from the Analog Comparator or

ADC is used. To reduce power consumption in Power-down mode, the user can avoid the three

conditions above to ensure that the reference is turned off before entering Power-down mode.

Note: 1. Values are guidelines only. Actual values are TBD.

8.2 Watchdog Timer

ATmega48/88/168 has an Enhanced Watchdog Timer (WDT). The main features are:

•Clocked from separate On-chip Oscillator

•3 Operating modes

–Interrupt

– System Reset

– Interrupt and System Reset

•Selectable Time-out period from 16ms to 8s

•Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode

Figure 8-7. Watchdog Timer

The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128 kHz oscillator.

The WDT gives an interrupt or a system reset when the counter reaches a given time-out value.

In normal operation mode, it is required that the system uses the WDR - Watchdog Timer Reset

- instruction to restart the counter before the time-out value is reached. If the system doesn't

restart the counter, an interrupt or system reset will be issued.

Table 8-4. Internal Voltage Reference Characteristics(1)

Symbol Parameter Condition Min Typ Max Units

VBG Bandgap reference voltage TBD 1.0 1.1 1.2 V

tBG Bandgap reference start-up time TBD 40 70 µs

IBG Bandgap reference current consumption TBD 10 TBD µA

128kHz

OSCILLATOR

OSC/2K

OSC/4K

OSC/8K

OSC/16K

OSC/32K

OSC/64K

OSC/128K

OSC/256K

OSC/512K

OSC/1024K

WDP0

WDP1

WDP2

WDP3

WATCHDOG

RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

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In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used

to wake the device from sleep-modes, and also as a general system timer. One example is to

limit the maximum time allowed for certain operations, giving an interrupt when the operation

has run longer than expected. In System Reset mode, the WDT gives a reset when the timer

expires. This is typically used to prevent system hang-up in case of runaway code. The third

mode, Interrupt and System Reset mode, combines the other two modes by first giving an inter-

rupt and then switch to System Reset mode. This mode will for instance allow a safe shutdown

by saving critical parameters before a system reset.

The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to Sys-

tem Reset mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt

mode bit (WDIE) are locked to 1 and 0 respectively. To further ensure program security, altera-

tions to the Watchdog set-up must follow timed sequences. The sequence for clearing WDE and

changing time-out configuration is as follows:

1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE)

and WDE. A logic one must be written to WDE regardless of the previous value of the

WDE bit.

2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as

desired, but with the WDCE bit cleared. This must be done in one operation.

The following code example shows one assembly and one C function for turning off the Watch-

dog Timer. The example assumes that interrupts are controlled (e.g. by disabling interrupts

globally) so that no interrupts will occur during the execution of these functions.

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Note: 1. The example code assumes that the part specific header file is included.

Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out

condition, the device will be reset and the Watchdog Timer will stay enabled. If the code is not

set up to handle the Watchdog, this might lead to an eternal loop of time-out resets. To avoid this

situation, the application software should always clear the Watchdog System Reset Flag

(WDRF) and the WDE control bit in the initialisation routine, even if the Watchdog is not in use.

The following code example shows one assembly and one C function for changing the time-out

value of the Watchdog Timer.

Assembly Code Example(1)

WDT_off:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Clear WDRF in MCUSR

in r16, MCUSR

andi r16, (0xff & (0<<WDRF))

out MCUSR, r16

; Write logical one to WDCE and WDE

; Keep old prescaler setting to prevent unintentional time-out

lds r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

sts WDTCSR, r16

; Turn off WDT

ldi r16, (0<<WDE)

sts WDTCSR, r16

; Turn on global interrupt

sei

ret

C Code Example(1)

void WDT_off(void)

{

__disable_interrupt();

__watchdog_reset();

/* Clear WDRF in MCUSR */

MCUSR &= ~(1<<WDRF);

/* Write logical one to WDCE and WDE */

/* Keep old prescaler setting to prevent unintentional time-out */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Turn off WDT */

WDTCSR = 0x00;

__enable_interrupt();

}

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Note: 1. The example code assumes that the part specific header file is included.

Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change

in the WDP bits can result in a time-out when switching to a shorter time-out period.

8.2.1 Watchdog Timer Control Register - WDTCSR

• Bit 7 - WDIF: Watchdog Interrupt Flag

This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is config-

ured for interrupt. WDIF is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, WDIF is cleared by writing a logic one to the flag. When the I-bit in

SREG and WDIE are set, the Watchdog Time-out Interrupt is executed.

• Bit 6 - WDIE: Watchdog Interrupt Enable

Assembly Code Example(1)

WDT_Prescaler_Change:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Start timed sequence

lds r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

sts WDTCSR, r16

; -- Got four cycles to set the new values from here -

; Set new prescaler(time-out) value = 64K cycles (~0.5 s)

ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)

sts WDTCSR, r16

; -- Finished setting new values, used 2 cycles -

; Turn on global interrupt

sei

ret

C Code Example(1)

void WDT_Prescaler_Change(void)

{

__disable_interrupt();

__watchdog_reset();

/* Start timed equence */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */

WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);

__enable_interrupt();

}

Bit 76543210

WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 WDTCSR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0000X000

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When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is

enabled. If WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt

Mode, and the corresponding interrupt is executed if time-out in the Watchdog Timer occurs.

If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first time-out in

the Watchdog Timer will set WDIF. Executing the corresponding interrupt vector will clear WDIE

and WDIF automatically by hardware (the Watchdog goes to System Reset Mode). This is use-

ful for keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and

System Reset Mode, WDIE must be set after each interrupt. This should however not be done

within the interrupt service routine itself, as this might compromise the safety-function of the

Watchdog System Reset mode. If the interrupt is not executed before the next time-out, a Sys-

tem Reset will be applied.

• Bit 4 - WDCE: Watchdog Change Enable

This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit,

and/or change the prescaler bits, WDCE must be set.

Once written to one, hardware will clear WDCE after four clock cycles.

• Bit 3 - WDE: Watchdog System Reset Enable

WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is

set. To clear WDE, WDRF must be cleared first. This feature ensures multiple resets during con-

ditions causing failure, and a safe start-up after the failure.

• Bit 5, 2..0 - WDP3..0: Watchdog Timer Prescaler 3, 2, 1 and 0

The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is run-

ning. The different prescaling values and their corresponding time-out periods are shown in

Table 8-6 on page 52.

Table 8-5. Watchdog Timer Configuration

WDTON WDE WDIE Mode Action on Time-out

0 0 0 Stopped None

0 0 1 Interrupt Mode Interrupt

0 1 0 System Reset Mode Reset

011

Interrupt and System Reset

Mode

Interrupt, then go to System

Reset Mode

1 x x System Reset Mode Reset

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9. Interrupts

This section describes the specifics of the interrupt handling as performed in ATmega48/88/168.

For a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling”

on page 12.

The interrupt vectors in ATmega48, ATmega88 and ATmega168 are generally the same, with

the following differences:

• Each Interrupt Vector occupies two instruction words in ATmega168, and one instruction

word in ATmega48 and ATmega88.

• ATmega48 does not have a separate Boot Loader Section. In ATmega88 and ATmega168,

the Reset Vector is affected by the BOOTRST fuse, and the Interrupt Vector start address is

affected by the IVSEL bit in MCUCR.

Table 8-6. Watchdog Timer Prescale Select

WDP3 WDP2 WDP1 WDP0

Number of WDT Oscillator

Cycles

Typical Time-out at

VCC = 5.0V

0000 2K (2048) cycles 16 ms

0001 4K (4096) cycles 32 ms

0010 8K (8192) cycles 64 ms

0011 16K (16384) cycles 0.125 s

0100 32K (32768) cycles 0.25 s

0101 64K (65536) cycles 0.5 s

0110 128K (131072) cycles 1.0 s

0111 256K (262144) cycles 2.0 s

1000 512K (524288) cycles 4.0 s

10011024K (1048576) cycles 8.0 s

1010

Reserved

1011

1100

1101

1110

1111

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9.1 Interrupt Vectors in ATmega48

The most typical and general program setup for the Reset and Interrupt Vector Addresses in

ATmega48 is:

Table 9-1. Reset and Interrupt Vectors in ATmega48

Vector

No.

Program

Address Source Interrupt Definition

1 0x000 RESET External Pin, Power-on Reset, Brown-out Reset and Watchdog

System Reset

2 0x001 INT0 External Interrupt Request 0

3 0x002 INT1 External Interrupt Request 1

4 0x003 PCINT0 Pin Change Interrupt Request 0

5 0x004 PCINT1 Pin Change Interrupt Request 1

6 0x005 PCINT2 Pin Change Interrupt Request 2

7 0x006 WDT Watchdog Time-out Interrupt

8 0x007 TIMER2

COMPA Timer/Counter2 Compare Match A

9 0x008 TIMER2

COMPB Timer/Counter2 Compare Match B

10 0x009 TIMER2 OVF Timer/Counter2 Overflow

11 0x00A TIMER1 CAPT Timer/Counter1 Capture Event

12 0x00B TIMER1

COMPA Timer/Counter1 Compare Match A

13 0x00C TIMER1

COMPB Timer/Coutner1 Compare Match B

14 0x00D TIMER1 OVF Timer/Counter1 Overflow

15 0x00E TIMER0

COMPA Timer/Counter0 Compare Match A

16 0x00F TIMER0

COMPB Timer/Counter0 Compare Match B

17 0x010 TIMER0 OVF Timer/Counter0 Overflow

18 0x011 SPI, STC SPI Serial Transfer Complete

19 0x012 USART, RX USART Rx Complete

20 0x013 USART, UDRE USART, Data Register Empty

21 0x014 USART, TX USART, Tx Complete

22 0x015 ADC ADC Conversion Complete

23 0x016 EE READY EEPROM Ready

24 0x017 ANALOG

COMP Analog Comparator

25 0x018 TWI 2-wire Serial Interface

26 0x019 SPM READY Store Program Memory Ready

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Address Labels Code Comments

0x000 rjmp RESET ; Reset Handler

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

0x003 rjmp PCINT0 ; PCINT0 Handler

0x004 rjmp PCINT1 ; PCINT1 Handler

0x005 rjmp PCINT2 ; PCINT2 Handler

0x006 rjmp WDT ; Watchdog Timer Handler

0x007 rjmp TIM2_COMPA ; Timer2 Compare A Handler

0x008 rjmp TIM2_COMPB ; Timer2 Compare B Handler

0x009 rjmp TIM2_OVF ; Timer2 Overflow Handler

0x00A rjmp TIM1_CAPT ; Timer1 Capture Handler

0x00B rjmp TIM1_COMPA ; Timer1 Compare A Handler

0x00C rjmp TIM1_COMPB ; Timer1 Compare B Handler

0x00D rjmp TIM1_OVF ; Timer1 Overflow Handler

0x00E rjmp TIM0_COMPA ; Timer0 Compare A Handler

0x00F rjmp TIM0_COMPB ; Timer0 Compare B Handler

0x010 rjmp TIM0_OVF ; Timer0 Overflow Handler

0x011 rjmp SPI_STC ; SPI Transfer Complete Handler

0x012 rjmp USART_RXC ; USART, RX Complete Handler

0x013 rjmp USART_UDRE ; USART, UDR Empty Handler

0x014 rjmp USART_TXC ; USART, TX Complete Handler

0x015 rjmp ADC ; ADC Conversion Complete Handler

0x016 rjmp EE_RDY ; EEPROM Ready Handler

0x017 rjmp ANA_COMP ; Analog Comparator Handler

0x018 rjmp TWI ; 2-wire Serial Interface Handler

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

0x01ARESET: ldi r16, high(RAMEND); Main program start

0x01B out SPH,r16 ; Set Stack Pointer to top of RAM

0x01C ldi r16, low(RAMEND)

0x01D out SPL,r16

0x01E sei ; Enable interrupts

0x01F <instr> xxx

... ... ... ...

9.2 Interrupt Vectors in ATmega88

Table 9-2. Reset and Interrupt Vectors in ATmega88

Vector

No.

Program

Address(2) Source Interrupt Definition

10x000

(1) RESET External Pin, Power-on Reset, Brown-out Reset and Watchdog

System Reset

2 0x001 INT0 External Interrupt Request 0

3 0x002 INT1 External Interrupt Request 1

4 0x003 PCINT0 Pin Change Interrupt Request 0

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Atmel ATmega48/88/168 Automotive

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at

reset, see “Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and

ATmega168” on page 263.

2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot

Flash Section. The address of each Interrupt Vector will then be the address in this table

added to the start address of the Boot Flash Section.

Table 9-3 shows reset and Interrupt Vectors placement for the various combinations of

BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt

Vectors are not used, and regular program code can be placed at these locations. This is also

the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the

Boot section or vice versa.

5 0x004 PCINT1 Pin Change Interrupt Request 1

6 0x005 PCINT2 Pin Change Interrupt Request 2

7 0x006 WDT Watchdog Time-out Interrupt

80x007

TIMER2

COMPA Timer/Counter2 Compare Match A

90x008

TIMER2

COMPB Timer/Counter2 Compare Match B

10 0x009 TIMER2 OVF Timer/Counter2 Overflow

11 0x00A TIMER1 CAPT Timer/Counter1 Capture Event

12 0x00B TIMER1

COMPA Timer/Counter1 Compare Match A

13 0x00C TIMER1

COMPB Timer/Coutner1 Compare Match B

14 0x00D TIMER1 OVF Timer/Counter1 Overflow

15 0x00E TIMER0

COMPA Timer/Counter0 Compare Match A

16 0x00F TIMER0

COMPB Timer/Counter0 Compare Match B

17 0x010 TIMER0 OVF Timer/Counter0 Overflow

18 0x011 SPI, STC SPI Serial Transfer Complete

19 0x012 USART, RX USART Rx Complete

20 0x013 USART, UDRE USART, Data Register Empty

21 0x014 USART, TX USART, Tx Complete

22 0x015 ADC ADC Conversion Complete

23 0x016 EE READY EEPROM Ready

24 0x017 ANALOG

COMP Analog Comparator

25 0x018 TWI 2-wire Serial Interface

26 0x019 SPM READY Store Program Memory Ready

Table 9-2. Reset and Interrupt Vectors in ATmega88

Vector

No.

Program

Address(2) Source Interrupt Definition

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Atmel ATmega48/88/168 Automotive

Note: 1. The Boot Reset Address is shown in Table 24-6 on page 276. For the BOOTRST Fuse “1”

means unprogrammed while “0” means programmed.

The most typical and general program setup for the Reset and Interrupt Vector Addresses in

ATmega88 is:

Address Labels Code Comments

0x000 rjmp RESET ; Reset Handler

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

0x003 rjmp PCINT0 ; PCINT0 Handler

0x004 rjmp PCINT1 ; PCINT1 Handler

0x005 rjmp PCINT2 ; PCINT2 Handler

0x006 rjmp WDT ; Watchdog Timer Handler

0x007 rjmp TIM2_COMPA ; Timer2 Compare A Handler

0X008 rjmp TIM2_COMPB ; Timer2 Compare B Handler

0x009 rjmp TIM2_OVF ; Timer2 Overflow Handler

0x00A rjmp TIM1_CAPT ; Timer1 Capture Handler

0x00B rjmp TIM1_COMPA ; Timer1 Compare A Handler

0x00C rjmp TIM1_COMPB ; Timer1 Compare B Handler

0x00D rjmp TIM1_OVF ; Timer1 Overflow Handler

0x00E rjmp TIM0_COMPA ; Timer0 Compare A Handler

0x00F rjmp TIM0_COMPB ; Timer0 Compare B Handler

0x010 rjmp TIM0_OVF ; Timer0 Overflow Handler

0x011 rjmp SPI_STC ; SPI Transfer Complete Handler

0x012 rjmp USART_RXC ; USART, RX Complete Handler

0x013 rjmp USART_UDRE ; USART, UDR Empty Handler

0x014 rjmp USART_TXC ; USART, TX Complete Handler

0x015 rjmp ADC ; ADC Conversion Complete Handler

0x016 rjmp EE_RDY ; EEPROM Ready Handler

0x017 rjmp ANA_COMP ; Analog Comparator Handler

0x018 rjmp TWI ; 2-wire Serial Interface Handler

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

0x01ARESET: ldi r16, high(RAMEND); Main program start

0x01B out SPH,r16 ; Set Stack Pointer to top of RAM

0x01C ldi r16, low(RAMEND)

0x01D out SPL,r16

0x01E sei ; Enable interrupts

0x01F <instr> xxx

Table 9-3. Reset and Interrupt Vectors Placement in ATmega88(1)

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x000 0x001

1 1 0x000 Boot Reset Address + 0x001

0 0 Boot Reset Address 0x001

0 1 Boot Reset Address Boot Reset Address + 0x001

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Atmel ATmega48/88/168 Automotive

... ... ... ...

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2K bytes and the

IVSEL bit in the MCUCR Register is set before any interrupts are enabled, the most typical and

general program setup for the Reset and Interrupt Vector Addresses in ATmega88 is:

Address Labels Code Comments

0x000 RESET: ldi r16,high(RAMEND); Main program start

0x001 out SPH,r16 ; Set Stack Pointer to top of RAM

0x002 ldi r16,low(RAMEND)

0x003 out SPL,r16

0x004 sei ; Enable interrupts

0x005 <instr> xxx

;

.org 0xC01

0xC01 rjmp EXT_INT0 ; IRQ0 Handler

0xC02 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0xC19 rjmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2K bytes, the most

typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega88

is:

Address Labels Code Comments

.org 0x001

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

.org 0xC00

0xC00 RESET: ldi r16,high(RAMEND); Main program start

0xC01 out SPH,r16 ; Set Stack Pointer to top of RAM

0xC02 ldi r16,low(RAMEND)

0xC03 out SPL,r16

0xC04 sei ; Enable interrupts

0xC05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2K bytes and the IVSEL

bit in the MCUCR Register is set before any interrupts are enabled, the most typical and general

program setup for the Reset and Interrupt Vector Addresses in ATmega88 is:

Address Labels Code Comments

;

.org 0xC00

0xC00 rjmp RESET ; Reset handler

0xC01 rjmp EXT_INT0 ; IRQ0 Handler

0xC02 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0xC19 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

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Atmel ATmega48/88/168 Automotive

0xC1A RESET: ldi r16,high(RAMEND); Main program start

0xC1B out SPH,r16 ; Set Stack Pointer to top of RAM

0xC1C ldi r16,low(RAMEND)

0xC1D out SPL,r16

0xC1E sei ; Enable interrupts

0xC1F <instr> xxx

9.3 Interrupt Vectors in ATmega168

Table 9-4. Reset and Interrupt Vectors in ATmega168

Vector

No.

Program

Address(2) Source Interrupt Definition

1 0x0000(1) RESET External Pin, Power-on Reset, Brown-out Reset and Watchdog

System Reset

2 0x0002 INT0 External Interrupt Request 0

3 0x0004 INT1 External Interrupt Request 1

4 0x0006 PCINT0 Pin Change Interrupt Request 0

5 0x0008 PCINT1 Pin Change Interrupt Request 1

6 0x000A PCINT2 Pin Change Interrupt Request 2

7 0x000C WDT Watchdog Time-out Interrupt

8 0x000E TIMER2

COMPA Timer/Counter2 Compare Match A

9 0x0010 TIMER2

COMPB Timer/Counter2 Compare Match B

10 0x0012 TIMER2 OVF Timer/Counter2 Overflow

11 0x0014 TIMER1 CAPT Timer/Counter1 Capture Event

12 0x0016 TIMER1

COMPA Timer/Counter1 Compare Match A

13 0x0018 TIMER1

COMPB Timer/Coutner1 Compare Match B

14 0x001A TIMER1 OVF Timer/Counter1 Overflow

15 0x001C TIMER0

COMPA Timer/Counter0 Compare Match A

16 0x001E TIMER0

COMPB Timer/Counter0 Compare Match B

17 0x0020 TIMER0 OVF Timer/Counter0 Overflow

18 0x0022 SPI, STC SPI Serial Transfer Complete

19 0x0024 USART, RX USART Rx Complete

20 0x0026 USART, UDRE USART, Data Register Empty

21 0x0028 USART, TX USART, Tx Complete

22 0x002A ADC ADC Conversion Complete

23 0x002C EE READY EEPROM Ready

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Atmel ATmega48/88/168 Automotive

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at

reset, see “Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and

ATmega168” on page 263.

2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot

Flash Section. The address of each Interrupt Vector will then be the address in this table

added to the start address of the Boot Flash Section.

Table 9-5 shows reset and Interrupt Vectors placement for the various combinations of

BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt

Vectors are not used, and regular program code can be placed at these locations. This is also

the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the

Boot section or vice versa.

Note: 1. The Boot Reset Address is shown in Table 24-6 on page 276. For the BOOTRST Fuse “1”

means unprogrammed while “0” means programmed.

The most typical and general program setup for the Reset and Interrupt Vector Addresses in

ATmega168 is:

Address Labels Code Comments

0x0000 jmp RESET ; Reset Handler

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

0x0006 jmp PCINT0 ; PCINT0 Handler

0x0008 jmp PCINT1 ; PCINT1 Handler

0x000A jmp PCINT2 ; PCINT2 Handler

0x000C jmp WDT ; Watchdog Timer Handler

0x000E jmp TIM2_COMPA ; Timer2 Compare A Handler

0x0010 jmp TIM2_COMPB ; Timer2 Compare B Handler

0x0012 jmp TIM2_OVF ; Timer2 Overflow Handler

0x0014 jmp TIM1_CAPT ; Timer1 Capture Handler

0x0016 jmp TIM1_COMPA ; Timer1 Compare A Handler

0x0018 jmp TIM1_COMPB ; Timer1 Compare B Handler

0x001A jmp TIM1_OVF ; Timer1 Overflow Handler

0x001C jmp TIM0_COMPA ; Timer0 Compare A Handler

0x001E jmp TIM0_COMPB ; Timer0 Compare B Handler

24 0x002E ANALOG

COMP Analog Comparator

25 0x0030 TWI 2-wire Serial Interface

26 0x0032 SPM READY Store Program Memory Ready

Table 9-5. Reset and Interrupt Vectors Placement in ATmega168(1)

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x000 0x001

1 1 0x000 Boot Reset Address + 0x0002

0 0 Boot Reset Address 0x001

0 1 Boot Reset Address Boot Reset Address + 0x0002

Table 9-4. Reset and Interrupt Vectors in ATmega168

Vector

No.

Program

Address(2) Source Interrupt Definition

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Atmel ATmega48/88/168 Automotive

0x0020 jmp TIM0_OVF ; Timer0 Overflow Handler

0x0022 jmp SPI_STC ; SPI Transfer Complete Handler

0x0024 jmp USART_RXC ; USART, RX Complete Handler

0x0026 jmp USART_UDRE ; USART, UDR Empty Handler

0x0028 jmp USART_TXC ; USART, TX Complete Handler

0x002A jmp ADC ; ADC Conversion Complete Handler

0x002C jmp EE_RDY ; EEPROM Ready Handler

0x002E jmp ANA_COMP ; Analog Comparator Handler

0x0030 jmp TWI ; 2-wire Serial Interface Handler

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

0x0033RESET: ldi r16, high(RAMEND); Main program start

0x0034 out SPH,r16 ; Set Stack Pointer to top of RAM

0x0035 ldi r16, low(RAMEND)

0x0036 out SPL,r16

0x0037 sei ; Enable interrupts

0x0038 <instr> xxx

... ... ... ...

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2K bytes and the

IVSEL bit in the MCUCR Register is set before any interrupts are enabled, the most typical and

general program setup for the Reset and Interrupt Vector Addresses in ATmega168 is:

Address Labels Code Comments

0x0000 RESET: ldi r16,high(RAMEND); Main program start

0x0001 out SPH,r16 ; Set Stack Pointer to top of RAM

0x0002 ldi r16,low(RAMEND)

0x0003 out SPL,r16

0x0004 sei ; Enable interrupts

0x0005 <instr> xxx

;

.org 0xC02

0x1C02 jmp EXT_INT0 ; IRQ0 Handler

0x1C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x1C32 jmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2K bytes, the most

typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega168

is:

Address Labels Code Comments

.org 0x0002

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

.org 0x1C00

0x1C00 RESET: ldi r16,high(RAMEND); Main program start

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Atmel ATmega48/88/168 Automotive

0x1C01 out SPH,r16 ; Set Stack Pointer to top of RAM

0x1C02 ldi r16,low(RAMEND)

0x1C03 out SPL,r16

0x1C04 sei ; Enable interrupts

0x1C05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2K bytes and the IVSEL

bit in the MCUCR Register is set before any interrupts are enabled, the most typical and general

program setup for the Reset and Interrupt Vector Addresses in ATmega168 is:

Address Labels Code Comments

;

.org 0x1C00

0x1C00 jmp RESET ; Reset handler

0x1C02 jmp EXT_INT0 ; IRQ0 Handler

0x1C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x1C32 jmp SPM_RDY ; Store Program Memory Ready Handler

;

0x1C33 RESET: ldi r16,high(RAMEND); Main program start

0x1C34 out SPH,r16 ; Set Stack Pointer to top of RAM

0x1C35 ldi r16,low(RAMEND)

0x1C36 out SPL,r16

0x1C37 sei ; Enable interrupts

0x1C38 <instr> xxx

9.3.1 Moving Interrupts Between Application and Boot Space, ATmega88 and ATmega168

The MCU Control Register controls the placement of the Interrupt Vector table.

9.3.2 MCU Control Register – MCUCR

• Bit 1 – IVSEL: Interrupt Vector Select

When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash

memory. When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot

Loader section of the Flash. The actual address of the start of the Boot Flash Section is deter-

mined by the BOOTSZ Fuses. Refer to the section “Boot Loader Support – Read-While-Write

Self-Programming, ATmega88 and ATmega168” on page 263 for details. To avoid unintentional

changes of Interrupt Vector tables, a special write procedure must be followed to change the

IVSEL bit:

a. Write the Interrupt Vector Change Enable (IVCE) bit to one.

b. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.

Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled

in the cycle IVCE is set, and they remain disabled until after the instruction following the write to

IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status

Bit 76543210

– – – PUD –– IVSEL IVCE MCUCR

Read/Write R R R R/W R R R/W R/W

Initial Value00000000

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Atmel ATmega48/88/168 Automotive

Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed,

interrupts are disabled while executing from the Application section. If Interrupt Vectors are placed

in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while

executing from the Boot Loader section. Refer to the section “Boot Loader Support –

Read-While-Write Self-Programming, ATmega88 and ATmega168” on page 263 for details on

Boot Lock bits.

This bit is not available in ATmega48.

• Bit 0 – IVCE: Interrupt Vector Change Enable

The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by

hardware four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable

interrupts, as explained in the IVSEL description above. See Code Example below.

This bit is not available in ATmega48.

10. I/O-Ports

10.1 Introduction

All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports.

This means that the direction of one port pin can be changed without unintentionally changing

the direction of any other pin with the SBI and CBI instructions. The same applies when chang-

ing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as

input). Each output buffer has symmetrical drive characteristics with both high sink and source

capability. The pin driver is strong enough to drive LED displays directly. All port pins have indi-

vidually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have

protection diodes to both VCC and Ground as indicated in Figure 10-1. Refer to “Electrical Char-

acteristics” on page 298 for a complete list of parameters.

Assembly Code Example

Move_interrupts:

; Enable change of Interrupt Vectors

ldi r16, (1<<IVCE)

out MCUCR, r16

; Move interrupts to Boot Flash section

ldi r16, (1<<IVSEL)

out MCUCR, r16

ret

C Code Example

void Move_interrupts(void)

{

/* Enable change of Interrupt Vectors */

MCUCR = (1<<IVCE);

/* Move interrupts to Boot Flash section */

MCUCR = (1<<IVSEL);

}

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Atmel ATmega48/88/168 Automotive

Figure 10-1. I/O Pin Equivalent Schematic

All registers and bit references in this section are written in general form. A lower case “x” repre-

sents the numbering letter for the port, and a lower case “n” represents the bit number. However,

when using the register or bit defines in a program, the precise form must be used. For example,

PORTB3 for bit no. 3 in Port B, here documented generally as PORTxn. The physical I/O Regis-

ters and bit locations are listed in “Register Description for I/O Ports” on page 80.

Three I/O memory address locations are allocated for each port, one each for the Data Register

– PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins

I/O location is read only, while the Data Register and the Data Direction Register are read/write.

However, writing a logic one to a bit in the PINx Register, will result in a toggle in the correspond-

ing bit in the Data Register. In addition, the Pull-up Disable – PUD bit in MCUCR disables the

pull-up function for all pins in all ports when set.

Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page

63. Most port pins are multiplexed with alternate functions for the peripheral features on the

device. How each alternate function interferes with the port pin is described in “Alternate Port

Functions” on page 68. Refer to the individual module sections for a full description of the alter-

nate functions.

Note that enabling the alternate function of some of the port pins does not affect the use of the

other pins in the port as general digital I/O.

10.2 Ports as General Digital I/O

The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a func-

tional description of one I/O-port pin, here generically called Pxn.

Cpin

Logic

Rpu

See Figure

"General Digital I/O" for

Details

Pxn

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Atmel ATmega48/88/168 Automotive

Figure 10-2. General Digital I/O(1)

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,

SLEEP, and PUD are common to all ports.

10.2.1 Configuring the Pin

Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register

Description for I/O Ports” on page 80, the DDxn bits are accessed at the DDRx I/O address, the

PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O address.

The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,

Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input

pin.

If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is

activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to

be configured as an output pin. The port pins are tri-stated when reset condition becomes active,

even if no clocks are running.

If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven

high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port

pin is driven low (zero).

10.2.2 Toggling the Pin

Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn.

Note that the SBI instruction can be used to toggle one single bit in a port.

clk

RPx

RRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

RESET

CLR

PORTxn

CLR

DDxn

PINxn

DATA BUS

SLEEP

SLEEP: SLEEP CONTROL

Pxn

I/O

WPx

WRx

WPx: WRITE PINx REGISTER

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Atmel ATmega48/88/168 Automotive

10.2.3 Switching Between Input and Output

When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn}

= 0b11), an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output

low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully accept-

able, as a high-impedant environment will not notice the difference between a strong high driver

and a pull-up. If this is not the case, the PUD bit in the MCUCR Register can be set to disable all

pull-ups in all ports.

Switching between input with pull-up and output low generates the same problem. The user

must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn}

= 0b11) as an intermediate step.

Table 10-1 summarizes the control signals for the pin value.

10.2.4 Reading the Pin Value

Independent of the setting of Data Direction bit DDxn, the port pin can be read through the

PINxn Register bit. As shown in Figure 10-2, the PINxn Register bit and the preceding latch con-

stitute a synchronizer. This is needed to avoid metastability if the physical pin changes value

near the edge of the internal clock, but it also introduces a delay. Figure 10-3 shows a timing dia-

gram of the synchronization when reading an externally applied pin value. The maximum and

minimum propagation delays are denoted tpd,max and tpd,min respectively.

Figure 10-3. Synchronization when Reading an Externally Applied Pin value

Table 10-1. Port Pin Configurations

DDxn PORTxn

PUD

(in MCUCR) I/O Pull-up Comment

0 0 X Input No Tri-state (Hi-Z)

0 1 0 Input Yes Pxn will source current if ext. pulled low.

0 1 1 Input No Tri-state (Hi-Z)

1 0 X Output No Output Low (Sink)

1 1 X Output No Output High (Source)

XXX in r17, PINx

0x00 0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

pd, max

pd, min

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Atmel ATmega48/88/168 Automotive

Consider the clock period starting shortly after the first falling edge of the system clock. The latch

is closed when the clock is low, and goes transparent when the clock is high, as indicated by the

shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock

goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indi-

cated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed

between ½ and 1½ system clock period depending upon the time of assertion.

When reading back a software assigned pin value, a nop instruction must be inserted as indi-

cated in Figure 10-4. The out instruction sets the “SYNC LATCH” signal at the positive edge of

the clock. In this case, the delay tpd through the synchronizer is 1 system clock period.

Figure 10-4. Synchronization when Reading a Software Assigned Pin Value

The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define

the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin

values are read back again, but as previously discussed, a nop instruction is included to be able

to read back the value recently assigned to some of the pins.

out PORTx, r16 nop in r17, PINx

0xFF

0x00 0xFF

SYSTEM CLK

r16

INSTRUCTIONS

SYNC LATCH

PINxn

r17

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Atmel ATmega48/88/168 Automotive

Note: 1. For the assembly program, two temporary registers are used to minimize the time from

pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and

3 as low and redefining bits 0 and 1 as strong high drivers.

10.2.5 Digital Input Enable and Sleep Modes

As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the

Schmitt Trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in

Power-down mode, Power-save mode, and Standby mode to avoid high power consumption if

some input signals are left floating, or have an analog signal level close to VCC/2.

SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt

request is not enabled, SLEEP is active also for these pins. SLEEP is also overridden by various

other alternate functions as described in “Alternate Port Functions” on page 68.

If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as

“Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt

is not enabled, the corresponding External Interrupt Flag will be set when resuming from the

above mentioned Sleep mode, as the clamping in these sleep mode produces the requested

logic change.

Assembly Code Example(1)

...

; Define pull-ups and set outputs high

; Define directions for port pins

ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)

ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)

out PORTB,r16

out DDRB,r17

; Insert nop for synchronization

nop

; Read port pins

in r16,PINB

...

C Code Example

unsigned char i;

...

/* Define pull-ups and set outputs high */

/* Define directions for port pins */

PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);

DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);

/* Insert nop for synchronization*/

__no_operation();

/* Read port pins */

i = PINB;

...

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10.2.6 Unconnected Pins

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even

though most of the digital inputs are disabled in the deep sleep modes as described above, float-

ing inputs should be avoided to reduce current consumption in all other modes where the digital

inputs are enabled (Reset, Active mode and Idle mode).

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.

In this case, the pull-up will be disabled during reset. If low power consumption during reset is

important, it is recommended to use an external pull-up or pull-down. Connecting unused pins

directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is

accidentally configured as an output.

10.3 Alternate Port Functions

Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5

shows how the port pin control signals from the simplified Figure 10-2 can be overridden by

alternate functions. The overriding signals may not be present in all port pins, but the figure

serves as a generic description applicable to all port pins in the AVR microcontroller family.

Figure 10-5. Alternate Port Functions(1)

clk

RPx

RRx

WRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

SET

CLR

DIxn

AIOxn

DIEOExn

PVOVxn

PVOExn

DDOVxn

DDOExn

PUOExn

PUOVxn

PUOExn: Pxn PULL-UP OVERRIDE ENABLE

PUOVxn: Pxn PULL-UP OVERRIDE VALUE

DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE

DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE

PVOExn: Pxn PORT VALUE OVERRIDE ENABLE

PVOVxn: Pxn PORT VALUE OVERRIDE VALUE

DIxn: DIGITAL INPUT PIN n ON PORTx

AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx

RESET

CLR

PINxn

PORTxn

DDxn

DATA BUS

DIEOVxn

SLEEP

DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE

DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE

SLEEP: SLEEP CONTROL

Pxn

I/O

PTOExn

PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE

WPx: WRITE PINx

WPx

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Atmel ATmega48/88/168 Automotive

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,

SLEEP, and PUD are common to all ports. All other signals are unique for each pin.

Table 10-2 summarizes the function of the overriding signals. The pin and port indexes from Fig-

ure 10-5 are not shown in the succeeding tables. The overriding signals are generated internally

in the modules having the alternate function.

The following subsections shortly describe the alternate functions for each port, and relate the

overriding signals to the alternate function. Refer to the alternate function description for further

details.

Table 10-2. Generic Description of Overriding Signals for Alternate Functions

Signal Name Full Name Description

PUOE Pull-up Override

Enable

If this signal is set, the pull-up enable is controlled by the PUOV

signal. If this signal is cleared, the pull-up is enabled when

{DDxn, PORTxn, PUD} = 0b010.

PUOV Pull-up Override

Value

If PUOE is set, the pull-up is enabled/disabled when PUOV is

set/cleared, regardless of the setting of the DDxn, PORTxn,

and PUD Register bits.

DDOE Data Direction

Override Enable

If this signal is set, the Output Driver Enable is controlled by the

DDOV signal. If this signal is cleared, the Output driver is

enabled by the DDxn Register bit.

DDOV Data Direction

Override Value

If DDOE is set, the Output Driver is enabled/disabled when

DDOV is set/cleared, regardless of the setting of the DDxn

PVOE Port Value

Override Enable

If this signal is set and the Output Driver is enabled, the port

value is controlled by the PVOV signal. If PVOE is cleared, and

the Output Driver is enabled, the port Value is controlled by the

PORTxn Register bit.

PVOV Port Value

Override Value

If PVOE is set, the port value is set to PVOV, regardless of the

setting of the PORTxn Register bit.

PTOE Port Toggle

Override Enable If PTOE is set, the PORTxn Register bit is inverted.

DIEOE

Digital Input

Enable Override

Enable

If this bit is set, the Digital Input Enable is controlled by the

DIEOV signal. If this signal is cleared, the Digital Input Enable

is determined by MCU state (Normal mode, sleep mode).

DIEOV

Digital Input

Enable Override

Value

If DIEOE is set, the Digital Input is enabled/disabled when

DIEOV is set/cleared, regardless of the MCU state (Normal

mode, sleep mode).

DI Digital Input

This is the Digital Input to alternate functions. In the figure, the

signal is connected to the output of the Schmitt Trigger but

before the synchronizer. Unless the Digital Input is used as a

clock source, the module with the alternate function will use its

own synchronizer.

AIO Analog

Input/Output

This is the Analog Input/output to/from alternate functions. The

signal is connected directly to the pad, and can be used

bi-directionally.

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10.3.1 MCU Control Register – MCUCR

• Bit 4 – PUD: Pull-up Disable

When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and

PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Con-

figuring the Pin” on page 64 for more details about this feature.

10.3.2 Alternate Functions of Port B

The Port B pins with alternate functions are shown in Table 10-3.

The alternate pin configuration is as follows:

• XTAL2/TOSC2/PCINT7 – Port B, Bit 7

XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-frequency

crystal Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.

TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected as chip

clock source, and the asynchronous timer is enabled by the correct setting in ASSR. When the

AS2 bit in ASSR is set (one) and the EXCLK bit is cleared (zero) to enable asynchronous clock-

ing of Timer/Counter2 using the Crystal Oscillator, pin PB7 is disconnected from the port, and

becomes the inverting output of the Oscillator amplifier. In this mode, a crystal Oscillator is con-

nected to this pin, and the pin cannot be used as an I/O pin.

Bit 7 6 5 4 3 2 1 0

– – –PUD– – IVSEL IVCE MCUCR

Read/Write R R R R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 10-3. Port B Pins Alternate Functions

Port Pin Alternate Functions

PB7

XTAL2 (Chip Clock Oscillator pin 2)

TOSC2 (Timer Oscillator pin 2)

PCINT7 (Pin Change Interrupt 7)

PB6

XTAL1 (Chip Clock Oscillator pin 1 or External clock input)

TOSC1 (Timer Oscillator pin 1)

PCINT6 (Pin Change Interrupt 6)

PB5 SCK (SPI Bus Master clock Input)

PCINT5 (Pin Change Interrupt 5)

PB4 MISO (SPI Bus Master Input/Slave Output)

PCINT4 (Pin Change Interrupt 4)

PB3

MOSI (SPI Bus Master Output/Slave Input)

OC2A (Timer/Counter2 Output Compare Match A Output)

PCINT3 (Pin Change Interrupt 3)

PB2

SS (SPI Bus Master Slave select)

OC1B (Timer/Counter1 Output Compare Match B Output)

PCINT2 (Pin Change Interrupt 2)

PB1 OC1A (Timer/Counter1 Output Compare Match A Output)

PCINT1 (Pin Change Interrupt 1)

PB0

ICP1 (Timer/Counter1 Input Capture Input)

CLKO (Divided System Clock Output)

PCINT0 (Pin Change Interrupt 0)

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Atmel ATmega48/88/168 Automotive

PCINT7: Pin Change Interrupt source 7. The PB7 pin can serve as an external interrupt source.

If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.

• XTAL1/TOSC1/PCINT6 – Port B, Bit 6

XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal calibrated RC

Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.

TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected as chip

clock source, and the asynchronous timer is enabled by the correct setting in ASSR. When the

AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PB6 is dis-

connected from the port, and becomes the input of the inverting Oscillator amplifier. In this

mode, a crystal Oscillator is connected to this pin, and the pin can not be used as an I/O pin.

PCINT6: Pin Change Interrupt source 6. The PB6 pin can serve as an external interrupt source.

If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.

• SCK/PCINT5 – Port B, Bit 5

SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a

Slave, this pin is configured as an input regardless of the setting of DDB5. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.

PCINT5: Pin Change Interrupt source 5. The PB5 pin can serve as an external interrupt source.

• MISO/PCINT4 – Port B, Bit 4

MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a

Master, this pin is configured as an input regardless of the setting of DDB4. When the SPI is

enabled as a Slave, the data direction of this pin is controlled by DDB4. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.

PCINT4: Pin Change Interrupt source 4. The PB4 pin can serve as an external interrupt source.

• MOSI/OC2/PCINT3 – Port B, Bit 3

MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a

Slave, this pin is configured as an input regardless of the setting of DDB3. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB3. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB3 bit.

OC2, Output Compare Match Output: The PB3 pin can serve as an external output for the

Timer/Counter2 Compare Match. The PB3 pin has to be configured as an output (DDB3 set

(one)) to serve this function. The OC2 pin is also the output pin for the PWM mode timer

function.

PCINT3: Pin Change Interrupt source 3. The PB3 pin can serve as an external interrupt source.

•SS

/OC1B/PCINT2 – Port B, Bit 2

SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input

regardless of the setting of DDB2. As a Slave, the SPI is activated when this pin is driven low.

When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB2. When

the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit.

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Atmel ATmega48/88/168 Automotive

OC1B, Output Compare Match output: The PB2 pin can serve as an external output for the

Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output (DDB2 set

(one)) to serve this function. The OC1B pin is also the output pin for the PWM mode timer

function.

PCINT2: Pin Change Interrupt source 2. The PB2 pin can serve as an external interrupt source.

• OC1A/PCINT1 – Port B, Bit 1

OC1A, Output Compare Match output: The PB1 pin can serve as an external output for the

Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output (DDB1 set

(one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer

function.

PCINT1: Pin Change Interrupt source 1. The PB1 pin can serve as an external interrupt source.

• ICP1/CLKO/PCINT0 – Port B, Bit 0

ICP1, Input Capture Pin: The PB0 pin can act as an Input Capture Pin for Timer/Counter1.

CLKO, Divided System Clock: The divided system clock can be output on the PB0 pin. The

divided system clock will be output if the CKOUT Fuse is programmed, regardless of the

PORTB0 and DDB0 settings. It will also be output during reset.

PCINT0: Pin Change Interrupt source 0. The PB0 pin can serve as an external interrupt source.

Table 10-4 and Table 10-5 relate the alternate functions of Port B to the overriding signals

shown in Figure 10-5 on page 68. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the

MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

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Notes: 1. INTRC means that one of the internal RC Oscillators are selected (by the CKSEL fuses),

EXTCK means that external clock is selected (by the CKSEL fuses).

Table 10-4. Overriding Signals for Alternate Functions in PB7..PB4

Signal

Name

PB7/XTAL2/

TOSC2/PCINT7(1) PB6/XTAL1/

TOSC1/PCINT6(1) PB5/SCK/

PCINT5

PB4/MISO/

PCINT4

PUOE INTRC • EXTCK+

AS2 INTRC + AS2 SPE • MSTR SPE • MSTR

PUOV 0 0 PORTB5 • PUD PORTB4 • PUD

DDOE INTRC • EXTCK+

AS2 INTRC + AS2 SPE • MSTR SPE • MSTR

DDOV 0 0 0 0

PVOE 0 0 SPE • MSTR SPE • MSTR

PVOV 0 0 SCK OUTPUT SPI SLAVE

OUTPUT

DIEOE

INTRC • EXTCK +

AS2 + PCINT7 •

PCIE0

INTRC + AS2 +

PCINT6 • PCIE0 PCINT5 • PCIE0 PCINT4 • PCIE0

DIEOV (INTRC + EXTCK) •

AS2 INTRC • AS2 11

DI PCINT7 INPUT PCINT6 INPUT PCINT5 INPUT

SCK INPUT

PCINT4 INPUT

SPI MSTR INPUT

AIO Oscillator Output Oscillator/Clock

Input ––

Table 10-5. Overriding Signals for Alternate Functions in PB3..PB0

Signal

Name

PB3/MOSI/

OC2/PCINT3

PB2/SS/

OC1B/PCINT2

PB1/OC1A/

PCINT1

PB0/ICP1/

PCINT0

PUOE SPE • MSTR SPE • MSTR 00

PUOV PORTB3 • PUD PORTB2 • PUD 00

DDOE SPE • MSTR SPE • MSTR 00

DDOV 0 0 0 0

PVOE SPE • MSTR +

OC2A ENABLE OC1B ENABLE OC1A ENABLE 0

PVOV SPI MSTR OUTPUT

+ OC2A OC1B OC1A 0

DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0

DIEOV1111

DI PCINT3 INPUT

SPI SLAVE INPUT

PCINT2 INPUT

SPI SS PCINT1 INPUT PCINT0 INPUT

ICP1 INPUT

AIO––––

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Atmel ATmega48/88/168 Automotive

10.3.3 Alternate Functions of Port C

The Port C pins with alternate functions are shown in Table 10-6.

The alternate pin configuration is as follows:

• RESET/PCINT14 – Port C, Bit 6

RESET, Reset pin: When the RSTDISBL Fuse is programmed, this pin functions as a normal I/O

pin, and the part will have to rely on Power-on Reset and Brown-out Reset as its reset sources.

When the RSTDISBL Fuse is unprogrammed, the reset circuitry is connected to the pin, and the

pin can not be used as an I/O pin.

If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.

PCINT14: Pin Change Interrupt source 14. The PC6 pin can serve as an external interrupt

source.

• SCL/ADC5/PCINT13 – Port C, Bit 5

SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the

2-wire Serial Interface, pin PC5 is disconnected from the port and becomes the Serial Clock I/O

pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress

spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver with

slew-rate limitation.

PC5 can also be used as ADC input Channel 5. Note that ADC input channel 5 uses digital

power.

PCINT13: Pin Change Interrupt source 13. The PC5 pin can serve as an external interrupt

source.

Table 10-6. Port C Pins Alternate Functions

Port Pin Alternate Function

PC6 RESET (Reset pin)

PCINT14 (Pin Change Interrupt 14)

PC5

ADC5 (ADC Input Channel 5)

SCL (2-wire Serial Bus Clock Line)

PCINT13 (Pin Change Interrupt 13)

PC4

ADC4 (ADC Input Channel 4)

SDA (2-wire Serial Bus Data Input/Output Line)

PCINT12 (Pin Change Interrupt 12)

PC3 ADC3 (ADC Input Channel 3)

PCINT11 (Pin Change Interrupt 11)

PC2 ADC2 (ADC Input Channel 2)

PCINT10 (Pin Change Interrupt 10)

PC1 ADC1 (ADC Input Channel 1)

PCINT9 (Pin Change Interrupt 9)

PC0 ADC0 (ADC Input Channel 0)

PCINT8 (Pin Change Interrupt 8)

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Atmel ATmega48/88/168 Automotive

• SDA/ADC4/PCINT12 – Port C, Bit 4

SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the 2-wire

Serial Interface, pin PC4 is disconnected from the port and becomes the Serial Data I/O pin for

the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes

shorter than 50 ns on the input signal, and the pin is driven by an open drain driver with

slew-rate limitation.

PC4 can also be used as ADC input Channel 4. Note that ADC input channel 4 uses digital

power.

PCINT12: Pin Change Interrupt source 12. The PC4 pin can serve as an external interrupt

source.

• ADC3/PCINT11 – Port C, Bit 3

PC3 can also be used as ADC input Channel 3. Note that ADC input channel 3 uses analog

power.

PCINT11: Pin Change Interrupt source 11. The PC3 pin can serve as an external interrupt

source.

• ADC2/PCINT10 – Port C, Bit 2

PC2 can also be used as ADC input Channel 2. Note that ADC input channel 2 uses analog

power.

PCINT10: Pin Change Interrupt source 10. The PC2 pin can serve as an external interrupt

source.

• ADC1/PCINT9 – Port C, Bit 1

PC1 can also be used as ADC input Channel 1. Note that ADC input channel 1 uses analog

power.

PCINT9: Pin Change Interrupt source 9. The PC1 pin can serve as an external interrupt source.

• ADC0/PCINT8 – Port C, Bit 0

PC0 can also be used as ADC input Channel 0. Note that ADC input channel 0 uses analog

power.

PCINT8: Pin Change Interrupt source 8. The PC0 pin can serve as an external interrupt source.

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Atmel ATmega48/88/168 Automotive

Table 10-7 and Table 10-8 relate the alternate functions of Port C to the overriding signals

shown in Figure 10-5 on page 68.

Note: 1. When enabled, the 2-wire Serial Interface enables slew-rate controls on the output pins PC4

and PC5. This is not shown in the figure. In addition, spike filters are connected between the

AIO outputs shown in the port figure and the digital logic of the TWI module.

Table 10-7. Overriding Signals for Alternate Functions in PC6..PC4(1)

Signal

Name PC6/RESET/PCINT14 PC5/SCL/ADC5/PCINT13 PC4/SDA/ADC4/PCINT12

PUOE RSTDISBL TWEN TWEN

PUOV 1 PORTC5 • PUD PORTC4 • PUD

DDOE RSTDISBL TWEN TWEN

DDOV 0 SCL_OUT SDA_OUT

PVOE 0 TWEN TWEN

PVOV 0 0 0

DIEOE RSTDISBL + PCINT14 •

PCIE1 PCINT13 • PCIE1 + ADC5D PCINT12 • PCIE1 + ADC4D

DIEOV RSTDISBL PCINT13 • PCIE1 PCINT12 • PCIE1

DI PCINT14 INPUT PCINT13 INPUT PCINT12 INPUT

AIO RESET INPUT ADC5 INPUT / SCL INPUT ADC4 INPUT / SDA INPUT

Table 10-8. Overriding Signals for Alternate Functions in PC3..PC0

Signal

Name

PC3/ADC3/

PCINT11

PC2/ADC2/

PCINT10

PC1/ADC1/

PCINT9

PC0/ADC0/

PCINT8

PUOE0000

PUOV0000

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE0000

PVOV0000

DIEOE PCINT11 • PCIE1 +

ADC3D

PCINT10 • PCIE1 +

ADC2D

PCINT9 • PCIE1 +

ADC1D

PCINT8 • PCIE1 +

ADC0D

DIEOV PCINT11 • PCIE1 PCINT10 • PCIE1 PCINT9 • PCIE1 PCINT8 • PCIE1

DI PCINT11 INPUT PCINT10 INPUT PCINT9 INPUT PCINT8 INPUT

AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT

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Atmel ATmega48/88/168 Automotive

10.3.4 Alternate Functions of Port D

The Port D pins with alternate functions are shown in Table 10-9.

The alternate pin configuration is as follows:

• AIN1/OC2B/PCINT23 – Port D, Bit 7

AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up

switched off to avoid the digital port function from interfering with the function of the Analog

Comparator.

PCINT23: Pin Change Interrupt source 23. The PD7 pin can serve as an external interrupt

source.

• AIN0/OC0A/PCINT22 – Port D, Bit 6

AIN0, Analog Comparator Positive Input. Configure the port pin as input with the internal pull-up

switched off to avoid the digital port function from interfering with the function of the Analog

Comparator.

OC0A, Output Compare Match output: The PD6 pin can serve as an external output for the

Timer/Counter0 Compare Match A. The PD6 pin has to be configured as an output (DDD6 set

(one)) to serve this function. The OC0A pin is also the output pin for the PWM mode timer

function.

PCINT22: Pin Change Interrupt source 22. The PD6 pin can serve as an external interrupt

source.

Table 10-9. Port D Pins Alternate Functions

Port Pin Alternate Function

PD7 AIN1 (Analog Comparator Negative Input)

PCINT23 (Pin Change Interrupt 23)

PD6

AIN0 (Analog Comparator Positive Input)

OC0A (Timer/Counter0 Output Compare Match A Output)

PCINT22 (Pin Change Interrupt 22)

PD5

T1 (Timer/Counter 1 External Counter Input)

OC0B (Timer/Counter0 Output Compare Match B Output)

PCINT21 (Pin Change Interrupt 21)

PD4

XCK (USART External Clock Input/Output)

T0 (Timer/Counter 0 External Counter Input)

PCINT20 (Pin Change Interrupt 20)

PD3

INT1 (External Interrupt 1 Input)

OC2B (Timer/Counter2 Output Compare Match B Output)

PCINT19 (Pin Change Interrupt 19)

PD2 INT0 (External Interrupt 0 Input)

PCINT18 (Pin Change Interrupt 18)

PD1 TXD (USART Output Pin)

PCINT17 (Pin Change Interrupt 17)

PD0 RXD (USART Input Pin)

PCINT16 (Pin Change Interrupt 16)

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• T1/OC0B/PCINT21 – Port D, Bit 5

T1, Timer/Counter1 counter source.

OC0B, Output Compare Match output: The PD5 pin can serve as an external output for the

Timer/Counter0 Compare Match B. The PD5 pin has to be configured as an output (DDD5 set

(one)) to serve this function. The OC0B pin is also the output pin for the PWM mode timer

function.

PCINT21: Pin Change Interrupt source 21. The PD5 pin can serve as an external interrupt

source.

• XCK/T0/PCINT20 – Port D, Bit 4

XCK, USART external clock.

T0, Timer/Counter0 counter source.

PCINT20: Pin Change Interrupt source 20. The PD4 pin can serve as an external interrupt

source.

• INT1/OC2B/PCINT19 – Port D, Bit 3

INT1, External Interrupt source 1: The PD3 pin can serve as an external interrupt source.

OC2B, Output Compare Match output: The PD3 pin can serve as an external output for the

Timer/Counter0 Compare Match B. The PD3 pin has to be configured as an output (DDD3 set

(one)) to serve this function. The OC2B pin is also the output pin for the PWM mode timer

function.

PCINT19: Pin Change Interrupt source 19. The PD3 pin can serve as an external interrupt

source.

• INT0/PCINT18 – Port D, Bit 2

INT0, External Interrupt source 0: The PD2 pin can serve as an external interrupt source.

PCINT18: Pin Change Interrupt source 18. The PD2 pin can serve as an external interrupt

source.

• TXD/PCINT17 – Port D, Bit 1

TXD, Transmit Data (Data output pin for the USART). When the USART Transmitter is enabled,

this pin is configured as an output regardless of the value of DDD1.

PCINT17: Pin Change Interrupt source 17. The PD1 pin can serve as an external interrupt

source.

• RXD/PCINT16 – Port D, Bit 0

RXD, Receive Data (Data input pin for the USART). When the USART Receiver is enabled this

pin is configured as an input regardless of the value of DDD0. When the USART forces this pin

to be an input, the pull-up can still be controlled by the PORTD0 bit.

PCINT16: Pin Change Interrupt source 16. The PD0 pin can serve as an external interrupt

source.

Table 10-10 and Table 10-11 relate the alternate functions of Port D to the overriding signals

shown in Figure 10-5 on page 68.

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Table 10-10. Overriding Signals for Alternate Functions PD7..PD4

Signal

Name

PD7/AIN1

/PCINT23

PD6/AIN0/

OC0A/PCINT22

PD5/T1/OC0B/

PCINT21

PD4/XCK/

T0/PCINT20

PUOE0000

PUO 0 0 0 0

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE 0 OC0A ENABLE OC0B ENABLE UMSEL

PVOV 0 OC0A OC0B XCK OUTPUT

DIEOE PCINT23 • PCIE2 PCINT22 • PCIE2 PCINT21 • PCIE2 PCINT20 • PCIE2

DIEOV 1 1 1 1

DI PCINT23 INPUT PCINT22 INPUT PCINT21 INPUT

T1 INPUT

PCINT20 INPUT

XCK INPUT

T0 INPUT

AIO AIN1 INPUT AIN0 INPUT – –

Table 10-11. Overriding Signals for Alternate Functions in PD3..PD0

Signal

Name

PD3/OC2B/INT1/

PCINT19

PD2/INT0/

PCINT18

PD1/TXD/

PCINT17

PD0/RXD/

PCINT16

PUOE 0 0 TXEN RXEN

PUO 0 0 0 PORTD0 • PUD

DDOE 0 0 TXEN RXEN

DDOV 0 0 1 0

PVOE OC2B ENABLE 0 TXEN 0

PVOV OC2B 0 TXD 0

DIEOE INT1 ENABLE +

PCINT19 • PCIE2

INT0 ENABLE +

PCINT18 • PCIE1 PCINT17 • PCIE2 PCINT16 • PCIE2

DIEOV 1 1 1 1

DI PCINT19 INPUT

INT1 INPUT

PCINT18 INPUT

INT0 INPUT PCINT17 INPUT PCINT16 INPUT

RXD

AIO – – – –

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Atmel ATmega48/88/168 Automotive

10.4 Register Description for I/O Ports

10.4.1 The Port B Data Register – PORTB

10.4.2 The Port B Data Direction Register – DDRB

10.4.3 The Port B Input Pins Address – PINB

10.4.4 The Port C Data Register – PORTC

10.4.5 The Port C Data Direction Register – DDRC

10.4.6 The Port C Input Pins Address – PINC

10.4.7 The Port D Data Register – PORTD

10.4.8 The Port D Data Direction Register – DDRD

Bit 76543210

PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB

Read/WriteRRRRRRRR

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

– PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 PORTC

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

– DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 DDRC

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

– PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 PINC

Read/WriteRRRRRRRR

Initial Value 0 N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 PORTD

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 DDRD

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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Atmel ATmega48/88/168 Automotive

10.4.9 The Port D Input Pins Address – PIND

11. External Interrupts

The External Interrupts are triggered by the INT0 and INT1 pins or any of the PCINT23..0 pins.

Observe that, if enabled, the interrupts will trigger even if the INT0 and INT1 or PCINT23..0 pins

are configured as outputs. This feature provides a way of generating a software interrupt. The

pin change interrupt PCI2 will trigger if any enabled PCINT23..16 pin toggles. The pin change

interrupt PCI1 will trigger if any enabled PCINT14..8 pin toggles. The pin change interrupt PCI0

will trigger if any enabled PCINT7..0 pin toggles. The PCMSK2, PCMSK1 and PCMSK0 Regis-

ters control which pins contribute to the pin change interrupts. Pin change interrupts on

PCINT23..0 are detected asynchronously. This implies that these interrupts can be used for

waking the part also from sleep modes other than Idle mode.

The INT0 and INT1 interrupts can be triggered by a falling or rising edge or a low level. This is

set up as indicated in the specification for the External Interrupt Control Register A – EICRA.

When the INT0 or INT1 interrupts are enabled and are configured as level triggered, the inter-

rupts will trigger as long as the pin is held low. Note that recognition of falling or rising edge

interrupts on INT0 or INT1 requires the presence of an I/O clock, described in “Clock Systems

and their Distribution” on page 24. Low level interrupt on INT0 and INT1 is detected asynchro-

nously. This implies that this interrupt can be used for waking the part also from sleep modes

other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.

Note that if a level triggered interrupt is used for wake-up from Power-down, the required level

must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If

the level disappears before the end of the Start-up Time, the MCU will still wake up, but no inter-

rupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described

in “System Clock and Clock Options” on page 24.

11.0.1 External Interrupt Control Register A – EICRA

The External Interrupt Control Register A contains control bits for interrupt sense control.

• Bit 7..4 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0

The External Interrupt 1 is activated by the external pin INT1 if the SREG I-flag and the corre-

sponding interrupt mask are set. The level and edges on the external INT1 pin that activate the

interrupt are defined in Table 11-1. The value on the INT1 pin is sampled before detecting

edges. If edge or toggle interrupt is selected, pulses that last longer than one clock period will

generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level

interrupt is selected, the low level must be held until the completion of the currently executing

instruction to generate an interrupt.

Bit 76543210

PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 PIND

Read/WriteRRRRRRRR

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

––––ISC11ISC10ISC01ISC00EICRA

Read/Write RRRRR/WR/WR/WR/W

Initial Value00000000

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Atmel ATmega48/88/168 Automotive

• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0

The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corre-

sponding interrupt mask are set. The level and edges on the external INT0 pin that activate the

interrupt are defined in Table 11-2. The value on the INT0 pin is sampled before detecting

edges. If edge or toggle interrupt is selected, pulses that last longer than one clock period will

generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level

interrupt is selected, the low level must be held until the completion of the currently executing

instruction to generate an interrupt.

11.0.2 External Interrupt Mask Register – EIMSK

• Bit 7..2 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 1 – INT1: External Interrupt Request 1 Enable

When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the

External Interrupt Control Register A (EICRA) define whether the external interrupt is activated

on rising and/or falling edge of the INT1 pin or level sensed. Activity on the pin will cause an

interrupt request even if INT1 is configured as an output. The corresponding interrupt of External

Interrupt Request 1 is executed from the INT1 Interrupt Vector.

Table 11-1. Interrupt 1 Sense Control

ISC11 ISC10 Description

0 0 The low level of INT1 generates an interrupt request.

0 1 Any logical change on INT1 generates an interrupt request.

1 0 The falling edge of INT1 generates an interrupt request.

1 1 The rising edge of INT1 generates an interrupt request.

Table 11-2. Interrupt 0 Sense Control

ISC01 ISC00 Description

0 0 The low level of INT0 generates an interrupt request.

0 1 Any logical change on INT0 generates an interrupt request.

1 0 The falling edge of INT0 generates an interrupt request.

1 1 The rising edge of INT0 generates an interrupt request.

Bit 76543210

––––––INT1INT0EIMSK

Read/Write RRRRRRR/WR/W

Initial Value00000000

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Atmel ATmega48/88/168 Automotive

• Bit 0 – INT0: External Interrupt Request 0 Enable

When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the

External Interrupt Control Register A (EICRA) define whether the external interrupt is activated

on rising and/or falling edge of the INT0 pin or level sensed. Activity on the pin will cause an

interrupt request even if INT0 is configured as an output. The corresponding interrupt of External

Interrupt Request 0 is executed from the INT0 Interrupt Vector.

11.0.3 External Interrupt Flag Register – EIFR

• Bit 7..2 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 1 – INTF1: External Interrupt Flag 1

When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 becomes set

(one). If the I-bit in SREG and the INT1 bit in EIMSK are set (one), the MCU will jump to the cor-

responding Interrupt Vector. The flag is cleared when the interrupt routine is executed.

Alternatively, the flag can be cleared by writing a logical one to it. This flag is always cleared

when INT1 is configured as a level interrupt.

• Bit 0 – INTF0: External Interrupt Flag 0

When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set

(one). If the I-bit in SREG and the INT0 bit in EIMSK are set (one), the MCU will jump to the cor-

responding Interrupt Vector. The flag is cleared when the interrupt routine is executed.

Alternatively, the flag can be cleared by writing a logical one to it. This flag is always cleared

when INT0 is configured as a level interrupt.

11.0.4 Pin Change Interrupt Control Register - PCICR

• Bit 7..3 - Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 2 - PCIE2: Pin Change Interrupt Enable 2

When the PCIE2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin

change interrupt 2 is enabled. Any change on any enabled PCINT23..16 pin will cause an inter-

rupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI2

Interrupt Vector. PCINT23..16 pins are enabled individually by the PCMSK2 Register.

Bit 76543210

––––––INTF1INTF0EIFR

Read/Write RRRRRRR/WR/W

Initial Value00000000

Bit 76543210

–––––PCIE2PCIE1PCIE0PCICR

Read/Write RRRRRR/WR/WR/W

Initial Value00000000

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• Bit 1 - PCIE1: Pin Change Interrupt Enable 1

When the PCIE1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin

change interrupt 1 is enabled. Any change on any enabled PCINT14..8 pin will cause an inter-

rupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI1

Interrupt Vector. PCINT14..8 pins are enabled individually by the PCMSK1 Register.

• Bit 0 - PCIE0: Pin Change Interrupt Enable 0

When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin

change interrupt 0 is enabled. Any change on any enabled PCINT7..0 pin will cause an interrupt.

The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI0 Inter-

rupt Vector. PCINT7..0 pins are enabled individually by the PCMSK0 Register.

11.0.5 Pin Change Interrupt Flag Register - PCIFR

• Bit 7..3 - Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 2 - PCIF2: Pin Change Interrupt Flag 2

When a logic change on any PCINT23..16 pin triggers an interrupt request, PCIF2 becomes set

(one). If the I-bit in SREG and the PCIE2 bit in PCICR are set (one), the MCU will jump to the

corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-

natively, the flag can be cleared by writing a logical one to it.

• Bit 1 - PCIF1: Pin Change Interrupt Flag 1

When a logic change on any PCINT14..8 pin triggers an interrupt request, PCIF1 becomes set

(one). If the I-bit in SREG and the PCIE1 bit in PCICR are set (one), the MCU will jump to the

corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-

natively, the flag can be cleared by writing a logical one to it.

• Bit 0 - PCIF0: Pin Change Interrupt Flag 0

When a logic change on any PCINT7..0 pin triggers an interrupt request, PCIF0 becomes set

(one). If the I-bit in SREG and the PCIE0 bit in PCICR are set (one), the MCU will jump to the

corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-

natively, the flag can be cleared by writing a logical one to it.

Bit 76543210

–––––PCIF2PCIF1PCIF0PCIFR

Read/Write RRRRRR/WR/WR/W

Initial Value00000000

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Atmel ATmega48/88/168 Automotive

11.0.6 Pin Change Mask Register 2 – PCMSK2

• Bit 7..0 – PCINT23..16: Pin Change Enable Mask 23..16

Each PCINT23..16-bit selects whether pin change interrupt is enabled on the corresponding I/O

pin. If PCINT23..16 is set and the PCIE2 bit in PCICR is set, pin change interrupt is enabled on

the corresponding I/O pin. If PCINT23..16 is cleared, pin change interrupt on the corresponding

I/O pin is disabled.

11.0.7 Pin Change Mask Register 1 – PCMSK1

• Bit 7 – Res: Reserved Bit

This bit is an unused bit in the ATmega48/88/168, and will always read as zero.

• Bit 6..0 – PCINT14..8: Pin Change Enable Mask 14..8

Each PCINT14..8-bit selects whether pin change interrupt is enabled on the corresponding I/O

pin. If PCINT14..8 is set and the PCIE1 bit in PCICR is set, pin change interrupt is enabled on

the corresponding I/O pin. If PCINT14..8 is cleared, pin change interrupt on the corresponding

I/O pin is disabled.

11.0.8 Pin Change Mask Register 0 – PCMSK0

• Bit 7..0 – PCINT7..0: Pin Change Enable Mask 7..0

Each PCINT7..0 bit selects whether pin change interrupt is enabled on the corresponding I/O

pin. If PCINT7..0 is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the

corresponding I/O pin. If PCINT7..0 is cleared, pin change interrupt on the corresponding I/O pin

is disabled.

Bit 76543210

PCINT23 PCINT22 PCINT21 PCINT20 PCINT19 PCINT18 PCINT17 PCINT16 PCMSK2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

– PCINT14 PCINT13 PCINT12 PCINT11 PCINT10 PCINT9 PCINT8 PCMSK1

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 PCMSK0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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12. 8-bit Timer/Counter0 with PWM

Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output

Compare Units, and with PWM support. It allows accurate program execution timing (event man-

agement) and wave generation. The main features are:

• Two Independent Output Compare Units

• Double Buffered Output Compare Registers

• Clear Timer on Compare Match (Auto Reload)

• Glitch Free, Phase Correct Pulse Width Modulator (PWM)

• Variable PWM Period

•Frequency Generator

• Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)

12.1 Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 12-1. For the actual

placement of I/O pins, refer to “Pinout ATmega48/88/168” on page 2. CPU accessible I/O Regis-

ters, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit

locations are listed in the “8-bit Timer/Counter Register Description” on page 97.

The PRTIM0 bit in “Power Reduction Register - PRR” on page 39 must be written to zero to

enable Timer/Counter0 module.

Figure 12-1. 8-bit Timer/Counter Block Diagram

Timer/Counter

DATA BUS

OCRnA

OCRnB

TCNTn

Waveform

Generation

Waveform

Generation

OCnA

OCnB

Fixed

TOP

Value

Control Logic

= 0

TOP BOTTOM

Count

Clear

Direction

TOVn

(Int.Req.)

OCnA

(Int.Req.)

OCnB

(Int.Req.)

TCCRnA TCCRnB

clk

Prescaler

T/C

Oscillator

clk

I/O

TOSC1

TOSC2

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Atmel ATmega48/88/168 Automotive

12.1.1 Definitions

Many register and bit references in this section are written in general form. A lower case “n”

replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Com-

pare Unit, in this case Compare Unit A or Compare Unit B. However, when using the register or

bit defines in a program, the precise form must be used, i.e., TCNT0 for accessing

Timer/Counter0 counter value and so on.

The definitions in the following table are also used extensively throughout the document.

12.1.2 Registers

The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit

registers. Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the

Timer Interrupt Flag Register (TIFR0). All interrupts are individually masked with the Timer Inter-

rupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on

the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter

uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source

is selected. The output from the Clock Select logic is referred to as the timer clock (clkT0).

The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the

Timer/Counter value at all times. The result of the compare can be used by the Waveform Gen-

erator to generate a PWM or variable frequency output on the Output Compare pins (OC0A and

OC0B). See “Using the Output Compare Unit” on page 115. for details. The compare match

event will also set the Compare Flag (OCF0A or OCF0B) which can be used to generate an Out-

put Compare interrupt request.

12.2 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the Clock Select logic which is controlled by the Clock Select (CS02:0) bits

located in the Timer/Counter Control Register (TCCR0B). For details on clock sources and pres-

caler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 103.

12.3 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

12-2 shows a block diagram of the counter and its surroundings.

BOTTOM The counter reaches the BOTTOM when it becomes 0x00.

MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

TOP The counter reaches the TOP when it becomes equal to the highest value in the

count sequence. The TOP value can be assigned to be the fixed value 0xFF

(MAX) or the value stored in the OCR0A Register. The assignment is depen-

dent on the mode of operation.

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Atmel ATmega48/88/168 Automotive

Figure 12-2. Counter Unit Block Diagram

Signal description (internal signals):

count Increment or decrement TCNT0 by 1.

direction Select between increment and decrement.

clear Clear TCNT0 (set all bits to zero).

clkTnTimer/Counter clock, referred to as clkT0 in the following.

top Signalize that TCNT0 has reached maximum value.

bottom Signalize that TCNT0 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clkT0). clkT0 can be generated from an external or internal clock source,

selected by the Clock Select bits (CS02:0). When no clock source is selected (CS02:0 = 0) the

timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of

whether clkT0 is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in

the Timer/Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter

Control Register B (TCCR0B). There are close connections between how the counter behaves

(counts) and how waveforms are generated on the Output Compare outputs OC0A and OC0B.

For more details about advanced counting sequences and waveform generation, see “Modes of

Operation” on page 91.

The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by

the WGM02:0 bits. TOV0 can be used for generating a CPU interrupt.

12.4 Output Compare Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers

(OCR0A and OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a

match. A match will set the Output Compare Flag (OCF0A or OCF0B) at the next timer clock

cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output

Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is exe-

cuted. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit

location. The Waveform Generator uses the match signal to generate an output according to

operating mode set by the WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max

and bottom signals are used by the Waveform Generator for handling the special cases of the

extreme values in some modes of operation (“Modes of Operation” on page 91).

Figure 12-3 shows a block diagram of the Output Compare unit.

DATA BUS

TCNTn Control Logic

count

TOVn

(Int.Req.)

Clock Select

top

Edge

Detector

( From Prescaler )

clk

bottom

direction

clear

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Figure 12-3. Output Compare Unit, Block Diagram

The OCR0x Registers are double buffered when using any of the Pulse Width Modulation

(PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the dou-

ble buffering is disabled. The double buffering synchronizes the update of the OCR0x Compare

Registers to either top or bottom of the counting sequence. The synchronization prevents the

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR0x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR0x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR0x directly.

12.4.1 Force Output Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC0x) bit. Forcing compare match will not set the

OCF0x Flag or reload/clear the timer, but the OC0x pin will be updated as if a real compare

match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set, cleared or

toggled).

12.4.2 Compare Match Blocking by TCNT0 Write

All CPU write operations to the TCNT0 Register will block any compare match that occur in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR0x to be initial-

ized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is

enabled.

12.4.3 Using the Output Compare Unit

Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT0 when using the Output Compare Unit,

independently of whether the Timer/Counter is running or not. If the value written to TCNT0

equals the OCR0x value, the compare match will be missed, resulting in incorrect waveform

generation. Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is

downcounting.

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnx1:0

bottom

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Atmel ATmega48/88/168 Automotive

The setup of the OC0x should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC0x value is to use the Force Output Com-

pare (FOC0x) strobe bits in Normal mode. The OC0x Registers keep their values even when

changing between Waveform Generation modes.

Be aware that the COM0x1:0 bits are not double buffered together with the compare value.

Changing the COM0x1:0 bits will take effect immediately.

12.5 Compare Match Output Unit

The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses

the COM0x1:0 bits for defining the Output Compare (OC0x) state at the next compare match.

Also, the COM0x1:0 bits control the OC0x pin output source. Figure 12-4 shows a simplified

schematic of the logic affected by the COM0x1:0 bit setting. The I/O Registers, I/O bits, and I/O

pins in the figure are shown in bold. Only the parts of the general I/O port control registers (DDR

and PORT) that are affected by the COM0x1:0 bits are shown. When referring to the OC0x

state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur,

the OC0x Register is reset to “0”.

Figure 12-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform

Generator if either of the COM0x1:0 bits are set. However, the OC0x pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ble on the pin. The port override function is independent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC0x state before the out-

put is enabled. Note that some COM0x1:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 97.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BUS

FOCn

clk

I/O

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Atmel ATmega48/88/168 Automotive

12.5.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes.

For all modes, setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the

OC0x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 12-1 on page 97. For fast PWM mode, refer to Table 12-2 on

page 97, and for phase correct PWM refer to Table 12-3 on page 98.

A change of the COM0x1:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC0x strobe bits.

12.6 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is

defined by the combination of the Waveform Generation mode (WGM02:0) and Compare Output

mode (COM0x1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM0x1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 90.).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 95.

12.6.1 Normal Mode

The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set in the same

timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt

that automatically clears the TOV0 Flag, the timer resolution can be increased by software.

There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Output Compare unit can be used to generate interrupts at some given time. Using the Out-

put Compare to generate waveforms in Normal mode is not recommended, since this will

occupy too much of the CPU time.

12.6.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to

manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter

value (TCNT0) matches the OCR0A. The OCR0A defines the top value for the counter, hence

also its resolution. This mode allows greater control of the compare match output frequency. It

also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 12-5. The counter value (TCNT0)

increases until a compare match occurs between TCNT0 and OCR0A, and then counter

(TCNT0) is cleared.

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

Figure 12-5. CTC Mode, Timing Diagram

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF0A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating

the TOP value. However, changing TOP to a value close to BOTTOM when the counter is run-

ning with none or a low prescaler value must be done with care since the CTC mode does not

have the double buffering feature. If the new value written to OCR0A is lower than the current

value of TCNT0, the counter will miss the compare match. The counter will then have to count to

its maximum value (0xFF) and wrap around starting at 0x00 before the compare match can

occur.

For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM0A1:0 = 1). The OC0A value will not be visible on the port pin unless the data direction for

the pin is set to output. The waveform generated will have a maximum frequency of fOC0 =

fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by the following

equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x00.

12.6.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high fre-

quency PWM waveform generation option. The fast PWM differs from the other PWM option by

its single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOT-

TOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when WGM2:0 = 7. In

non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the compare

match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode,

the output is set on compare match and cleared at BOTTOM. Due to the single-slope operation,

the operating frequency of the fast PWM mode can be twice as high as the phase correct PWM

mode that use dual-slope operation. This high frequency makes the fast PWM mode well suited

for power regulation, rectification, and DAC applications. High frequency allows physically small

sized external components (coils, capacitors), and therefore reduces total system cost.

TCNTn

OCn

(Toggle)

OCnx Interrupt Flag Set

1 4

Period

2 3

(COMnx1:0 = 1)

fOCnx

fclk_I/O

2N1OCRnx+()⋅⋅

-------------------------------------------------------=

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

In fast PWM mode, the counter is incremented until the counter value matches the TOP value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 12-6. The TCNT0 value is in the timing diagram shown as a his-

togram for illustrating the single-slope operation. The diagram includes non-inverted and

inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare

matches between OCR0x and TCNT0.

Figure 12-6. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins.

Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows

the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available

for the OC0B pin (see Table 12-5 on page 98). The actual OC0x value will only be visible on the

port pin if the data direction for the port pin is set as output. The PWM waveform is generated by

setting (or clearing) the OC0x Register at the compare match between OCR0x and TCNT0, and

clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes

from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0A Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will

be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result

in a constantly high or low output (depending on the polarity of the output set by the COM0A1:0

bits.)

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

2 3

OCn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

4 5 6 7

fOCnxPWM

fclk_I/O

N256⋅

---------------------=

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC0x to toggle its logical level on each compare match (COM0x1:0 = 1). The waveform

generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This

feature is similar to the OC0A toggle in CTC mode, except the double buffer feature of the Out-

put Compare unit is enabled in the fast PWM mode.

12.6.4 Phase Correct PWM Mode

The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct

PWM waveform generation option. The phase correct PWM mode is based on a dual-slope

operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOT-

TOM. TOP is defined as 0xFF when WGM2:0 = 1, and OCR0A when WGM2:0 = 5. In

non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the compare

match between TCNT0 and OCR0x while upcounting, and set on the compare match while

downcounting. In inverting Output Compare mode, the operation is inverted. The dual-slope

operation has lower maximum operation frequency than single slope operation. However, due to

the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor con-

trol applications.

In phase correct PWM mode the counter is incremented until the counter value matches TOP.

When the counter reaches TOP, it changes the count direction. The TCNT0 value will be equal

to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown

on Figure 12-7. The TCNT0 value is in the timing diagram shown as a histogram for illustrating

the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The

small horizontal line marks on the TCNT0 slopes represent compare matches between OCR0x

and TCNT0.

Figure 12-7. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The

Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM

value.

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

1 2 3

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

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Atmel ATmega48/88/168 Automotive

In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the

OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted

PWM output can be generated by setting the COM0x1:0 to three: Setting the COM0A0 bits to

one allows the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is

not available for the OC0B pin (see Table 12-6 on page 99). The actual OC0x value will only be

visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is

generated by clearing (or setting) the OC0x Register at the compare match between OCR0x and

TCNT0 when the counter increments, and setting (or clearing) the OC0x Register at compare

match between OCR0x and TCNT0 when the counter decrements. The PWM frequency for the

output when using phase correct PWM can be calculated by the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0A Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR0A is set equal to BOTTOM, the

output will be continuously low and if set equal to MAX the output will be continuously high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.

At the very start of period 2 in Figure 12-7 OCnx has a transition from high to low even though

there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-

TOM. There are two cases that give a transition without Compare Match.

• OCRnx changes its value from MAX, like in Figure 12-7. When the OCR0A value is MAX the

OCn pin value is the same as the result of a down-counting Compare Match. To ensure

symmetry around BOTTOM the OCnx value at MAX must correspond to the result of an

up-counting Compare Match.

• The timer starts counting from a value higher than the one in OCRnx, and for that reason

misses the Compare Match and hence the OCnx change that would have happened on the

way up.

12.7 Timer/Counter Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a

clock enable signal in the following figures. The figures include information on when interrupt

flags are set. Figure 12-8 contains timing data for basic Timer/Counter operation. The figure

shows the count sequence close to the MAX value in all modes other than phase correct PWM

mode.

Figure 12-8. Timer/Counter Timing Diagram, no Prescaling

Figure 12-9 shows the same timing data, but with the prescaler enabled.

fOCnxPCPWM

fclk_I/O

N510⋅

---------------------=

clk

(clk

I/O

/1)

TOVn

clk

I/O

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

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Atmel ATmega48/88/168 Automotive

Figure 12-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

Figure 12-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC

mode and PWM mode, where OCR0A is TOP.

Figure 12-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)

Figure 12-11 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast

PWM mode where OCR0A is TOP.

Figure 12-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Pres-

caler (fclk_I/O/8)

TOVn

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clkI/O

clkTn

(clk

I/O

/8)

OCFnx

OCRnx

TCNTn

(CTC)

TOP

TOP - 1 TOP BOTTOM BOTTOM + 1

clkI/O

clkTn

(clk

I/O

/8)

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Atmel ATmega48/88/168 Automotive

12.8 8-bit Timer/Counter Register Description

12.8.1 Timer/Counter Control Register A – TCCR0A

• Bits 7:6 – COM0A1:0: Compare Match Output A Mode

These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0

bits are set, the OC0A output overrides the normal port functionality of the I/O pin it is connected

to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0A pin

must be set in order to enable the output driver.

When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the

WGM02:0 bit setting. Table 12-1 shows the COM0A1:0 bit functionality when the WGM02:0 bits

are set to a normal or CTC mode (non-PWM).

Table 12-2 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM

mode.

Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 92

for more details.

Bit 7 6 5 4 3 210

COM0A1 COM0A0 COM0B1 COM0B0 – – WGM01 WGM00 TCCR0A

Read/Write R/W R/W R/W R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 12-1. Compare Output Mode, non-PWM Mode

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected.

0 1 Toggle OC0A on Compare Match

1 0 Clear OC0A on Compare Match

1 1 Set OC0A on Compare Match

Table 12-2. Compare Output Mode, Fast PWM Mode(1)

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected.

WGM02 = 0: Normal Port Operation, OC0A Disconnected.

WGM02 = 1: Toggle OC0A on Compare Match.

1 0 Clear OC0A on Compare Match, set OC0A at TOP

1 1 Set OC0A on Compare Match, clear OC0A at TOP

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Atmel ATmega48/88/168 Automotive

Table 12-3 shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to phase cor-

rect PWM mode.

Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on

page 121 for more details.

• Bits 5:4 – COM0B1:0: Compare Match Output B Mode

These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B1:0

bits are set, the OC0B output overrides the normal port functionality of the I/O pin it is connected

to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0B pin

must be set in order to enable the output driver.

When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the

WGM02:0 bit setting. Table 12-4 shows the COM0B1:0 bit functionality when the WGM02:0 bits

are set to a normal or CTC mode (non-PWM).

Table 12-5 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast PWM

mode.

Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 92

for more details.

Table 12-3. Compare Output Mode, Phase Correct PWM Mode(1)

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected.

WGM02 = 0: Normal Port Operation, OC0A Disconnected.

WGM02 = 1: Toggle OC0A on Compare Match.

Clear OC0A on Compare Match when up-counting. Set OC0A on

Compare Match when down-counting.

Set OC0A on Compare Match when up-counting. Clear OC0A on

Compare Match when down-counting.

Table 12-4. Compare Output Mode, non-PWM Mode

COM0B1 COM0B0 Description

0 0 Normal port operation, OC0B disconnected.

0 1 Toggle OC0B on Compare Match

1 0 Clear OC0B on Compare Match

1 1 Set OC0B on Compare Match

Table 12-5. Compare Output Mode, Fast PWM Mode(1)

COM0B1 COM0B0 Description

0 0 Normal port operation, OC0B disconnected.

01Reserved

1 0 Clear OC0B on Compare Match, set OC0B at TOP

1 1 Set OC0B on Compare Match, clear OC0B at TOP

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Atmel ATmega48/88/168 Automotive

Table 12-6 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to phase cor-

rect PWM mode.

Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on

page 94 for more details.

• Bits 3, 2 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bits 1:0 – WGM01:0: Waveform Generation Mode

Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 12-7. Modes of operation supported by the Timer/Counter

unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and two types of

Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page 91).

Notes: 1. MAX = 0xFF

2. BOTTOM = 0x00

Table 12-6. Compare Output Mode, Phase Correct PWM Mode(1)

COM0B1 COM0B0 Description

0 0 Normal port operation, OC0B disconnected.

01Reserved

Clear OC0B on Compare Match when up-counting. Set OC0B on

Compare Match when down-counting.

Set OC0B on Compare Match when up-counting. Clear OC0B on

Compare Match when down-counting.

Table 12-7. Waveform Generation Mode Bit Description

Mode WGM02 WGM01 WGM00

Timer/Counter

Mode of

Operation TOP

Update of

OCRx at

TOV Flag

Set on(1)(2)

0 0 0 0 Normal 0xFF Immediate MAX

10 0 1

PWM, Phase

Correct 0xFF TOP BOTTOM

2 0 1 0 CTC OCRA Immediate MAX

3 0 1 1 Fast PWM 0xFF TOP MAX

4 1 0 0 Reserved – – –

51 0 1

PWM, Phase

Correct OCRA TOP BOTTOM

6 1 1 0 Reserved – – –

7 1 1 1 Fast PWM OCRA TOP TOP

100

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Atmel ATmega48/88/168 Automotive

12.8.2 Timer/Counter Control Register B – TCCR0B

• Bit 7 – FOC0A: Force Output Compare A

The FOC0A bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when

TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0A bit,

an immediate Compare Match is forced on the Waveform Generation unit. The OC0A output is

changed according to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a

strobe. Therefore it is the value present in the COM0A1:0 bits that determines the effect of the

forced compare.

A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0A as TOP.

The FOC0A bit is always read as zero.

• Bit 6 – FOC0B: Force Output Compare B

The FOC0B bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when

TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0B bit,

an immediate Compare Match is forced on the Waveform Generation unit. The OC0B output is

changed according to its COM0B1:0 bits setting. Note that the FOC0B bit is implemented as a

strobe. Therefore it is the value present in the COM0B1:0 bits that determines the effect of the

forced compare.

A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0B as TOP.

The FOC0B bit is always read as zero.

• Bits 5:4 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bit 3 – WGM02: Waveform Generation Mode

See the description in the “Timer/Counter Control Register A – TCCR0A” on page 97.

• Bits 2:0 – CS02:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter.

Bit 7 6 5 4 3 210

FOC0A FOC0B – – WGM02 CS02 CS01 CS00 TCCR0B

Read/Write W W R R R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

101

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

12.8.3 Timer/Counter Register – TCNT0

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare

Match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,

introduces a risk of missing a Compare Match between TCNT0 and the OCR0x Registers.

12.8.4 Output Compare Register A – OCR0A

The Output Compare Register A contains an 8-bit value that is continuously compared with the

counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC0A pin.

12.8.5 Output Compare Register B – OCR0B

The Output Compare Register B contains an 8-bit value that is continuously compared with the

counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC0B pin.

Table 12-8. Clock Select Bit Description

CS02 CS01 CS00 Description

0 0 0 No clock source (Timer/Counter stopped)

001clk

I/O/(No prescaling)

010clk

I/O/8 (From prescaler)

011clk

I/O/64 (From prescaler)

100clk

I/O/256 (From prescaler)

101clk

I/O/1024 (From prescaler)

1 1 0 External clock source on T0 pin. Clock on falling edge.

1 1 1 External clock source on T0 pin. Clock on rising edge.

Bit 76543210

TCNT0[7:0] TCNT0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR0A[7:0] OCR0A

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR0B[7:0] OCR0B

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

102

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

12.8.6 Timer/Counter Interrupt Mask Register – TIMSK0

• Bits 7..3 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bit 2 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable

When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the

Timer/Counter Compare Match B interrupt is enabled. The corresponding interrupt is executed if

a Compare Match in Timer/Counter occurs, i.e., when the OCF0B bit is set in the Timer/Counter

Interrupt Flag Register – TIFR0.

• Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable

When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the

Timer/Counter0 Compare Match A interrupt is enabled. The corresponding interrupt is executed

if a Compare Match in Timer/Counter0 occurs, i.e., when the OCF0A bit is set in the

Timer/Counter 0 Interrupt Flag Register – TIFR0.

• Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable

When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the

Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter0 occurs, i.e., when the TOV0 bit is set in the Timer/Counter 0 Inter-

rupt Flag Register – TIFR0.

12.8.7 Timer/Counter 0 Interrupt Flag Register – TIFR0

• Bits 7..3 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bit 2 – OCF0B: Timer/Counter 0 Output Compare B Match Flag

The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in

OCR0B – Output Compare Register0 B. OCF0B is cleared by hardware when executing the cor-

responding interrupt handling vector. Alternatively, OCF0B is cleared by writing a logic one to

the flag. When the I-bit in SREG, OCIE0B (Timer/Counter Compare B Match Interrupt Enable),

and OCF0B are set, the Timer/Counter Compare Match Interrupt is executed.

• Bit 1 – OCF0A: Timer/Counter 0 Output Compare A Match Flag

The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data

in OCR0A – Output Compare Register0. OCF0A is cleared by hardware when executing the cor-

responding interrupt handling vector. Alternatively, OCF0A is cleared by writing a logic one to

the flag. When the I-bit in SREG, OCIE0A (Timer/Counter0 Compare Match Interrupt Enable),

and OCF0A are set, the Timer/Counter0 Compare Match Interrupt is executed.

Bit 76543 210

–––––OCIE0BOCIE0ATOIE0TIMSK0

Read/Write R R R R R R/W R/W R/W

Initial Value00000000

Bit 76543210

–––––OCF0BOCF0ATOV0TIFR0

Read/Write RRRRRR/WR/WR/W

Initial Value00000000

103

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Atmel ATmega48/88/168 Automotive

• Bit 0 – TOV0: Timer/Counter0 Overflow Flag

The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware

when executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by

writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt

Enable), and TOV0 are set, the Timer/Counter0 Overflow interrupt is executed.

The setting of this flag is dependent of the WGM02:0 bit setting. Refer to Table 12-7, “Waveform

Generation Mode Bit Description” on page 99.

13. Timer/Counter0 and Timer/Counter1 Prescalers

Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters

can have different prescaler settings. The description below applies to both Timer/Counter1 and

Timer/Counter0.

13.0.1 Internal Clock Source

The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This

provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system

clock frequency (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a

clock source. The prescaled clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or

fCLK_I/O/1024.

13.0.2 Prescaler Reset

The prescaler is free running, i.e., operates independently of the Clock Select logic of the

Timer/Counter, and it is shared by Timer/Counter1 and Timer/Counter0. Since the prescaler is

not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications

for situations where a prescaled clock is used. One example of prescaling artifacts occurs when

the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock

cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system

clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).

It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execu-

tion. However, care must be taken if the other Timer/Counter that shares the same prescaler

also uses prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is

connected to.

13.0.3 External Clock Source

An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock

(clkT1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin synchronization

logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 13-1

shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector

logic. The registers are clocked at the positive edge of the internal system clock (clkI/O). The latch

is transparent in the high period of the internal system clock.

The edge detector generates one clkT1/clkT0 pulse for each positive (CSn2:0 = 7) or negative

(CSn2:0 = 6) edge it detects.

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Figure 13-1. T1/T0 Pin Sampling

The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles

from an edge has been applied to the T1/T0 pin to the counter is updated.

Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least

one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.

Each half period of the external clock applied must be longer than one system clock cycle to

ensure correct sampling. The external clock must be guaranteed to have less than half the sys-

tem clock frequency (fExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses

sampling, the maximum frequency of an external clock it can detect is half the sampling fre-

quency (Nyquist sampling theorem). However, due to variation of the system clock frequency

and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is

recommended that maximum frequency of an external clock source is less than fclk_I/O/2.5.

An external clock source can not be prescaled.

Figure 13-2. Prescaler for Timer/Counter0 and Timer/Counter1(1)

Note: 1. The synchronization logic on the input pins (T1/T0) is shown in Figure 13-1.

Tn_sync

(To Clock

Select Logic)

Edge DetectorSynchronization

DQDQ

clkI/O

PSRSYNC

Clear

clk

I/O

Synchronization

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13.0.4 General Timer/Counter Control Register – GTCCR

• Bit 7 – TSM: Timer/Counter Synchronization Mode

Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the

value that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the correspond-

ing prescaler reset signals asserted. This ensures that the corresponding Timer/Counters are

halted and can be configured to the same value without the risk of one of them advancing during

configuration. When the TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared

by hardware, and the Timer/Counters start counting simultaneously.

• Bit 0 – PSRSYNC: Prescaler Reset

When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is nor-

mally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter1

and Timer/Counter0 share the same prescaler and a reset of this prescaler will affect both

timers.

14. 16-bit Timer/Counter1 with PWM

The 16-bit Timer/Counter unit allows accurate program execution timing (event management),

wave generation, and signal timing measurement. The main features are:

•True 16-bit Design (i.e., Allows 16-bit PWM)

•Two independent Output Compare Units

•Double Buffered Output Compare Registers

•One Input Capture Unit

•Input Capture Noise Canceler

•Clear Timer on Compare Match (Auto Reload)

•Glitch-free, Phase Correct Pulse Width Modulator (PWM)

•Variable PWM Period

•Frequency Generator

•External Event Counter

•Four independent interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)

14.1 Overview

Most register and bit references in this section are written in general form. A lower case “n”

replaces the Timer/Counter number, and a lower case “x” replaces the Output Compare unit

channel. However, when using the register or bit defines in a program, the precise form must be

used, i.e., TCNT1 for accessing Timer/Counter1 counter value and so on.

A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 14-1. For the actual

placement of I/O pins, refer to “Pinout ATmega48/88/168” on page 2. CPU accessible I/O Regis-

ters, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit

locations are listed in the “16-bit Timer/Counter Register Description” on page 127.

The PRTIM1 bit in “Power Reduction Register - PRR” on page 39 must be written to zero to

enable Timer/Counter1 module.

Bit 765432 1 0

TSM – – – – – PSRASY PSRSYNC GTCCR

Read/Write R/W R R R R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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Figure 14-1. 16-bit Timer/Counter Block Diagram(1)

Note: 1. Refer to Figure 1-1 on page 2, Table 10-3 on page 70 and Table 10-9 on page 77 for

Timer/Counter1 pin placement and description.

14.1.1 Registers

The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Regis-

ter (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the

16-bit registers. These procedures are described in the section “Accessing 16-bit Registers” on

page 107. The Timer/Counter Control Registers (TCCR1A/B) are 8-bit registers and have no

CPU access restrictions. Interrupt requests (abbreviated to Int.Req. in the figure) signals are all

visible in the Timer Interrupt Flag Register (TIFR1). All interrupts are individually masked with

the Timer Interrupt Mask Register (TIMSK1). TIFR1 and TIMSK1 are not shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on

the T1 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter

uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source

is selected. The output from the Clock Select logic is referred to as the timer clock (clkT1).

Clock Select

Timer/Counter

DATA BUS

OCRnA

OCRnB

ICRn

TCNTn

Waveform

Generation

Waveform

Generation

OCnA

OCnB

Noise

Canceler ICPn

Fixed

TOP

Values

Edge

Detector

Control Logic

TOP BOTTOM

Count

Clear

Direction

TOVn

(Int.Req.)

OCnA

(Int.Req.)

OCnB

(Int.Req.)

ICFn (Int.Req.)

TCCRnA TCCRnB

( From Analog

Comparator Ouput )

Edge

Detector

( From Prescaler )

clk

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The double buffered Output Compare Registers (OCR1A/B) are compared with the Timer/Coun-

ter value at all time. The result of the compare can be used by the Waveform Generator to

generate a PWM or variable frequency output on the Output Compare pin (OC1A/B). See “Out-

put Compare Units” on page 114.. The compare match event will also set the Compare Match

Flag (OCF1A/B) which can be used to generate an Output Compare interrupt request.

The Input Capture Register can capture the Timer/Counter value at a given external (edge trig-

gered) event on either the Input Capture pin (ICP1) or on the Analog Comparator pins (See

“Analog Comparator” on page 233.) The Input Capture unit includes a digital filtering unit (Noise

Canceler) for reducing the chance of capturing noise spikes.

The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined

by either the OCR1A Register, the ICR1 Register, or by a set of fixed values. When using

OCR1A as TOP value in a PWM mode, the OCR1A Register can not be used for generating a

PWM output. However, the TOP value will in this case be double buffered allowing the TOP

value to be changed in run time. If a fixed TOP value is required, the ICR1 Register can be used

as an alternative, freeing the OCR1A to be used as PWM output.

14.1.2 Definitions

The following definitions are used extensively throughout the section:

14.2 Accessing 16-bit Registers

The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via

the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations.

Each 16-bit timer has a single 8-bit register for temporary storing of the high byte of the 16-bit

access. The same temporary register is shared between all 16-bit registers within each 16-bit

timer. Accessing the low byte triggers the 16-bit read or write operation. When the low byte of a

16-bit register is written by the CPU, the high byte stored in the temporary register, and the low

byte written are both copied into the 16-bit register in the same clock cycle. When the low byte of

a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the tempo-

rary register in the same clock cycle as the low byte is read.

Not all 16-bit accesses uses the temporary register for the high byte. Reading the OCR1A/B

16-bit registers does not involve using the temporary register.

To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the low

byte must be read before the high byte.

The following code examples show how to access the 16-bit Timer Registers assuming that no

interrupts updates the temporary register. The same principle can be used directly for accessing

the OCR1A/B and ICR1 Registers. Note that when using “C”, the compiler handles the 16-bit

access.

BOTTOM The counter reaches the BOTTOM when it becomes 0x0000.

MAX The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).

TOP

The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, 0x01FF,

or 0x03FF, or to the value stored in the OCR1A or ICR1 Register. The assignment is

dependent of the mode of operation.

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Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The assembly code example returns the TCNT1 value in the r17:r16 register pair.

It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt

occurs between the two instructions accessing the 16-bit register, and the interrupt code

updates the temporary register by accessing the same or any other of the 16-bit Timer Regis-

ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both

the main code and the interrupt code update the temporary register, the main code must disable

the interrupts during the 16-bit access.

Assembly Code Examples(1)

...

; Set TCNT1 to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCNT1H,r17

out TCNT1L,r16

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

...

C Code Examples(1)

unsigned int i;

...

/* Set TCNT1 to 0x01FF */

TCNT1 = 0x1FF;

/* Read TCNT1 into i */

i = TCNT1;

...

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The following code examples show how to do an atomic read of the TCNT1 Register contents.

Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The assembly code example returns the TCNT1 value in the r17:r16 register pair.

Assembly Code Example(1)

TIM16_ReadTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

unsigned int TIM16_ReadTCNT1( void )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNT1 into i */

i = TCNT1;

/* Restore global interrupt flag */

SREG = sreg;

return i;

}

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The following code examples show how to do an atomic write of the TCNT1 Register contents.

Writing any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The assembly code example requires that the r17:r16 register pair contains the value to be writ-

ten to TCNT1.

14.2.1 Reusing the Temporary High Byte Register

If writing to more than one 16-bit register where the high byte is the same for all registers written,

then the high byte only needs to be written once. However, note that the same rule of atomic

operation described previously also applies in this case.

14.3 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the Clock Select logic which is controlled by the Clock Select (CS12:0) bits

located in the Timer/Counter control Register B (TCCR1B). For details on clock sources and

prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 103.

Assembly Code Example(1)

TIM16_WriteTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNT1 to r17:r16

out TCNT1H,r17

out TCNT1L,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

void TIM16_WriteTCNT1( unsigned int i )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Set TCNT1 to i */

TCNT1 = i;

/* Restore global interrupt flag */

SREG = sreg;

}

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14.4 Counter Unit

The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.

Figure 14-2 shows a block diagram of the counter and its surroundings.

Figure 14-2. Counter Unit Block Diagram

Signal description (internal signals):

Count Increment or decrement TCNT1 by 1.

Direction Select between increment and decrement.

Clear Clear TCNT1 (set all bits to zero).

clkT1Timer/Counter clock.

TOP Signalize that TCNT1 has reached maximum value.

BOTTOM Signalize that TCNT1 has reached minimum value (zero).

The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) con-

taining the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight

bits. The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an

access to the TCNT1H I/O location, the CPU accesses the high byte temporary register (TEMP).

The temporary register is updated with the TCNT1H value when the TCNT1L is read, and

TCNT1H is updated with the temporary register value when TCNT1L is written. This allows the

CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data bus.

It is important to notice that there are special cases of writing to the TCNT1 Register when the

counter is counting that will give unpredictable results. The special cases are described in the

sections where they are of importance.

Depending on the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clkT1). The clkT1 can be generated from an external or internal clock source,

selected by the Clock Select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the

timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of

whether clkT1 is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the Waveform Generation mode bits

(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).

There are close connections between how the counter behaves (counts) and how waveforms

are generated on the Output Compare outputs OC1x. For more details about advanced counting

sequences and waveform generation, see “Modes of Operation” on page 117.

TEMP (8-bit)

DATA BUS

(8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit) Control Logic

Count

Clear

Direction

TOVn

(Int.Req.)

Clock Select

TOP BOTTOM

Edge

Detector

( From Prescaler )

clkTn

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The Timer/Counter Overflow Flag (TOV1) is set according to the mode of operation selected by

the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.

14.5 Input Capture Unit

The Timer/Counter incorporates an Input Capture unit that can capture external events and give

them a time-stamp indicating time of occurrence. The external signal indicating an event, or mul-

tiple events, can be applied via the ICP1 pin or alternatively, via the analog-comparator unit. The

time-stamps can then be used to calculate frequency, duty-cycle, and other features of the sig-

nal applied. Alternatively the time-stamps can be used for creating a log of the events.

The Input Capture unit is illustrated by the block diagram shown in Figure 14-3. The elements of

the block diagram that are not directly a part of the Input Capture unit are gray shaded. The

small “n” in register and bit names indicates the Timer/Counter number.

Figure 14-3. Input Capture Unit Block Diagram

When a change of the logic level (an event) occurs on the Input Capture pin (ICP1), alternatively

on the Analog Comparator output (ACO), and this change confirms to the setting of the edge

detector, a capture will be triggered. When a capture is triggered, the 16-bit value of the counter

(TCNT1) is written to the Input Capture Register (ICR1). The Input Capture Flag (ICF1) is set at

the same system clock as the TCNT1 value is copied into ICR1 Register. If enabled (ICIE1 = 1),

the Input Capture Flag generates an Input Capture interrupt. The ICF1 Flag is automatically

cleared when the interrupt is executed. Alternatively the ICF1 Flag can be cleared by software

by writing a logical one to its I/O bit location.

Reading the 16-bit value in the Input Capture Register (ICR1) is done by first reading the low

byte (ICR1L) and then the high byte (ICR1H). When the low byte is read the high byte is copied

into the high byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will

access the TEMP Register.

ICFn (Int.Req.)

Analog

Comparator

WRITE ICRn (16-bit Register)

ICRnH (8-bit)

Noise

Canceler

ICPn

Edge

Detector

TEMP (8-bit)

DATA BUS

(8-bit)

ICRnL (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit)

ACIC* ICNC ICES

ACO*

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The ICR1 Register can only be written when using a Waveform Generation mode that utilizes

the ICR1 Register for defining the counter’s TOP value. In these cases the Waveform Genera-

tion mode (WGM13:0) bits must be set before the TOP value can be written to the ICR1

before the low byte is written to ICR1L.

For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers”

on page 107.

14.5.1 Input Capture Trigger Source

The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).

Timer/Counter1 can alternatively use the Analog Comparator output as trigger source for the

Input Capture unit. The Analog Comparator is selected as trigger source by setting the Analog

Comparator Input Capture (ACIC) bit in the Analog Comparator Control and Status Register

(ACSR). Be aware that changing trigger source can trigger a capture. The Input Capture Flag

must therefore be cleared after the change.

Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled

using the same technique as for the T1 pin (Figure 13-1 on page 104). The edge detector is also

identical. However, when the noise canceler is enabled, additional logic is inserted before the

edge detector, which increases the delay by four system clock cycles. Note that the input of the

noise canceler and edge detector is always enabled unless the Timer/Counter is set in a Wave-

form Generation mode that uses ICR1 to define TOP.

An Input Capture can be triggered by software by controlling the port of the ICP1 pin.

14.5.2 Noise Canceler

The noise canceler improves noise immunity by using a simple digital filtering scheme. The

noise canceler input is monitored over four samples, and all four must be equal for changing the

output that in turn is used by the edge detector.

The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNC1) bit in

Timer/Counter Control Register B (TCCR1B). When enabled the noise canceler introduces addi-

tional four system clock cycles of delay from a change applied to the input, to the update of the

ICR1 Register. The noise canceler uses the system clock and is therefore not affected by the

prescaler.

14.5.3 Using the Input Capture Unit

The main challenge when using the Input Capture unit is to assign enough processor capacity

for handling the incoming events. The time between two events is critical. If the processor has

not read the captured value in the ICR1 Register before the next event occurs, the ICR1 will be

overwritten with a new value. In this case the result of the capture will be incorrect.

When using the Input Capture interrupt, the ICR1 Register should be read as early in the inter-

rupt handler routine as possible. Even though the Input Capture interrupt has relatively high

priority, the maximum interrupt response time is dependent on the maximum number of clock

cycles it takes to handle any of the other interrupt requests.

Using the Input Capture unit in any mode of operation when the TOP value (resolution) is

actively changed during operation, is not recommended.

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Measurement of an external signal’s duty cycle requires that the trigger edge is changed after

each capture. Changing the edge sensing must be done as early as possible after the ICR1

cleared by software (writing a logical one to the I/O bit location). For measuring frequency only,

the clearing of the ICF1 Flag is not required (if an interrupt handler is used).

14.6 Output Compare Units

The 16-bit comparator continuously compares TCNT1 with the Output Compare Register

(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set the Output

Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Com-

pare Flag generates an Output Compare interrupt. The OCF1x Flag is automatically cleared

when the interrupt is executed. Alternatively the OCF1x Flag can be cleared by software by writ-

ing a logical one to its I/O bit location. The Waveform Generator uses the match signal to

generate an output according to operating mode set by the Waveform Generation mode

(WGM13:0) bits and Compare Output mode (COM1x1:0) bits. The TOP and BOTTOM signals

are used by the Waveform Generator for handling the special cases of the extreme values in

some modes of operation (See “Modes of Operation” on page 117.)

A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e.,

counter resolution). In addition to the counter resolution, the TOP value defines the period time

for waveforms generated by the Waveform Generator.

Figure 14-4 shows a block diagram of the Output Compare unit. The small “n” in the register and

bit names indicates the device number (n = 1 for Timer/Counter 1), and the “x” indicates Output

Compare unit (A/B). The elements of the block diagram that are not directly a part of the Output

Compare unit are gray shaded.

Figure 14-4. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

(16-bit Comparator )

OCRnx Buffer (16-bit Register)

OCRnxH Buf. (8-bit)

OCnx

TEMP (8-bit)

DATA BUS

(8-bit)

OCRnxL Buf. (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit)

COMnx1:0WGMn3:0

OCRnx (16-bit Register)

OCRnxH (8-bit) OCRnxL (8-bit)

Waveform Generator

TOP

BOTTOM

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The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation

(PWM) modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the

double buffering is disabled. The double buffering synchronizes the update of the OCR1x Com-

pare Register to either TOP or BOTTOM of the counting sequence. The synchronization

prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the out-

put glitch-free.

The OCR1x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)

automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the high byte

temporary register (TEMP). However, it is a good practice to read the low byte first as when

accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP Reg-

ister since the compare of all 16 bits is done continuously. The high byte (OCR1xH) has to be

written first. When the high byte I/O location is written by the CPU, the TEMP Register will be

updated by the value written. Then when the low byte (OCR1xL) is written to the lower eight bits,

the high byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Compare

For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers”

on page 107.

14.6.1 Force Output Compare

In non-PWM Waveform Generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC1x) bit. Forcing compare match will not set the

OCF1x Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare

match had occurred (the COM11:0 bits settings define whether the OC1x pin is set, cleared or

toggled).

14.6.2 Compare Match Blocking by TCNT1 Write

All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer

clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the

same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.

14.6.3 Using the Output Compare Unit

Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT1 when using any of the Output Compare

channels, independent of whether the Timer/Counter is running or not. If the value written to

TCNT1 equals the OCR1x value, the compare match will be missed, resulting in incorrect wave-

form generation. Do not write the TCNT1 equal to TOP in PWM modes with variable TOP

values. The compare match for the TOP will be ignored and the counter will continue to 0xFFFF.

Similarly, do not write the TCNT1 value equal to BOTTOM when the counter is downcounting.

The setup of the OC1x should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC1x value is to use the Force Output Com-

pare (FOC1x) strobe bits in Normal mode. The OC1x Register keeps its value even when

changing between Waveform Generation modes.

Be aware that the COM1x1:0 bits are not double buffered together with the compare value.

Changing the COM1x1:0 bits will take effect immediately.

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14.7 Compare Match Output Unit

The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Generator uses

the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare match.

Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 14-5 shows a simplified

schematic of the logic affected by the COM1x1:0 bit setting. The I/O Registers, I/O bits, and I/O

pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers

(DDR and PORT) that are affected by the COM1x1:0 bits are shown. When referring to the

OC1x state, the reference is for the internal OC1x Register, not the OC1x pin. If a system reset

occur, the OC1x Register is reset to “0”.

Figure 14-5. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform

Generator if either of the COM1x1:0 bits are set. However, the OC1x pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ble on the pin. The port override function is generally independent of the Waveform Generation

mode, but there are some exceptions. Refer to Table 14-1, Table 14-2 and Table 14-3 for

details.

The design of the Output Compare pin logic allows initialization of the OC1x state before the out-

put is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of

operation. See “16-bit Timer/Counter Register Description” on page 127.

The COM1x1:0 bits have no effect on the Input Capture unit.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BUS

FOCnx

clk

I/O

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14.7.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM1x1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM1x1:0 = 0 tells the Waveform Generator that no action on the

OC1x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 14-1 on page 127. For fast PWM mode refer to Table 14-2 on

page 127, and for phase correct and phase and frequency correct PWM refer to Table 14-3 on

page 128.

A change of the COM1x1:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC1x strobe bits.

14.8 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is

defined by the combination of the Waveform Generation mode (WGM13:0) and Compare Output

mode (COM1x1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM1x1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM1x1:0 bits control whether the output should be set, cleared or toggle at a compare

match (See “Compare Match Output Unit” on page 116.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 125.

14.8.1 Normal Mode

The simplest mode of operation is the Normal mode (WGM13:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the

BOTTOM (0x0000). In normal operation the Timer/Counter Overflow Flag (TOV1) will be set in

the same timer clock cycle as the TCNT1 becomes zero. The TOV1 Flag in this case behaves

like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow

interrupt that automatically clears the TOV1 Flag, the timer resolution can be increased by soft-

ware. There are no special cases to consider in the Normal mode, a new counter value can be

written anytime.

The Input Capture unit is easy to use in Normal mode. However, observe that the maximum

interval between the external events must not exceed the resolution of the counter. If the interval

between events are too long, the timer overflow interrupt or the prescaler must be used to

extend the resolution for the capture unit.

The Output Compare units can be used to generate interrupts at some given time. Using the

Output Compare to generate waveforms in Normal mode is not recommended, since this will

occupy too much of the CPU time.

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14.8.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 Register

are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when

the counter value (TCNT1) matches either the OCR1A (WGM13:0 = 4) or the ICR1 (WGM13:0 =

12). The OCR1A or ICR1 define the top value for the counter, hence also its resolution. This

mode allows greater control of the compare match output frequency. It also simplifies the opera-

tion of counting external events.

The timing diagram for the CTC mode is shown in Figure 14-6. The counter value (TCNT1)

increases until a compare match occurs with either OCR1A or ICR1, and then counter (TCNT1)

is cleared.

Figure 14-6. CTC Mode, Timing Diagram

An interrupt can be generated at each time the counter value reaches the TOP value by either

using the OCF1A or ICF1 Flag according to the register used to define the TOP value. If the

interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. How-

ever, changing the TOP to a value close to BOTTOM when the counter is running with none or a

low prescaler value must be done with care since the CTC mode does not have the double buff-

ering feature. If the new value written to OCR1A or ICR1 is lower than the current value of

TCNT1, the counter will miss the compare match. The counter will then have to count to its max-

imum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.

In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode

using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.

For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction for

the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum fre-

quency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). The waveform frequency is

defined by the following equation:

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV1 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x0000.

TCNTn

OCnA

(Toggle)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 4

Period

2 3

(COMnA1:0 = 1)

fOCnA

fclk_I/O

2N1OCRnA+()⋅⋅

--------------------------------------------------------=

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14.8.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5, 6, 7, 14, or 15) provides a

high frequency PWM waveform generation option. The fast PWM differs from the other PWM

options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts

from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is set on

the compare match between TCNT1 and OCR1x, and cleared at TOP. In inverting Compare

Output mode output is cleared on compare match and set at TOP. Due to the single-slope oper-

ation, the operating frequency of the fast PWM mode can be twice as high as the phase correct

and phase and frequency correct PWM modes that use dual-slope operation. This high fre-

quency makes the fast PWM mode well suited for power regulation, rectification, and DAC

applications. High frequency allows physically small sized external components (coils, capaci-

tors), hence reduces total system cost.

The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICR1 or

OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the max-

imum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be

calculated by using the following equation:

In fast PWM mode the counter is incremented until the counter value matches either one of the

fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 =

14), or the value in OCR1A (WGM13:0 = 15). The counter is then cleared at the following timer

clock cycle. The timing diagram for the fast PWM mode is shown in Figure 14-7. The figure

shows fast PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the

timing diagram shown as a histogram for illustrating the single-slope operation. The diagram

includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1

slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will

be set when a compare match occurs.

Figure 14-7. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition

the OC1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A

or ICR1 is used for defining the TOP value. If one of the interrupts are enabled, the interrupt han-

dler routine can be used for updating the TOP and compare values.

RFPWM TOP 1+()log

2()log

-----------------------------------=

TCNTn

OCRnx/TOP Update and

TOVn Interrupt Flag Set and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 7

Period

2 3 4 5 6 8

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

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When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

Note that when using fixed TOP values the unused bits are masked to zero when any of the

OCR1x Registers are written.

The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP

value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low

value when the counter is running with none or a low prescaler value, there is a risk that the new

ICR1 value written is lower than the current value of TCNT1. The result will then be that the

counter will miss the compare match at the TOP value. The counter will then have to count to the

MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.

The OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location

to be written anytime. When the OCR1A I/O location is written the value written will be put into

the OCR1A Buffer Register. The OCR1A Compare Register will then be updated with the value

in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The update is done

at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.

Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed (by changing the TOP value), using the OCR1A

as TOP is clearly a better choice due to its double buffer feature.

In fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins.

Setting the COM1x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM1x1:0 to three (see Table on page 127). The actual OC1x

value will only be visible on the port pin if the data direction for the port pin is set as output

(DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at

the compare match between OCR1x and TCNT1, and clearing (or setting) the OC1x Register at

the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the out-

put will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP

will result in a constant high or low output (depending on the polarity of the output set by the

COM1x1:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). This applies only

if OCR1A is used to define the TOP value (WGM13:0 = 15). The waveform generated will have

a maximum frequency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is

similar to the OC1A toggle in CTC mode, except the double buffer feature of the Output Com-

pare unit is enabled in the fast PWM mode.

fOCnxPWM

fclk_I/O

N1TOP+()⋅

-------------------------------------=

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14.8.4 Phase Correct PWM Mode

The phase correct Pulse Width Modulation or phase correct PWM mode (WGM13:0 = 1, 2, 3,

10, or 11) provides a high resolution phase correct PWM waveform generation option. The

phase correct PWM mode is, like the phase and frequency correct PWM mode, based on a

dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then

from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is

cleared on the compare match between TCNT1 and OCR1x while upcounting, and set on the

compare match while downcounting. In inverting Output Compare mode, the operation is

inverted. The dual-slope operation has lower maximum operation frequency than single slope

operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes

are preferred for motor control applications.

The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined

by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to

0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolu-

tion in bits can be calculated by using the following equation:

In phase correct PWM mode the counter is incremented until the counter value matches either

one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 1, 2, or 3), the value in ICR1

(WGM13:0 = 10), or the value in OCR1A (WGM13:0 = 11). The counter has then reached the

TOP and changes the count direction. The TCNT1 value will be equal to TOP for one timer clock

cycle. The timing diagram for the phase correct PWM mode is shown on Figure 14-8. The figure

shows phase correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1

value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The

diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on

the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Inter-

rupt Flag will be set when a compare match occurs.

Figure 14-8. Phase Correct PWM Mode, Timing Diagram

RPCPWM TOP 1+()log

2()log

-----------------------------------=

OCRnx/TOP Update and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 2 3 4

TOVn Interrupt Flag Set

(Interrupt on Bottom)

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

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The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOTTOM. When

either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accord-

ingly at the same timer clock cycle as the OCR1x Registers are updated with the double buffer

value (at TOP). The Interrupt Flags can be used to generate an interrupt each time the counter

reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

Note that when using fixed TOP values, the unused bits are masked to zero when any of the

OCR1x Registers are written. As the third period shown in Figure 14-8 illustrates, changing the

TOP actively while the Timer/Counter is running in the phase correct mode can result in an

unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Reg-

ister. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP. This

implies that the length of the falling slope is determined by the previous TOP value, while the

length of the rising slope is determined by the new TOP value. When these two values differ the

two slopes of the period will differ in length. The difference in length gives the unsymmetrical

result on the output.

It is recommended to use the phase and frequency correct mode instead of the phase correct

mode when changing the TOP value while the Timer/Counter is running. When using a static

TOP value there are practically no differences between the two modes of operation.

In phase correct PWM mode, the compare units allow generation of PWM waveforms on the

OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and an inverted

PWM output can be generated by setting the COM1x1:0 to three (See Table on page 128). The

actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as

output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Regis-

ter at the compare match between OCR1x and TCNT1 when the counter increments, and

clearing (or setting) the OC1x Register at compare match between OCR1x and TCNT1 when

the counter decrements. The PWM frequency for the output when using phase correct PWM can

be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the

output will be continuously low and if set equal to TOP the output will be continuously high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If

OCR1A is used to define the TOP value (WGM13:0 = 11) and COM1A1:0 = 1, the OC1A output

will toggle with a 50% duty cycle.

fOCnxPCPWM

fclk_I/O

2NTOP⋅⋅

---------------------------------=

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14.8.5 Phase and Frequency Correct PWM Mode

The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM

mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency correct PWM wave-

form generation option. The phase and frequency correct PWM mode is, like the phase correct

PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM

(0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the

Output Compare (OC1x) is cleared on the compare match between TCNT1 and OCR1x while

upcounting, and set on the compare match while downcounting. In inverting Compare Output

mode, the operation is inverted. The dual-slope operation gives a lower maximum operation fre-

quency compared to the single-slope operation. However, due to the symmetric feature of the

dual-slope PWM modes, these modes are preferred for motor control applications.

The main difference between the phase correct, and the phase and frequency correct PWM

mode is the time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure

14-8 and Figure 14-9).

The PWM resolution for the phase and frequency correct PWM mode can be defined by either

ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and

the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can

be calculated using the following equation:

In phase and frequency correct PWM mode the counter is incremented until the counter value

matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The

counter has then reached the TOP and changes the count direction. The TCNT1 value will be

equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency

correct PWM mode is shown on Figure 14-9. The figure shows phase and frequency correct

PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the timing dia-

gram shown as a histogram for illustrating the dual-slope operation. The diagram includes

non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1 slopes

represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set

when a compare match occurs.

RPFCPWM TOP 1+()log

2()log

-----------------------------------=

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Figure 14-9. Phase and Frequency Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x

Registers are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1

is used for defining the TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP.

The Interrupt Flags can then be used to generate an interrupt each time the counter reaches the

TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

As Figure 14-9 shows the output generated is, in contrast to the phase correct mode, symmetri-

cal in all periods. Since the OCR1x Registers are updated at BOTTOM, the length of the rising

and the falling slopes will always be equal. This gives symmetrical output pulses and is therefore

frequency correct.

Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A as

TOP is clearly a better choice due to its double buffer feature.

In phase and frequency correct PWM mode, the compare units allow generation of PWM wave-

forms on the OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and

an inverted PWM output can be generated by setting the COM1x1:0 to three (See Table on

page 128). The actual OC1x value will only be visible on the port pin if the data direction for the

port pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing)

the OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-

ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and

TCNT1 when the counter decrements. The PWM frequency for the output when using phase

and frequency correct PWM can be calculated by the following equation:

OCRnx/TOP Updateand

TOVn Interrupt Flag Set

(Interrupt on Bottom)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 2 3 4

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

fOCnxPFCPWM

fclk_I/O

2NTOP⋅⋅

---------------------------------=

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The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the

output will be continuously low and if set equal to TOP the output will be set to high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If

OCR1A is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A output

will toggle with a 50% duty cycle.

14.9 Timer/Counter Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkT1) is therefore shown as a

clock enable signal in the following figures. The figures include information on when Interrupt

Flags are set, and when the OCR1x Register is updated with the OCR1x buffer value (only for

modes utilizing double buffering). Figure 14-10 shows a timing diagram for the setting of OCF1x.

Figure 14-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling

Figure 14-11 shows the same timing data, but with the prescaler enabled.

Figure 14-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)

clk

(clkI/O/1)

OCFnx

clk

I/O

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clk

I/O

clk

(clk

I/O

/8)

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Figure 14-12 shows the count sequence close to TOP in various modes. When using phase and

frequency correct PWM mode the OCR1x Register is updated at BOTTOM. The timing diagrams

will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on.

The same renaming applies for modes that set the TOV1 Flag at BOTTOM.

Figure 14-12. Timer/Counter Timing Diagram, no Prescaling

Figure 14-13 shows the same timing data, but with the prescaler enabled.

Figure 14-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

TOVn (FPWM)

and ICFn (if used

as TOP)

OCRnx

(Update at TOP)

TCNTn

(CTC and FPWM)

TCNTn

(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2

Old OCRnx Value New OCRnx Value

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

(clk

I/O

/1)

clkI/O

TOVn (FPWM)

and ICFn (if used

as TOP)

OCRnx

(Update at TOP)

TCNTn

(CTC and FPWM)

TCNTn

(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2

Old OCRnx Value New OCRnx Value

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

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14.10 16-bit Timer/Counter Register Description

14.10.1 Timer/Counter1 Control Register A – TCCR1A

• Bit 7:6 – COM1A1:0: Compare Output Mode for Channel A

• Bit 5:4 – COM1B1:0: Compare Output Mode for Channel B

The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B respec-

tively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A output

overrides the normal port functionality of the I/O pin it is connected to. If one or both of the

COM1B1:0 bit are written to one, the OC1B output overrides the normal port functionality of the

I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-

ing to the OC1A or OC1B pin must be set in order to enable the output driver.

When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is depen-

dent of the WGM13:0 bits setting. Table 14-1 shows the COM1x1:0 bit functionality when the

WGM13:0 bits are set to a Normal or a CTC mode (non-PWM).

Table 14-2 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast

PWM mode.

Note: 1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In

this case the compare match is ignored, but the set or clear is done at TOP. See “Fast PWM

Mode” on page 119. for more details.

Bit 7 6 5 4 3210

COM1A1 COM1A0 COM1B1 COM1B0 – – WGM11 WGM10 TCCR1A

Read/Write R/W R/W R/W R/W R R R/W R/W

Initial Value0 0 0 00000

Table 14-1. Compare Output Mode, non-PWM

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B disconnected.

0 1 Toggle OC1A/OC1B on Compare Match.

Clear OC1A/OC1B on Compare Match (Set output to

low level).

Set OC1A/OC1B on Compare Match (Set output to

high level).

Table 14-2. Compare Output Mode, Fast PWM(1)

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B disconnected.

WGM13:0 = 14 or 15: Toggle OC1A on Compare

Match, OC1B disconnected (normal port operation).

For all other WGM1 settings, normal port operation,

OC1A/OC1B disconnected.

Clear OC1A/OC1B on Compare Match, set

OC1A/OC1B at TOP

Set OC1A/OC1B on Compare Match, clear

OC1A/OC1B at TOP

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Table 14-3 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase

correct or the phase and frequency correct, PWM mode.

Note: 1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See

“Phase Correct PWM Mode” on page 121. for more details.

• Bit 1:0 – WGM11:0: Waveform Generation Mode

Combined with the WGM13:2 bits found in the TCCR1B Register, these bits control the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 14-4. Modes of operation supported by the Timer/Counter

unit are: Normal mode (counter), Clear Timer on Compare match (CTC) mode, and three types

of Pulse Width Modulation (PWM) modes. (See “Modes of Operation” on page 117.).

Table 14-3. Compare Output Mode, Phase Correct and Phase and Frequency Correct

PWM(1)

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B disconnected.

WGM13:0 = 8, 9, 10 or 11: Toggle OC1A on Compare

Match, OC1B disconnected (normal port operation).

For all other WGM1 settings, normal port operation,

OC1A/OC1B disconnected.

Clear OC1A/OC1B on Compare Match when

up-counting. Set OC1A/OC1B on Compare Match

when downcounting.

Set OC1A/OC1B on Compare Match when

up-counting. Clear OC1A/OC1B on Compare Match

when downcounting.

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Note: 1. The CTC1 and PWM11:0 bit definition names are obsolete. Use the WGM12:0 definitions. However, the functionality and

location of these bits are compatible with previous versions of the timer.

Table 14-4. Waveform Generation Mode Bit Description(1)

WGM1

(CTC1

)

WGM1

(PWM

11)

WGM1

(PWM

10)

Timer/Counter Mode

of Operation TOP

Update

OCR1x

TOV1

Flag

Set on

00000Normal 0xFFF

Immedi

ate MAX

10001

PWM, Phase Correct,

8-bit

0x00F

FTOP BOTTO

20010

PWM, Phase Correct,

9-bit

0x01F

FTOP BOTTO

30011

PWM, Phase Correct,

10-bit

0x03F

FTOP BOTTO

40100CTC OCR1

Immedi

ate MAX

5 0 1 0 1 Fast PWM, 8-bit 0x00F

FTOP TOP

6 0 1 1 0 Fast PWM, 9-bit 0x01F

FTOP TOP

7 0 1 1 1 Fast PWM, 10-bit 0x03F

FTOP TOP

81000

PWM, Phase and

Frequency Correct ICR1 BOTTO

BOTTO

91001

PWM, Phase and

Frequency Correct

OCR1

BOTTO

10 1 0 1 0 PWM, Phase Correct ICR1 TOP BOTTO

11 1 0 1 1 PWM, Phase Correct OCR1

ATOP BOTTO

121100CTC ICR1

Immedi

ate MAX

131101(Reserved) –– –

14 1 1 1 0 Fast PWM ICR1 TOP TOP

151111Fast PWM OCR1

ATOP TOP

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14.10.2 Timer/Counter1 Control Register B – TCCR1B

• Bit 7 – ICNC1: Input Capture Noise Canceler

Setting this bit (to one) activates the Input Capture Noise Canceler. When the noise canceler is

activated, the input from the Input Capture pin (ICP1) is filtered. The filter function requires four

successive equal valued samples of the ICP1 pin for changing its output. The Input Capture is

therefore delayed by four Oscillator cycles when the noise canceler is enabled.

• Bit 6 – ICES1: Input Capture Edge Select

This bit selects which edge on the Input Capture pin (ICP1) that is used to trigger a capture

event. When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and

when the ICES1 bit is written to one, a rising (positive) edge will trigger the capture.

When a capture is triggered according to the ICES1 setting, the counter value is copied into the

Input Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this

can be used to cause an Input Capture Interrupt, if this interrupt is enabled.

When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the

TCCR1A and the TCCR1B Register), the ICP1 is disconnected and consequently the Input Cap-

ture function is disabled.

• Bit 5 – Reserved Bit

This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be

written to zero when TCCR1B is written.

• Bit 4:3 – WGM13:2: Waveform Generation Mode

See TCCR1A Register description.

• Bit 2:0 – CS12:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter, see Figure

14-10 and Figure 14-11.

Bit 7654 3210

ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 TCCR1B

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 14-5. Clock Select Bit Description

CS12 CS11 CS10 Description

0 0 0 No clock source (Timer/Counter stopped).

001clk

I/O/1 (No prescaling)

010clk

I/O/8 (From prescaler)

011clk

I/O/64 (From prescaler)

100clk

I/O/256 (From prescaler)

101clk

I/O/1024 (From prescaler)

1 1 0 External clock source on T1 pin. Clock on falling edge.

1 1 1 External clock source on T1 pin. Clock on rising edge.

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If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

14.10.3 Timer/Counter1 Control Register C – TCCR1C

• Bit 7 – FOC1A: Force Output Compare for Channel A

• Bit 6 – FOC1B: Force Output Compare for Channel B

The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.

However, for ensuring compatibility with future devices, these bits must be set to zero when

TCCR1A is written when operating in a PWM mode. When writing a logical one to the

FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.

The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the

FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the

COM1x1:0 bits that determine the effect of the forced compare.

A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer

on Compare match (CTC) mode using OCR1A as TOP.

The FOC1A/FOC1B bits are always read as zero.

14.10.4 Timer/Counter1 – TCNT1H and TCNT1L

The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct

access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To

ensure that both the high and low bytes are read and written simultaneously when the CPU

accesses these registers, the access is performed using an 8-bit temporary High Byte Register

(TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit

Registers” on page 107.

Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a com-

pare match between TCNT1 and one of the OCR1x Registers.

Writing to the TCNT1 Register blocks (removes) the compare match on the following timer clock

for all compare units.

Bit 7654 3210

FOC1A FOC1B – – – – – – TCCR1C

Read/Write R/W R/W R R R R R R

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

TCNT1[15:8] TCNT1H

TCNT1[7:0] TCNT1L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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14.10.5 Output Compare Register 1 A – OCR1AH and OCR1AL

14.10.6 Output Compare Register 1 B – OCR1BH and OCR1BL

The Output Compare Registers contain a 16-bit value that is continuously compared with the

counter value (TCNT1). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC1x pin.

The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are

written simultaneously when the CPU writes to these registers, the access is performed using an

8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other

16-bit registers. See “Accessing 16-bit Registers” on page 107.

14.10.7 Input Capture Register 1 – ICR1H and ICR1L

The Input Capture is updated with the counter (TCNT1) value each time an event occurs on the

ICP1 pin (or optionally on the Analog Comparator output for Timer/Counter1). The Input Capture

can be used for defining the counter TOP value.

The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read

simultaneously when the CPU accesses these registers, the access is performed using an 8-bit

temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit

registers. See “Accessing 16-bit Registers” on page 107.

14.10.8 Timer/Counter1 Interrupt Mask Register – TIMSK1

• Bit 7, 6 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 5 – ICIE1: Timer/Counter1, Input Capture Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Input Capture interrupt is enabled. The corresponding Interrupt

Vector (see “Interrupts” on page 52) is executed when the ICF1 Flag, located in TIFR1, is set.

Bit 76543210

OCR1A[15:8] OCR1AH

OCR1A[7:0] OCR1AL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR1B[15:8] OCR1BH

OCR1B[7:0] OCR1BL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

ICR1[15:8] ICR1H

ICR1[7:0] ICR1L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

– – ICIE1 – – OCIE1B OCIE1A TOIE1 TIMSK1

Read/Write R R R/W R R R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 4, 3 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 2 – OCIE1B: Timer/Counter1, Output Compare B Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Output Compare B Match interrupt is enabled. The corresponding

Interrupt Vector (see “Interrupts” on page 52) is executed when the OCF1B Flag, located in

TIFR1, is set.

• Bit 1 – OCIE1A: Timer/Counter1, Output Compare A Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Output Compare A Match interrupt is enabled. The corresponding

Interrupt Vector (see “Interrupts” on page 52) is executed when the OCF1A Flag, located in

TIFR1, is set.

• Bit 0 – TOIE1: Timer/Counter1, Overflow Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Overflow interrupt is enabled. The corresponding Interrupt Vector

(See “Watchdog Timer” on page 47.) is executed when the TOV1 Flag, located in TIFR1, is set.

14.10.9 Timer/Counter1 Interrupt Flag Register – TIFR1

• Bit 7, 6 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 5 – ICF1: Timer/Counter1, Input Capture Flag

This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register

(ICR1) is set by the WGM13:0 to be used as the TOP value, the ICF1 Flag is set when the coun-

ter reaches the TOP value.

ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,

ICF1 can be cleared by writing a logic one to its bit location.

• Bit 4, 3 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 2 – OCF1B: Timer/Counter1, Output Compare B Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register B (OCR1B).

Note that a Forced Output Compare (FOC1B) strobe will not set the OCF1B Flag.

OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is exe-

cuted. Alternatively, OCF1B can be cleared by writing a logic one to its bit location.

Bit 76543210

– – ICF1 – – OCF1B OCF1A TOV1 TIFR1

Read/Write R R R/W R R R/W R/W R/W

Initial Value00000000

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• Bit 1 – OCF1A: Timer/Counter1, Output Compare A Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register A (OCR1A).

Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.

OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is exe-

cuted. Alternatively, OCF1A can be cleared by writing a logic one to its bit location.

• Bit 0 – TOV1: Timer/Counter1, Overflow Flag

The setting of this flag is dependent of the WGM13:0 bits setting. In Normal and CTC modes,

the TOV1 Flag is set when the timer overflows. Refer to Table 14-4 on page 129 for the TOV1

Flag behavior when using another WGM13:0 bit setting.

TOV1 is automatically cleared when the Timer/Counter1 Overflow Interrupt Vector is executed.

Alternatively, TOV1 can be cleared by writing a logic one to its bit location.

15. 8-bit Timer/Counter2 with PWM and Asynchronous Operation

Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. The main

features are:

•Single Channel Counter

•Clear Timer on Compare Match (Auto Reload)

•Glitch-free, Phase Correct Pulse Width Modulator (PWM)

•Frequency Generator

•10-bit Clock Prescaler

•Overflow and Compare Match Interrupt Sources (TOV2, OCF2A and OCF2B)

•Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock

15.1 Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 15-1. For the actual

placement of I/O pins, refer to “Pinout ATmega48/88/168” on page 2. CPU accessible I/O Regis-

ters, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit

locations are listed in the “8-bit Timer/Counter Register Description” on page 146.

The PRTIM2 bit in “Power Reduction Register - PRR” on page 39 must be written to zero to

enable Timer/Counter2 module.

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Figure 15-1. 8-bit Timer/Counter Block Diagram

15.1.1 Registers

The Timer/Counter (TCNT2) and Output Compare Register (OCR2A and OCR2B) are 8-bit reg-

isters. Interrupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag

(TIMSK2). TIFR2 and TIMSK2 are not shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from

the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by

the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock

source he Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-

tive when no clock source is selected. The output from the Clock Select logic is referred to as the

timer clock (clkT2).

The double buffered Output Compare Register (OCR2A and OCR2B) are compared with the

Timer/Counter value at all times. The result of the compare can be used by the Waveform Gen-

erator to generate a PWM or variable frequency output on the Output Compare pins (OC2A and

OC2B). See “Output Compare Unit” on page 137. for details. The compare match event will also

set the Compare Flag (OCF2A or OCF2B) which can be used to generate an Output Compare

interrupt request.

Timer/Counter

DATA BUS

OCRnA

OCRnB

TCNTn

Waveform

Generation

Waveform

Generation

OCnA

OCnB

Fixed

TOP

Value

Control Logic

TOP BOTTOM

Count

Clear

Direction

TOVn

(Int.Req.)

OCnA

(Int.Req.)

OCnB

(Int.Req.)

TCCRnA TCCRnB

clk

Prescaler

T/C

Oscillator

clk

I/O

TOSC1

TOSC2

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15.1.2 Definitions

Many register and bit references in this document are written in general form. A lower case “n”

replaces the Timer/Counter number, in this case 2. However, when using the register or bit

defines in a program, the precise form must be used, i.e., TCNT2 for accessing Timer/Counter2

counter value and so on.

The definitions in the following table are also used extensively throughout the section.

15.2 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal synchronous or an external asynchronous

clock source. The clock source clkT2 is by default equal to the MCU clock, clkI/O. When the AS2

bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter

Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see “Asyn-

chronous Status Register – ASSR” on page 154. For details on clock sources and prescaler, see

“Timer/Counter Prescaler” on page 155.

15.3 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

15-2 shows a block diagram of the counter and its surrounding environment.

Figure 15-2. Counter Unit Block Diagram

Signal description (internal signals):

count Increment or decrement TCNT2 by 1.

direction Selects between increment and decrement.

clear Clear TCNT2 (set all bits to zero).

clkTn Timer/Counter clock, referred to as clkT2 in the following.

top Signalizes that TCNT2 has reached maximum value.

bottom Signalizes that TCNT2 has reached minimum value (zero).

BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).

MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

TOP The counter reaches the TOP when it becomes equal to the highest value in the

count sequence. The TOP value can be assigned to be the fixed value 0xFF

(MAX) or the value stored in the OCR2A Register. The assignment is depen-

dent on the mode of operation.

DATA BUS

TCNTn Control Logic

count

TOVn

(Int.Req.)

topbottom

direction

clear

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

clk Tn

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Depending on the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clkT2). clkT2 can be generated from an external or internal clock source,

selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the

timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of

whether clkT2 is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in

the Timer/Counter Control Register (TCCR2A) and the WGM22 located in the Timer/Counter

Control Register B (TCCR2B). There are close connections between how the counter behaves

(counts) and how waveforms are generated on the Output Compare outputs OC2A and OC2B.

For more details about advanced counting sequences and waveform generation, see “Modes of

Operation” on page 140.

The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by

the WGM22:0 bits. TOV2 can be used for generating a CPU interrupt.

15.4 Output Compare Unit

The 8-bit comparator continuously compares TCNT2 with the Output Compare Register

(OCR2A and OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator signals a

match. A match will set the Output Compare Flag (OCF2A or OCF2B) at the next timer clock

cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output

Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is exe-

cuted. Alternatively, the Output Compare Flag can be cleared by software by writing a logical

one to its I/O bit location. The Waveform Generator uses the match signal to generate an output

according to operating mode set by the WGM22:0 bits and Compare Output mode (COM2x1:0)

bits. The max and bottom signals are used by the Waveform Generator for handling the special

cases of the extreme values in some modes of operation (“Modes of Operation” on page 140).

Figure 15-3 shows a block diagram of the Output Compare unit.

Figure 15-3. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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The OCR2x Register is double buffered when using any of the Pulse Width Modulation (PWM)

modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the double

buffering is disabled. The double buffering synchronizes the update of the OCR2x Compare

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR2x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR2x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR2x directly.

15.4.1 Force Output Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC2x) bit. Forcing compare match will not set the

OCF2x Flag or reload/clear the timer, but the OC2x pin will be updated as if a real compare

match had occurred (the COM2x1:0 bits settings define whether the OC2x pin is set, cleared or

toggled).

15.4.2 Compare Match Blocking by TCNT2 Write

All CPU write operations to the TCNT2 Register will block any compare match that occurs in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR2x to be initial-

ized to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is

enabled.

15.4.3 Using the Output Compare Unit

Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT2 when using the Output Compare channel,

independently of whether the Timer/Counter is running or not. If the value written to TCNT2

equals the OCR2x value, the compare match will be missed, resulting in incorrect waveform

generation. Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is

downcounting.

The setup of the OC2x should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC2x value is to use the Force Output Com-

pare (FOC2x) strobe bit in Normal mode. The OC2x Register keeps its value even when

changing between Waveform Generation modes.

Be aware that the COM2x1:0 bits are not double buffered together with the compare value.

Changing the COM2x1:0 bits will take effect immediately.

15.5 Compare Match Output Unit

The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Generator uses

the COM2x1:0 bits for defining the Output Compare (OC2x) state at the next compare match.

Also, the COM2x1:0 bits control the OC2x pin output source. Figure 15-4 shows a simplified

schematic of the logic affected by the COM2x1:0 bit setting. The I/O Registers, I/O bits, and I/O

pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers

(DDR and PORT) that are affected by the COM2x1:0 bits are shown. When referring to the

OC2x state, the reference is for the internal OC2x Register, not the OC2x pin.

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Figure 15-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2x) from the Waveform

Generator if either of the COM2x1:0 bits are set. However, the OC2x pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ble on the pin. The port override function is independent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC2x state before the out-

put is enabled. Note that some COM2x1:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 146.

15.5.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM2x1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM2x1:0 = 0 tells the Waveform Generator that no action on the

OC2x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 15-4 on page 147. For fast PWM mode, refer to Table 15-5 on

page 147, and for phase correct PWM refer to Table 15-6 on page 148.

A change of the COM2x1:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC2x strobe bits.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BUS

FOCnx

clk

I/O

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15.6 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is

defined by the combination of the Waveform Generation mode (WGM22:0) and Compare Output

mode (COM2x1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM2x1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM2x1:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 138.).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 144.

15.6.1 Normal Mode

The simplest mode of operation is the Normal mode (WGM22:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same

timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt

that automatically clears the TOV2 Flag, the timer resolution can be increased by software.

There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Output Compare unit can be used to generate interrupts at some given time. Using the Out-

put Compare to generate waveforms in Normal mode is not recommended, since this will

occupy too much of the CPU time.

15.6.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM22:0 = 2), the OCR2A Register is used to

manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter

value (TCNT2) matches the OCR2A. The OCR2A defines the top value for the counter, hence

also its resolution. This mode allows greater control of the compare match output frequency. It

also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 15-5. The counter value (TCNT2)

increases until a compare match occurs between TCNT2 and OCR2A, and then counter

(TCNT2) is cleared.

Figure 15-5. CTC Mode, Timing Diagram

TCNTn

OCnx

(Toggle)

OCnx Interrupt Flag Set

1 4

Period 2 3

(COMnx1:0 = 1)

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An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF2A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating

the TOP value. However, changing TOP to a value close to BOTTOM when the counter is run-

ning with none or a low prescaler value must be done with care since the CTC mode does not

have the double buffering feature. If the new value written to OCR2A is lower than the current

value of TCNT2, the counter will miss the compare match. The counter will then have to count to

its maximum value (0xFF) and wrap around starting at 0x00 before the compare match can

occur.

For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM2A1:0 = 1). The OC2A value will not be visible on the port pin unless the data direction for

the pin is set to output. The waveform generated will have a maximum frequency of fOC2A =

fclk_I/O/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following

equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x00.

15.6.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM22:0 = 3 or 7) provides a high fre-

quency PWM waveform generation option. The fast PWM differs from the other PWM option by

its single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOT-

TOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR2A when MGM2:0 = 7. In

non-inverting Compare Output mode, the Output Compare (OC2x) is cleared on the compare

match between TCNT2 and OCR2x, and set at BOTTOM. In inverting Compare Output mode,

the output is set on compare match and cleared at BOTTOM. Due to the single-slope operation,

the operating frequency of the fast PWM mode can be twice as high as the phase correct PWM

mode that uses dual-slope operation. This high frequency makes the fast PWM mode well suited

for power regulation, rectification, and DAC applications. High frequency allows physically small

sized external components (coils, capacitors), and therefore reduces total system cost.

In fast PWM mode, the counter is incremented until the counter value matches the TOP value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 15-6. The TCNT2 value is in the timing diagram shown as a his-

togram for illustrating the single-slope operation. The diagram includes non-inverted and

inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare

matches between OCR2x and TCNT2.

fOCnx

fclk_I/O

2N1OCRnx+()⋅⋅

-------------------------------------------------------=

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Figure 15-6. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches TOP. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin.

Setting the COM2x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM2x1:0 to three. TOP is defined as 0xFF when WGM2:0 = 3,

and OCR2A when MGM2:0 = 7. (See Table 15-2 on page 146). The actual OC2x value will only

be visible on the port pin if the data direction for the port pin is set as output. The PWM wave-

form is generated by setting (or clearing) the OC2x Register at the compare match between

OCR2x and TCNT2, and clearing (or setting) the OC2x Register at the timer clock cycle the

counter is cleared (changes from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2A Register represent special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will

be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result

in a constantly high or low output (depending on the polarity of the output set by the COM2A1:0

bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC2x to toggle its logical level on each compare match (COM2x1:0 = 1). The waveform

generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2A is set to zero. This fea-

ture is similar to the OC2A toggle in CTC mode, except the double buffer feature of the Output

Compare unit is enabled in the fast PWM mode.

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

2 3

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

4 5 6 7

fOCnxPWM

fclk_I/O

N256⋅

---------------------=

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15.6.4 Phase Correct PWM Mode

The phase correct PWM mode (WGM22:0 = 1 or 5) provides a high resolution phase correct

PWM waveform generation option. The phase correct PWM mode is based on a dual-slope

operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOT-

TOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR2A when MGM2:0 = 7. In

non-inverting Compare Output mode, the Output Compare (OC2x) is cleared on the compare

match between TCNT2 and OCR2x while upcounting, and set on the compare match while

downcounting. In inverting Output Compare mode, the operation is inverted. The dual-slope

operation has lower maximum operation frequency than single slope operation. However, due to

the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor con-

trol applications.

In phase correct PWM mode the counter is incremented until the counter value matches TOP.

When the counter reaches TOP, it changes the count direction. The TCNT2 value will be equal

to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown

on Figure 15-7. The TCNT2 value is in the timing diagram shown as a histogram for illustrating

the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The

small horizontal line marks on the TCNT2 slopes represent compare matches between OCR2x

and TCNT2.

Figure 15-7. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The

Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM

value.

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

1 2 3

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

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In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the

OC2x pin. Setting the COM2x1:0 bits to two will produce a non-inverted PWM. An inverted PWM

output can be generated by setting the COM2x1:0 to three. TOP is defined as 0xFF when

WGM2:0 = 3, and OCR2A when MGM2:0 = 7 (See Table 15-3 on page 147). The actual OC2x

value will only be visible on the port pin if the data direction for the port pin is set as output. The

PWM waveform is generated by clearing (or setting) the OC2x Register at the compare match

between OCR2x and TCNT2 when the counter increments, and setting (or clearing) the OC2x

PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2A Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the

output will be continuously low and if set equal to MAX the output will be continuously high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.

At the very start of period 2 in Figure 15-7 OCnx has a transition from high to low even though

there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-

TOM. There are two cases that give a transition without Compare Match.

• OCR2A changes its value from MAX, like in Figure 15-7. When the OCR2A value is MAX the

OCn pin value is the same as the result of a down-counting compare match. To ensure

symmetry around BOTTOM the OCn value at MAX must correspond to the result of an

up-counting Compare Match.

• The timer starts counting from a value higher than the one in OCR2A, and for that reason

misses the Compare Match and hence the OCn change that would have happened on the

way up.

15.7 Timer/Counter Timing Diagrams

The following figures show the Timer/Counter in synchronous mode, and the timer clock (clkT2)

is therefore shown as a clock enable signal. In asynchronous mode, clkI/O should be replaced by

the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are

set. Figure 15-8 contains timing data for basic Timer/Counter operation. The figure shows the

count sequence close to the MAX value in all modes other than phase correct PWM mode.

Figure 15-8. Timer/Counter Timing Diagram, no Prescaling

Figure 15-9 shows the same timing data, but with the prescaler enabled.

fOCnxPCPWM

fclk_I/O

N510⋅

---------------------=

clk

(clk

I/O

/1)

TOVn

clk

I/O

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

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Figure 15-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

Figure 15-10 shows the setting of OCF2A in all modes except CTC mode.

Figure 15-10. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (fclk_I/O/8)

Figure 15-11 shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.

Figure 15-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Pres-

caler (fclk_I/O/8)

TOVn

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

clkI/O

clkTn

(clk

I/O

/8)

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clk

I/O

clk

(clk

I/O

/8)

OCFnx

OCRnx

TCNTn

(CTC)

TOP

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

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15.8 8-bit Timer/Counter Register Description

15.8.1 Timer/Counter Control Register A – TCCR2A

• Bits 7:6 – COM2A1:0: Compare Match Output A Mode

These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A1:0

bits are set, the OC2A output overrides the normal port functionality of the I/O pin it is connected

to. However, note that the Data Direction Register (DDR) bit corresponding to the OC2A pin

must be set in order to enable the output driver.

When OC2A is connected to the pin, the function of the COM2A1:0 bits depends on the

WGM22:0 bit setting. Table 15-1 shows the COM2A1:0 bit functionality when the WGM22:0 bits

are set to a normal or CTC mode (non-PWM).

Table 15-2 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set to fast PWM

mode.

Note: 1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 141

for more details.

Bit 7 6 5 4 3 210

COM2A1 COM2A0 COM2B1 COM2B0 – – WGM21 WGM20 TCCR2A

Read/Write R/W R/W R/W R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 15-1. Compare Output Mode, non-PWM Mode

COM2A1 COM2A0 Description

0 0 Normal port operation, OC0A disconnected.

0 1 Toggle OC2A on Compare Match

1 0 Clear OC2A on Compare Match

1 1 Set OC2A on Compare Match

Table 15-2. Compare Output Mode, Fast PWM Mode(1)

COM2A1 COM2A0 Description

0 0 Normal port operation, OC2A disconnected.

WGM22 = 0: Normal Port Operation, OC0A Disconnected.

WGM22 = 1: Toggle OC2A on Compare Match.

1 0 Clear OC2A on Compare Match, set OC2A at TOP

1 1 Set OC2A on Compare Match, clear OC2A at TOP

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Table 15-3 shows the COM2A1:0 bit functionality when the WGM22:0 bits are set to phase cor-
rect PWM mode.
Note: 1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on 
page 143 for more details.
• Bits 5:4 – COM2B1:0: Compare Match Output B Mode
These bits control the Output Compare pin (OC2B) behavior. If one or both of the COM2B1:0
bits are set, the OC2B output overrides the normal port functionality of the I/O pin it is connected
to. However, note that the Data Direction Register (DDR) bit corresponding to the OC2B pin
must be set in order to enable the output driver.
When OC2B is connected to the pin, the function of the COM2B1:0 bits depends on the
WGM22:0 bit setting. Table 15-4 shows the COM2B1:0 bit functionality when the WGM22:0 bits
are set to a normal or CTC mode (non-PWM).
Table 15-5 shows the COM2B1:0 bit functionality when the WGM22:0 bits are set to fast PWM
mode.
Note: 1. A special case occurs when OCR2B equals TOP and COM2B1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on 
page 143 for more details.
Table 15-3. Compare Output Mode, Phase Correct PWM Mode(1)
COM2A1 COM2A0 Description
0 0 Normal port operation, OC2A disconnected.
01
WGM22 = 0: Normal Port Operation, OC2A Disconnected.
WGM22 = 1: Toggle OC2A on Compare Match.
10
Clear OC2A on Compare Match when up-counting. Set OC2A on 
Compare Match when down-counting.
11
Set OC2A on Compare Match when up-counting. Clear OC2A on 
Compare Match when down-counting.
Table 15-4. Compare Output Mode, non-PWM Mode
COM2B1 COM2B0 Description
0 0 Normal port operation, OC2B disconnected.
0 1 Toggle OC2B on Compare Match
1 0 Clear OC2B on Compare Match
1 1 Set OC2B on Compare Match
Table 15-5. Compare Output Mode, Fast PWM Mode(1)
COM2B1 COM2B0 Description
0 0 Normal port operation, OC2B disconnected.
01Reserved
1 0 Clear OC2B on Compare Match, set OC2B at TOP
1 1 Set OC2B on Compare Match, clear OC2B at TOP

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Table 15-6 shows the COM2B1:0 bit functionality when the WGM22:0 bits are set to phase cor-

rect PWM mode.

Note: 1. A special case occurs when OCR2B equals TOP and COM2B1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on

page 143 for more details.

• Bits 3, 2 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bits 1:0 – WGM21:0: Waveform Generation Mode

Combined with the WGM22 bit found in the TCCR2B Register, these bits control the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 15-7. Modes of operation supported by the Timer/Counter

unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and two types of

Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page 140).

Notes: 1. MAX= 0xFF

2. BOTTOM= 0x00

Table 15-6. Compare Output Mode, Phase Correct PWM Mode(1)

COM2B1 COM2B0 Description

0 0 Normal port operation, OC2B disconnected.

01Reserved

Clear OC2B on Compare Match when up-counting. Set OC2B on

Compare Match when down-counting.

Set OC2B on Compare Match when up-counting. Clear OC2B on

Compare Match when down-counting.

Table 15-7. Waveform Generation Mode Bit Description

Mode WGM2 WGM1 WGM0

Timer/Counter

Mode of

Operation TOP

Update of

OCRx at

TOV Flag

Set on(1)(2)

0 0 0 0 Normal 0xFF Immediate MAX

10 0 1

PWM, Phase

Correct 0xFF TOP BOTTOM

2 0 1 0 CTC OCRA Immediate MAX

3 0 1 1 Fast PWM 0xFF TOP MAX

4 1 0 0 Reserved – – –

51 0 1

PWM, Phase

Correct OCRA TOP BOTTOM

6 1 1 0 Reserved – – –

7 1 1 1 Fast PWM OCRA TOP TOP

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15.8.2 Timer/Counter Control Register B – TCCR2B

• Bit 7 – FOC2A: Force Output Compare A

The FOC2A bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when

TCCR2B is written when operating in PWM mode. When writing a logical one to the FOC2A bit,

an immediate Compare Match is forced on the Waveform Generation unit. The OC2A output is

changed according to its COM2A1:0 bits setting. Note that the FOC2A bit is implemented as a

strobe. Therefore it is the value present in the COM2A1:0 bits that determines the effect of the

forced compare.

A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR2A as TOP.

The FOC2A bit is always read as zero.

• Bit 6 – FOC2B: Force Output Compare B

The FOC2B bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when

TCCR2B is written when operating in PWM mode. When writing a logical one to the FOC2B bit,

an immediate Compare Match is forced on the Waveform Generation unit. The OC2B output is

changed according to its COM2B1:0 bits setting. Note that the FOC2B bit is implemented as a

strobe. Therefore it is the value present in the COM2B1:0 bits that determines the effect of the

forced compare.

A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR2B as TOP.

The FOC2B bit is always read as zero.

• Bits 5:4 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bit 3 – WGM22: Waveform Generation Mode

See the description in the “Timer/Counter Control Register A – TCCR2A” on page 146.

• Bit 2:0 – CS22:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter, see Table

15-8.

Bit 7 6 5 4 3 210

FOC2A FOC2B – – WGM22 CS22 CS21 CS20 TCCR2B

Read/Write W W R R R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

15.8.3 Timer/Counter Register – TCNT2

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the Compare

Match on the following timer clock. Modifying the counter (TCNT2) while the counter is running,

introduces a risk of missing a Compare Match between TCNT2 and the OCR2x Registers.

15.8.4 Output Compare Register A – OCR2A

The Output Compare Register A contains an 8-bit value that is continuously compared with the

counter value (TCNT2). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC2A pin.

15.8.5 Output Compare Register B – OCR2B

The Output Compare Register B contains an 8-bit value that is continuously compared with the

counter value (TCNT2). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC2B pin.

Table 15-8. Clock Select Bit Description

CS22 CS21 CS20 Description

0 0 0 No clock source (Timer/Counter stopped).

001clk

T2S/(No prescaling)

010clk

T2S/8 (From prescaler)

011clk

T2S/32 (From prescaler)

100clk

T2S/64 (From prescaler)

101clk

T2S/128 (From prescaler)

110clk

T2S/256 (From prescaler)

111clk

T2S/1024 (From prescaler)

Bit 76543210

TCNT2[7:0] TCNT2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR2A[7:0] OCR2A

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR2B[7:0] OCR2B

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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15.8.6 Timer/Counter2 Interrupt Mask Register – TIMSK2

• Bit 2 – OCIE2B: Timer/Counter2 Output Compare Match B Interrupt Enable

When the OCIE2B bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Compare Match B interrupt is enabled. The corresponding interrupt is executed

if a compare match in Timer/Counter2 occurs, i.e., when the OCF2B bit is set in the Timer/Coun-

ter 2 Interrupt Flag Register – TIFR2.

• Bit 1 – OCIE2A: Timer/Counter2 Output Compare Match A Interrupt Enable

When the OCIE2A bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Compare Match A interrupt is enabled. The corresponding interrupt is executed

if a compare match in Timer/Counter2 occurs, i.e., when the OCF2A bit is set in the Timer/Coun-

ter 2 Interrupt Flag Register – TIFR2.

• Bit 0 – TOIE2: Timer/Counter2 Overflow Interrupt Enable

When the TOIE2 bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter2 occurs, i.e., when the TOV2 bit is set in the Timer/Counter2 Interrupt

Flag Register – TIFR2.

15.8.7 Timer/Counter2 Interrupt Flag Register – TIFR2

• Bit 2 – OCF2B: Output Compare Flag 2 B

The OCF2B bit is set (one) when a compare match occurs between the Timer/Counter2 and the

data in OCR2B – Output Compare Register2. OCF2B is cleared by hardware when executing

the corresponding interrupt handling vector. Alternatively, OCF2B is cleared by writing a logic

one to the flag. When the I-bit in SREG, OCIE2B (Timer/Counter2 Compare match Interrupt

Enable), and OCF2B are set (one), the Timer/Counter2 Compare match Interrupt is executed.

• Bit 1 – OCF2A: Output Compare Flag 2 A

The OCF2A bit is set (one) when a compare match occurs between the Timer/Counter2 and the

data in OCR2A – Output Compare Register2. OCF2A is cleared by hardware when executing

the corresponding interrupt handling vector. Alternatively, OCF2A is cleared by writing a logic

one to the flag. When the I-bit in SREG, OCIE2A (Timer/Counter2 Compare match Interrupt

Enable), and OCF2A are set (one), the Timer/Counter2 Compare match Interrupt is executed.

• Bit 0 – TOV2: Timer/Counter2 Overflow Flag

The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hard-

ware when executing the corresponding interrupt handling vector. Alternatively, TOV2 is cleared

by writing a logic one to the flag. When the SREG I-bit, TOIE2A (Timer/Counter2 Overflow Inter-

rupt Enable), and TOV2 are set (one), the Timer/Counter2 Overflow interrupt is executed. In

PWM mode, this bit is set when Timer/Counter2 changes counting direction at 0x00.

Bit 76543 2 1 0

– – – – – OCIE2B OCIE2A TOIE2 TIMSK2

Read/Write R R R R R R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

–––––OCF2BOCF2ATOV2TIFR2

Read/Write RRRRRR/WR/WR/W

Initial Value00000000

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15.9 Asynchronous operation of the Timer/Counter

15.9.1 Asynchronous Operation of Timer/Counter2

When Timer/Counter2 operates asynchronously, some considerations must be taken.

• Warning: When switching between asynchronous and synchronous clocking of

Timer/Counter2, the Timer Registers TCNT2, OCR2x, and TCCR2x might be corrupted. A

safe procedure for switching clock source is:

a. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.

b. Select clock source by setting AS2 as appropriate.

c. Write new values to TCNT2, OCR2x, and TCCR2x.

d. To switch to asynchronous operation: Wait for TCN2xUB, OCR2xUB, and

TCR2xUB.

e. Clear the Timer/Counter2 Interrupt Flags.

f. Enable interrupts, if needed.

• The CPU main clock frequency must be more than four times the Oscillator frequency.

• When writing to one of the registers TCNT2, OCR2x, or TCCR2x, the value is transferred to a

temporary register, and latched after two positive edges on TOSC1. The user should not

write a new value before the contents of the temporary register have been transferred to its

destination. Each of the five mentioned registers have their individual temporary register,

which means that e.g. writing to TCNT2 does not disturb an OCR2x write in progress. To

detect that a transfer to the destination register has taken place, the Asynchronous Status

• When entering Power-save or ADC Noise Reduction mode after having written to TCNT2,

OCR2x, or TCCR2x, the user must wait until the written register has been updated if

Timer/Counter2 is used to wake up the device. Otherwise, the MCU will enter sleep mode

before the changes are effective. This is particularly important if any of the Output Compare2

interrupt is used to wake up the device, since the Output Compare function is disabled during

writing to OCR2x or TCNT2. If the write cycle is not finished, and the MCU enters sleep mode

before the corresponding OCR2xUB bit returns to zero, the device will never receive a

compare match interrupt, and the MCU will not wake up.

• If Timer/Counter2 is used to wake the device up from Power-save or ADC Noise Reduction

mode, precautions must be taken if the user wants to re-enter one of these modes: The

interrupt logic needs one TOSC1 cycle to be reset. If the time between wake-up and

re-entering sleep mode is less than one TOSC1 cycle, the interrupt will not occur, and the

device will fail to wake up. If the user is in doubt whether the time before re-entering

Power-save or ADC Noise Reduction mode is sufficient, the following algorithm can be used

to ensure that one TOSC1 cycle has elapsed:

a. Write a value to TCCR2x, TCNT2, or OCR2x.

b. Wait until the corresponding Update Busy Flag in ASSR returns to zero.

c. Enter Power-save or ADC Noise Reduction mode.

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• When the asynchronous operation is selected, the 32.768 kHz Oscillator for Timer/Counter2

is always running, except in Power-down and Standby modes. After a Power-up Reset or

wake-up from Power-down or Standby mode, the user should be aware of the fact that this

Oscillator might take as long as one second to stabilize. The user is advised to wait for at

least one second before using Timer/Counter2 after power-up or wake-up from Power-down

or Standby mode. The contents of all Timer/Counter2 Registers must be considered lost after

a wake-up from Power-down or Standby mode due to unstable clock signal upon start-up, no

matter whether the Oscillator is in use or a clock signal is applied to the TOSC1 pin.

• Description of wake up from Power-save or ADC Noise Reduction mode when the timer is

clocked asynchronously: When the interrupt condition is met, the wake up process is started

on the following cycle of the timer clock, that is, the timer is always advanced by at least one

before the processor can read the counter value. After wake-up, the MCU is halted for four

cycles, it executes the interrupt routine, and resumes execution from the instruction following

SLEEP.

• Reading of the TCNT2 Register shortly after wake-up from Power-save may give an incorrect

result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2 must be

done through a register synchronized to the internal I/O clock domain. Synchronization takes

place for every rising TOSC1 edge. When waking up from Power-save mode, and the I/O

clock (clkI/O) again becomes active, TCNT2 will read as the previous value (before entering

sleep) until the next rising TOSC1 edge. The phase of the TOSC clock after waking up from

Power-save mode is essentially unpredictable, as it depends on the wake-up time. The

recommended procedure for reading TCNT2 is thus as follows:

a. Write any value to either of the registers OCR2x or TCCR2x.

b. Wait for the corresponding Update Busy Flag to be cleared.

c. Read TCNT2.

During asynchronous operation, the synchronization of the Interrupt Flags for the asynchronous

timer takes 3 processor cycles plus one timer cycle. The timer is therefore advanced by at least

one before the processor can read the timer value causing the setting of the Interrupt Flag. The

Output Compare pin is changed on the timer clock and is not synchronized to the processor

clock.

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15.9.2 Asynchronous Status Register – ASSR

• Bit 6 – EXCLK: Enable External Clock Input

When EXCLK is written to one, and asynchronous clock is selected, the external clock input buf-

fer is enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a

32 kHz crystal. Writing to EXCLK should be done before asynchronous operation is selected.

Note that the crystal Oscillator will only run when this bit is zero.

• Bit 5 – AS2: Asynchronous Timer/Counter2

When AS2 is written to zero, Timer/Counter2 is clocked from the I/O clock, clkI/O. When AS2 is

written to one, Timer/Counter2 is clocked from a crystal Oscillator connected to the Timer Oscil-

lator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2A,

OCR2B, TCCR2A and TCCR2B might be corrupted.

• Bit 4 – TCN2UB: Timer/Counter2 Update Busy

When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set.

When TCNT2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.

• Bit 3 – OCR2AUB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2A is written, this bit becomes set.

When OCR2A has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2A is ready to be updated with a new value.

• Bit 2 – OCR2BUB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2B is written, this bit becomes set.

When OCR2B has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2B is ready to be updated with a new value.

• Bit 1 – TCR2AUB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2A is written, this bit becomes set.

When TCCR2A has been updated from the temporary storage register, this bit is cleared by

hardware. A logical zero in this bit indicates that TCCR2A is ready to be updated with a new

value.

• Bit 0 – TCR2BUB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2B is written, this bit becomes set.

When TCCR2B has been updated from the temporary storage register, this bit is cleared by

hardware. A logical zero in this bit indicates that TCCR2B is ready to be updated with a new

value.

If a write is performed to any of the five Timer/Counter2 Registers while its update busy flag is

set, the updated value might get corrupted and cause an unintentional interrupt to occur.

The mechanisms for reading TCNT2, OCR2A, OCR2B, TCCR2A and TCCR2B are different.

When reading TCNT2, the actual timer value is read. When reading OCR2A, OCR2B, TCCR2A

and TCCR2B the value in the temporary storage register is read.

Bit 7 6 5 4 3 2 1 0

– EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB ASSR

Read/Write R R/W R/W R R R R R

Initial Value 0 0 0 0 0 0 0 0

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15.10 Timer/Counter Prescaler

Figure 15-12. Prescaler for Timer/Counter2

The clock source for Timer/Counter2 is named clkT2S. clkT2S is by default connected to the main

system I/O clock clkIO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously

clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter

(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can

then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock

source for Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal. Apply-

ing an external clock source to TOSC1 is not recommended.

For Timer/Counter2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64,

clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected.

Setting the PSRASY bit in GTCCR resets the prescaler. This allows the user to operate with a

predictable prescaler.

15.10.1 General Timer/Counter Control Register – GTCCR

• Bit 1 – PSRASY: Prescaler Reset Timer/Counter2

When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared

immediately by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous

mode, the bit will remain one until the prescaler has been reset. The bit will not be cleared by

hardware if the TSM bit is set. Refer to the description of the “Bit 7 – TSM: Timer/Counter Syn-

chronization Mode” on page 105 for a description of the Timer/Counter Synchronization mode.

10-BIT T/C PRESCALER

TIMER/COUNTER2 CLOCK SOURCE

clkI/O clkT2S

TOSC1

AS2

CS20

CS21

CS22

clkT2S

/64

clkT2S

/128

clkT2S

/1024

clkT2S

/256

clkT2S

/32

PSRASY

Clear

clkT2

Bit 7 6 5 4 3 2 1 0

TSM – – – – – PSRASY PSRSYNC GTCCR

Read/Write R/W R R R R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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16. Serial Peripheral Interface – SPI

The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the

ATmega48/88/168 and peripheral devices or between several AVR devices. The

ATmega48/88/168 SPI includes the following features:

•Full-duplex, Three-wire Synchronous Data Transfer

•Master or Slave Operation

•LSB First or MSB First Data Transfer

•Seven Programmable Bit Rates

•End of Transmission Interrupt Flag

•Write Collision Flag Protection

•Wake-up from Idle Mode

•Double Speed (CK/2) Master SPI Mode

The USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 192. The

PRSPI bit in “Power Reduction Register - PRR” on page 39 must be written to zero to enable

SPI module.

Figure 16-1. SPI Block Diagram(1)

Note: 1. Refer to Figure 1-1 on page 2, and Table 10-3 on page 70 for SPI pin placement.

SPI2X

DIVIDER

/2/4/8/16/32/64/128

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The interconnection between Master and Slave CPUs with SPI is shown in Figure 16-2. The sys-

tem consists of two shift Registers, and a Master clock generator. The SPI Master initiates the

communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and

Slave prepare the data to be sent in their respective shift Registers, and the Master generates

the required clock pulses on the SCK line to interchange data. Data is always shifted from Mas-

ter to Slave on the Master Out – Slave In, MOSI, line, and from Slave to Master on the Master In

– Slave Out, MISO, line. After each data packet, the Master will synchronize the Slave by pulling

high the Slave Select, SS, line.

When configured as a Master, the SPI interface has no automatic control of the SS line. This

must be handled by user software before communication can start. When this is done, writing a

byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the eight

bits into the Slave. After shifting one byte, the SPI clock generator stops, setting the end of

Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in the SPCR Register is set, an

interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or

signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be

kept in the Buffer Register for later use.

When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long

as the SS pin is driven high. In this state, software may update the contents of the SPI Data

until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission

Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt

is requested. The Slave may continue to place new data to be sent into SPDR before reading

the incoming data. The last incoming byte will be kept in the Buffer Register for later use.

Figure 16-2. SPI Master-slave Interconnection

The system is single buffered in the transmit direction and double buffered in the receive direc-

tion. This means that bytes to be transmitted cannot be written to the SPI Data Register before

the entire shift cycle is completed. When receiving data, however, a received character must be

read from the SPI Data Register before the next character has been completely shifted in. Oth-

erwise, the first byte is lost.

In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure

correct sampling of the clock signal, the frequency of the SPI clock should never exceed fosc/4.

When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden

according to Table 16-1 on page 158. For more details on automatic port overrides, refer to

“Alternate Port Functions” on page 68.

SHIFT

ENABLE

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Note: 1. See “Alternate Functions of Port B” on page 70 for a detailed description of how to define the

direction of the user defined SPI pins.

The following code examples show how to initialize the SPI as a Master and how to perform a

simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction

actual data direction bits for these pins. E.g. if MOSI is placed on pin PB5, replace DD_MOSI

with DDB5 and DDR_SPI with DDRB.

Table 16-1. SPI Pin Overrides(1)

Pin Direction, Master SPI Direction, Slave SPI

MOSI User Defined Input

MISO Input User Defined

SCK User Defined Input

SS User Defined Input

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Note: 1. The example code assumes that the part specific header file is included.

Assembly Code Example(1)

SPI_MasterInit:

; Set MOSI and SCK output, all others input

ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)

out DDR_SPI,r17

; Enable SPI, Master, set clock rate fck/16

ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)

out SPCR,r17

ret

SPI_MasterTransmit:

; Start transmission of data (r16)

out SPDR,r16

Wait_Transmit:

; Wait for transmission complete

sbis SPSR,SPIF

rjmp Wait_Transmit

ret

C Code Example(1)

void SPI_MasterInit(void)

{

/* Set MOSI and SCK output, all others input */

DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);

/* Enable SPI, Master, set clock rate fck/16 */

SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);

}

void SPI_MasterTransmit(char cData)

{

/* Start transmission */

SPDR = cData;

/* Wait for transmission complete */

while(!(SPSR & (1<<SPIF)))

;

}

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The following code examples show how to initialize the SPI as a Slave and how to perform a

simple reception.

Note: 1. The example code assumes that the part specific header file is included.

Assembly Code Example(1)

SPI_SlaveInit:

; Set MISO output, all others input

ldi r17,(1<<DD_MISO)

out DDR_SPI,r17

; Enable SPI

ldi r17,(1<<SPE)

out SPCR,r17

ret

SPI_SlaveReceive:

; Wait for reception complete

sbis SPSR,SPIF

rjmp SPI_SlaveReceive

; Read received data and return

in r16,SPDR

ret

C Code Example(1)

void SPI_SlaveInit(void)

{

/* Set MISO output, all others input */

DDR_SPI = (1<<DD_MISO);

/* Enable SPI */

SPCR = (1<<SPE);

}

char SPI_SlaveReceive(void)

{

/* Wait for reception complete */

while(!(SPSR & (1<<SPIF)))

;

/* Return Data Register */

return SPDR;

}

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16.1 SS Pin Functionality

16.1.1 Slave Mode

When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is

held low, the SPI is activated, and MISO becomes an output if configured so by the user. All

other pins are inputs. When SS is driven high, all pins are inputs, and the SPI is passive, which

means that it will not receive incoming data. Note that the SPI logic will be reset once the SS pin

is driven high.

The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous

with the master clock generator. When the SS pin is driven high, the SPI slave will immediately

reset the send and receive logic, and drop any partially received data in the Shift Register.

16.1.2 Master Mode

When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the

direction of the SS pin.

If SS is configured as an output, the pin is a general output pin which does not affect the SPI

system. Typically, the pin will be driving the SS pin of the SPI Slave.

If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin

is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin

defined as an input, the SPI system interprets this as another master selecting the SPI as a

slave and starting to send data to it. To avoid bus contention, the SPI system takes the following

actions:

1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of

the SPI becoming a Slave, the MOSI and SCK pins become inputs.

2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG

is set, the interrupt routine will be executed.

Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possi-

bility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the

MSTR bit has been cleared by a slave select, it must be set by the user to re-enable SPI Master

mode.

16.1.3 SPI Control Register – SPCR

• Bit 7 – SPIE: SPI Interrupt Enable

This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if

the Global Interrupt Enable bit in SREG is set.

• Bit 6 – SPE: SPI Enable

When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI

operations.

• Bit 5 – DORD: Data Order

When the DORD bit is written to one, the LSB of the data word is transmitted first.

Bit 76543210

SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 SPCR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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When the DORD bit is written to zero, the MSB of the data word is transmitted first.

• Bit 4 – MSTR: Master/Slave Select

This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic

zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared,

and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI Mas-

ter mode.

• Bit 3 – CPOL: Clock Polarity

When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low

when idle. Refer to Figure 16-3 and Figure 16-4 for an example. The CPOL functionality is sum-

marized below:

• Bit 2 – CPHA: Clock Phase

The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or

trailing (last) edge of SCK. Refer to Figure 16-3 and Figure 16-4 for an example. The CPOL

functionality is summarized below:

• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0

These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have

no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is

shown in the following table:

Table 16-2. CPOL Functionality

CPOL Leading Edge Trailing Edge

0 Rising Falling

1 Falling Rising

Table 16-3. CPHA Functionality

CPHA Leading Edge Trailing Edge

0 Sample Setup

1 Setup Sample

Table 16-4. Relationship Between SCK and the Oscillator Frequency

SPI2X SPR1 SPR0 SCK Frequency

000

fosc/4

001

fosc/16

010

fosc/64

011

fosc/128

100

fosc/2

101

fosc/8

110fosc/32

111fosc/64

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16.1.4 SPI Status Register – SPSR

• Bit 7 – SPIF: SPI Interrupt Flag

When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in

SPCR is set and global interrupts are enabled. If SS is an input and is driven low when the SPI is

in Master mode, this will also set the SPIF Flag. SPIF is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, the SPIF bit is cleared by first reading the

SPI Status Register with SPIF set, then accessing the SPI Data Register (SPDR).

• Bit 6 – WCOL: Write COLlision Flag

The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The

WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set,

and then accessing the SPI Data Register.

• Bit 5..1 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bit 0 – SPI2X: Double SPI Speed Bit

When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI

is in Master mode (see Table 16-4). This means that the minimum SCK period will be two CPU

clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4

or lower.

The SPI interface on the ATmega48/88/168 is also used for program memory and EEPROM

downloading or uploading. See page 293 for serial programming and verification.

16.1.5 SPI Data Register – SPDR

The SPI Data Register is a read/write register used for data transfer between the Register File

and the SPI Shift Register. Writing to the register initiates data transmission. Reading the regis-

ter causes the Shift Register Receive buffer to be read.

16.2 Data Modes

There are four combinations of SCK phase and polarity with respect to serial data, which are

determined by control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure

16-3 and Figure 16-4. Data bits are shifted out and latched in on opposite edges of the SCK sig-

nal, ensuring sufficient time for data signals to stabilize. This is clearly seen by summarizing

Table 16-2 and Table 16-3, as done below.

Bit 76543210

SPIFWCOL–––––SPI2XSPSR

Read/Write RRRRRRRR/W

Initial Value00000000

Bit 76543210

MSB LSB SPDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial ValueXXXXXXXXUndefined

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Figure 16-3. SPI Transfer Format with CPHA = 0

Figure 16-4. SPI Transfer Format with CPHA = 1

Table 16-5. CPOL Functionality

Leading Edge Trailing eDge SPI Mode

CPOL=0, CPHA=0 Sample (Rising) Setup (Falling) 0

CPOL=0, CPHA=1 Setup (Rising) Sample (Falling) 1

CPOL=1, CPHA=0 Sample (Falling) Setup (Rising) 2

CPOL=1, CPHA=1 Setup (Falling) Sample (Rising) 3

Bit 1

Bit 6

LSB

MSB

SCK (CPOL = 0)

mode 0

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SCK (CPOL = 1)

mode 2

MSB

LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

MSB first (DORD = 0)

LSB first (DORD = 1)

SCK (CPOL = 0)

mode 1

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SCK (CPOL = 1)

mode 3

MSB

LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

Bit 1

Bit 6

LSB

MSB

MSB first (DORD = 0)

LSB first (DORD = 1)

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17. USART0

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a

highly flexible serial communication device. The main features are:

•Full Duplex Operation (Independent Serial Receive and Transmit Registers)

•Asynchronous or Synchronous Operation

•Master or Slave Clocked Synchronous Operation

•High Resolution Baud Rate Generator

•Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits

•Odd or Even Parity Generation and Parity Check Supported by Hardware

•Data OverRun Detection

•Framing Error Detection

•Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter

•Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete

•Multi-processor Communication Mode

•Double Speed Asynchronous Communication Mode

The USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 192. The

Power Reduction USART bit, PRUSART0, in “Power Reduction Register - PRR” on page 39

must be disabled by writing a logical zero to it.

17.1 Overview

A simplified block diagram of the USART Transmitter is shown in Figure 17-1. CPU accessible

I/O Registers and I/O pins are shown in bold.

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Figure 17-1. USART Block Diagram(1)

Note: 1. Refer to Figure 1-1 on page 2 and Table 10-9 on page 77 for USART0 pin placement.

The dashed boxes in the block diagram separate the three main parts of the USART (listed from

the top): Clock Generator, Transmitter and Receiver. Control Registers are shared by all units.

The Clock Generation logic consists of synchronization logic for external clock input used by

synchronous slave operation, and the baud rate generator. The XCKn (Transfer Clock) pin is

only used by synchronous transfer mode. The Transmitter consists of a single write buffer, a

serial Shift Register, Parity Generator and Control logic for handling different serial frame for-

mats. The write buffer allows a continuous transfer of data without any delay between frames.

The Receiver is the most complex part of the USART module due to its clock and data recovery

units. The recovery units are used for asynchronous data reception. In addition to the recovery

units, the Receiver includes a Parity Checker, Control logic, a Shift Register and a two level

receive buffer (UDRn). The Receiver supports the same frame formats as the Transmitter, and

can detect Frame Error, Data OverRun and Parity Errors.

PARITY

GENERATOR

UBRRn [H:L]

UDRn(Transmit)

UCSRnA UCSRnB UCSRnC

BAUD RATE GENERATOR

TRANSMIT SHIFT REGISTER

RECEIVE SHIFT REGISTER RxDn

TxDn

CONTROL

UDRn (Receive)

CONTROL

XCKn

DATA

RECOVERY

CLOCK

RECOVERY

CONTROL

PARITY

CHECKER

DATA BUS

OSC

SYNC LOGIC

Clock Generator

Transmitter

Receiver

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17.2 Clock Generation

The Clock Generation logic generates the base clock for the Transmitter and Receiver. The

USART supports four modes of clock operation: Normal asynchronous, Double Speed asyn-

chronous, Master synchronous and Slave synchronous mode. The UMSELn bit in USART

Control and Status Register C (UCSRnC) selects between asynchronous and synchronous

operation. Double Speed (asynchronous mode only) is controlled by the U2Xn found in the

UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction Register

for the XCKn pin (DDR_XCKn) controls whether the clock source is internal (Master mode) or

external (Slave mode). The XCKn pin is only active when using synchronous mode.

Figure 17-2 shows a block diagram of the clock generation logic.

Figure 17-2. Clock Generation Logic, Block Diagram

Signal description:

txclk Transmitter clock (Internal Signal).

rxclk Receiver base clock (Internal Signal).

xcki Input from XCK pin (internal Signal). Used for synchronous slave

operation.

xcko Clock output to XCK pin (Internal Signal). Used for synchronous master

operation.

fosc XTAL pin frequency (System Clock).

17.2.1 Internal Clock Generation – The Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous master modes of

operation. The description in this section refers to Figure 17-2.

The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a

programmable prescaler or baud rate generator. The down-counter, running at system clock

(fosc), is loaded with the UBRRn value each time the counter has counted down to zero or when

the UBRRnL Register is written. A clock is generated each time the counter reaches zero. This

clock is the baud rate generator clock output (= fosc/(UBRRn+1)). The Transmitter divides the

baud rate generator clock output by 2, 8 or 16 depending on mode. The baud rate generator out-

put is used directly by the Receiver’s clock and data recovery units. However, the recovery units

use a state machine that uses 2, 8 or 16 states depending on mode set by the state of the

UMSELn, U2Xn and DDR_XCKn bits.

Prescaling

Down-Counter /2

UBRRn

/4 /2

foscn

UBRRn+1

Sync

OSC

XCKn

txclk

U2Xn

UMSELn

DDR_XCKn

xcki

xcko

DDR_XCKn rxclk

Edge

Detector

UCPOLn

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Table 17-1 contains equations for calculating the baud rate (in bits per second) and for calculat-

ing the UBRRn value for each mode of operation using an internally generated clock source.

Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)

BAUD Baud rate (in bits per second, bps)

fOSC System Oscillator clock frequency

UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095)

Some examples of UBRRn values for some system clock frequencies are found in Table 17-9

(see page 188).

17.2.2 Double Speed Operation (U2Xn)

The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has

effect for the asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling

the transfer rate for asynchronous communication. Note however that the Receiver will in this

case only use half the number of samples (reduced from 16 to 8) for data sampling and clock

recovery, and therefore a more accurate baud rate setting and system clock are required when

this mode is used. For the Transmitter, there are no downsides.

Table 17-1. Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating Baud

Rate(1) Equation for Calculating UBRRn

Value

Asynchronous Normal

mode (U2Xn = 0)

Asynchronous Double

Speed mode (U2Xn = 1)

Synchronous Master

mode

BAUD fOSC

16 UBRRn1+()

------------------------------------------=

UBRRnfOSC

16BAUD

------------------------1–=

BAUD fOSC

8UBRRn1+()

---------------------------------------=

UBRRnfOSC

8BAUD

-------------------- 1–=

BAUD fOSC

2UBRRn1+()

---------------------------------------=

UBRRnfOSC

2BAUD

-------------------- 1–=

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17.2.3 External Clock

External clocking is used by the synchronous slave modes of operation. The description in this

section refers to Figure 17-2 for details.

External clock input from the XCKn pin is sampled by a synchronization register to minimize the

chance of meta-stability. The output from the synchronization register must then pass through

an edge detector before it can be used by the Transmitter and Receiver. This process intro-

duces a two CPU clock period delay and therefore the maximum external XCKn clock frequency

is limited by the following equation:

Note that fosc depends on the stability of the system clock source. It is therefore recommended to

add some margin to avoid possible loss of data due to frequency variations.

17.2.4 Synchronous Clock Operation

When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input

(Slave) or clock output (Master). The dependency between the clock edges and data sampling

or data change is the same. The basic principle is that data input (on RxDn) is sampled at the

opposite XCKn clock edge of the edge the data output (TxDn) is changed.

Figure 17-3. Synchronous Mode XCKn Timing.

The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is

used for data change. As Figure 17-3 shows, when UCPOLn is zero the data will be changed at

rising XCKn edge and sampled at falling XCKn edge. If UCPOLn is set, the data will be changed

at falling XCKn edge and sampled at rising XCKn edge.

fXCK

fOSC

-----------

RxD / TxD

XCK

RxD / TxD

XCK

UCPOL = 0

UCPOL = 1

Sample

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17.3 Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start and stop

bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of

the following as valid frame formats:

• 1 start bit

• 5, 6, 7, 8, or 9 data bits

• no, even or odd parity bit

• 1 or 2 stop bits

A frame starts with the start bit followed by the least significant data bit. Then the next data bits,

up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit

is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can

be directly followed by a new frame, or the communication line can be set to an idle (high) state.

Figure 17-4 illustrates the possible combinations of the frame formats. Bits inside brackets are

optional.

Figure 17-4. Frame Formats

St Start bit, always low.

(n) Data bits (0 to 8).

PParity bit. Can be odd or even.

Sp Stop bit, always high.

IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line

must be

high.

The frame format used by the USART is set by the UCSZn2:0, UPMn1:0 and USBSn bits in

UCSRnB and UCSRnC. The Receiver and Transmitter use the same setting. Note that changing

the setting of any of these bits will corrupt all ongoing communication for both the Receiver and

Transmitter.

The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The

USART Parity mode (UPMn1:0) bits enable and set the type of parity bit. The selection between

one or two stop bits is done by the USART Stop Bit Select (USBSn) bit. The Receiver ignores

the second stop bit. An FE (Frame Error) will therefore only be detected in the cases where the

first stop bit is zero.

10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)

FRAME

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17.3.1 Parity Bit Calculation

The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the

result of the exclusive or is inverted. The relation between the parity bit and data bits is as

follows:

Peven Parity bit using even parity

Podd Parity bit using odd parity

dnData bit n of the character

If used, the parity bit is located between the last data bit and first stop bit of a serial frame.

17.4 USART Initialization

The USART has to be initialized before any communication can take place. The initialization pro-

cess normally consists of setting the baud rate, setting frame format and enabling the

Transmitter or the Receiver depending on the usage. For interrupt driven USART operation, the

Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the

initialization.

Before doing a re-initialization with changed baud rate or frame format, be sure that there are no

ongoing transmissions during the period the registers are changed. The TXCn Flag can be used

to check that the Transmitter has completed all transfers, and the RXC Flag can be used to

check that there are no unread data in the receive buffer. Note that the TXCn Flag must be

cleared before each transmission (before UDRn is written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C func-

tion that are equal in functionality. The examples assume asynchronous operation using polling

(no interrupts enabled) and a fixed frame format. The baud rate is given as a function parameter.

For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16

Registers.

Peven dn1–…d3d2d1d00

Podd

⊕⊕⊕⊕⊕⊕

dn1–…d3d2d1d01⊕⊕⊕⊕⊕⊕

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Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

More advanced initialization routines can be made that include frame format as parameters, dis-

able interrupts and so on. However, many applications use a fixed setting of the baud and

control registers, and for these types of applications the initialization code can be placed directly

in the main routine, or be combined with initialization code for other I/O modules.

17.5 Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB

den by the USART and given the function as the Transmitter’s serial output. The baud rate,

mode of operation and frame format must be set up once before doing any transmissions. If syn-

chronous operation is used, the clock on the XCKn pin will be overridden and used as

transmission clock.

Assembly Code Example(1)

USART_Init:

; Set baud rate

out UBRRnH, r17

out UBRRnL, r16

; Enable receiver and transmitter

ldi r16, (1<<RXENn)|(1<<TXENn)

out UCSRnB,r16

; Set frame format: 8data, 2stop bit

ldi r16, (1<<USBSn)|(3<<UCSZn0)

out UCSRnC,r16

ret

C Code Example(1)

void USART_Init( unsigned int baud )

{

/* Set baud rate */

UBRRnH = (unsigned char)(baud>>8);

UBRRnL = (unsigned char)baud;

/* Enable receiver and transmitter */

UCSRnB = (1<<RXENn)|(1<<TXENn);

/* Set frame format: 8data, 2stop bit */

UCSRnC = (1<<USBSn)|(3<<UCSZn0);

}

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17.5.1 Sending Frames with 5 to 8 Data Bit

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The

CPU can load the transmit buffer by writing to the UDRn I/O location. The buffered data in the

transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new

frame. The Shift Register is loaded with new data if it is in idle state (no ongoing transmission) or

immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is

loaded with new data, it will transfer one complete frame at the rate given by the Baud Register,

U2Xn bit or by XCKn depending on mode of operation.

The following code examples show a simple USART transmit function based on polling of the

Data Register Empty (UDREn) Flag. When using frames with less than eight bits, the most sig-

nificant bits written to the UDRn are ignored. The USART has to be initialized before the function

can be used. For the assembly code, the data to be sent is assumed to be stored in Register

R16

Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,

before loading it with new data to be transmitted. If the Data Register Empty interrupt is utilized,

the interrupt routine writes the data into the buffer.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRnA,UDREn

rjmp USART_Transmit

; Put data (r16) into buffer, sends the data

out UDRn,r16

ret

C Code Example(1)

void USART_Transmit( unsigned char data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn)) )

;

/* Put data into buffer, sends the data */

UDRn = data;

}

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17.5.2 Sending Frames with 9 Data Bit

If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCS-

RnB before the low byte of the character is written to UDRn. The following code examples show

a transmit function that handles 9-bit characters. For the assembly code, the data to be sent is

assumed to be stored in registers R17:R16.

Notes: 1. These transmit functions are written to be general functions. They can be optimized if the con-

tents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used

after initialization.

2. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The ninth bit can be used for indicating an address frame when using multi processor communi-

cation mode or for other protocol handling as for example synchronization.

Assembly Code Example(1)(2)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRnA,UDREn

rjmp USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi UCSRnB,TXB8

sbrc r17,0

sbi UCSRnB,TXB8

; Put LSB data (r16) into buffer, sends the data

out UDRn,r16

ret

C Code Example(1)(2)

void USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn))) )

;

/* Copy 9th bit to TXB8 */

UCSRnB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRnB |= (1<<TXB8);

/* Put data into buffer, sends the data */

UDRn = data;

}

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17.5.3 Transmitter Flags and Interrupts

The USART Transmitter has two flags that indicate its state: USART Data Register Empty

(UDREn) and Transmit Complete (TXCn). Both flags can be used for generating interrupts.

The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive

new data. This bit is set when the transmit buffer is empty, and cleared when the transmit buffer

contains data to be transmitted that has not yet been moved into the Shift Register. For compat-

ibility with future devices, always write this bit to zero when writing the UCSRnA Register.

When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the

USART Data Register Empty Interrupt will be executed as long as UDREn is set (provided that

global interrupts are enabled). UDREn is cleared by writing UDRn. When interrupt-driven data

transmission is used, the Data Register Empty interrupt routine must either write new data to

UDRn in order to clear UDREn or disable the Data Register Empty interrupt, otherwise a new

interrupt will occur once the interrupt routine terminates.

The Transmit Complete (TXCn) Flag bit is set one when the entire frame in the Transmit Shift

The TXCn Flag bit is automatically cleared when a transmit complete interrupt is executed, or it

can be cleared by writing a one to its bit location. The TXCn Flag is useful in half-duplex commu-

nication interfaces (like the RS-485 standard), where a transmitting application must enter

receive mode and free the communication bus immediately after completing the transmission.

When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART

Transmit Complete Interrupt will be executed when the TXCn Flag becomes set (provided that

global interrupts are enabled). When the transmit complete interrupt is used, the interrupt han-

dling routine does not have to clear the TXCn Flag, this is done automatically when the interrupt

is executed.

17.5.4 Parity Generator

The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled

(UPMn1 = 1), the transmitter control logic inserts the parity bit between the last data bit and the

first stop bit of the frame that is sent.

17.5.5 Disabling the Transmitter

The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongo-

ing and pending transmissions are completed, i.e., when the Transmit Shift Register and

Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter

will no longer override the TxDn pin.

17.6 Data Reception – The USART Receiver

The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the

UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the RxDn

pin is overridden by the USART and given the function as the Receiver’s serial input. The baud

rate, mode of operation and frame format must be set up once before any serial reception can

be done. If synchronous operation is used, the clock on the XCKn pin will be used as transfer

clock.

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17.6.1 Receiving Frames with 5 to 8 Data Bits

The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start

bit will be sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register

until the first stop bit of a frame is received. A second stop bit will be ignored by the Receiver.

When the first stop bit is received, i.e., a complete serial frame is present in the Receive Shift

buffer can then be read by reading the UDRn I/O location.

The following code example shows a simple USART receive function based on polling of the

Receive Complete (RXCn) Flag. When using frames with less than eight bits the most significant

bits of the data read from the UDRn will be masked to zero. The USART has to be initialized

before the function can be used.

Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The function simply waits for data to be present in the receive buffer by checking the RXCn Flag,

before reading the buffer and returning the value.

17.6.2 Receiving Frames with 9 Data Bits

If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCS-

RnB before reading the low bits from the UDRn. This rule applies to the FEn, DORn and UPEn

Status Flags as well. Read status from UCSRnA, then data from UDRn. Reading the UDRn I/O

location will change the state of the receive buffer FIFO and consequently the TXB8n, FEn,

DORn and UPEn bits, which all are stored in the FIFO, will change.

The following code example shows a simple USART receive function that handles both nine bit

characters and the status bits.

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRnA, RXCn

rjmp USART_Receive

; Get and return received data from buffer

in r16, UDRn

ret

C Code Example(1)

unsigned char USART_Receive( void )

{

/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) )

;

/* Get and return received data from buffer */

return UDRn;

}

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Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The receive function example reads all the I/O Registers into the Register File before any com-

putation is done. This gives an optimal receive buffer utilization since the buffer location read will

be free to accept new data as early as possible.

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRnA, RXCn

rjmp USART_Receive

; Get status and 9th bit, then data from buffer

in r18, UCSRnA

in r17, UCSRnB

in r16, UDRn

; If error, return -1

andi r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)

breq USART_ReceiveNoError

ldi r17, HIGH(-1)

ldi r16, LOW(-1)

USART_ReceiveNoError:

; Filter the 9th bit, then return

lsr r17

andi r17, 0x01

ret

C Code Example(1)

unsigned int USART_Receive( void )

{

unsigned char status, resh, resl;

/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) )

;

/* Get status and 9th bit, then data */

/* from buffer */

status = UCSRnA;

resh = UCSRnB;

resl = UDRn;

/* If error, return -1 */

if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )

return -1;

/* Filter the 9th bit, then return */

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

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17.6.3 Receive Compete Flag and Interrupt

The USART Receiver has one flag that indicates the Receiver state.

The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buf-

fer. This flag is one when unread data exist in the receive buffer, and zero when the receive

buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled (RXENn = 0),

the receive buffer will be flushed and consequently the RXCn bit will become zero.

When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive

Complete interrupt will be executed as long as the RXCn Flag is set (provided that global inter-

rupts are enabled). When interrupt-driven data reception is used, the receive complete routine

must read the received data from UDRn in order to clear the RXCn Flag, otherwise a new inter-

rupt will occur once the interrupt routine terminates.

17.6.4 Receiver Error Flags

The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn) and

Parity Error (UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags is

that they are located in the receive buffer together with the frame for which they indicate the

error status. Due to the buffering of the Error Flags, the UCSRnA must be read before the

receive buffer (UDRn), since reading the UDRn I/O location changes the buffer read location.

Another equality for the Error Flags is that they can not be altered by software doing a write to

the flag location. However, all flags must be set to zero when the UCSRnA is written for upward

compatibility of future USART implementations. None of the Error Flags can generate interrupts.

The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame

stored in the receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one),

and the FEn Flag will be one when the stop bit was incorrect (zero). This flag can be used for

detecting out-of-sync conditions, detecting break conditions and protocol handling. The FEn

Flag is not affected by the setting of the USBSn bit in UCSRnC since the Receiver ignores all,

except for the first, stop bits. For compatibility with future devices, always set this bit to zero

when writing to UCSRnA.

The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A

Data OverRun occurs when the receive buffer is full (two characters), it is a new character wait-

ing in the Receive Shift Register, and a new start bit is detected. If the DORn Flag is set there

was one or more serial frame lost between the frame last read from UDRn, and the next frame

read from UDRn. For compatibility with future devices, always write this bit to zero when writing

to UCSRnA. The DORn Flag is cleared when the frame received was successfully moved from

the Shift Register to the receive buffer.

The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity

Error when received. If Parity Check is not enabled the UPEn bit will always be read zero. For

compatibility with future devices, always set this bit to zero when writing to UCSRnA. For more

details see “Parity Bit Calculation” on page 171 and “Parity Checker” on page 179.

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17.6.5 Parity Checker

The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Par-

ity Check to be performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity

Checker calculates the parity of the data bits in incoming frames and compares the result with

the parity bit from the serial frame. The result of the check is stored in the receive buffer together

with the received data and stop bits. The Parity Error (UPEn) Flag can then be read by software

to check if the frame had a Parity Error.

The UPEn bit is set if the next character that can be read from the receive buffer had a Parity

Error when received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is

valid until the receive buffer (UDRn) is read.

17.6.6 Disabling the Receiver

In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing

receptions will therefore be lost. When disabled (i.e., the RXENn is set to zero) the Receiver will

no longer override the normal function of the RxDn port pin. The Receiver buffer FIFO will be

flushed when the Receiver is disabled. Remaining data in the buffer will be lost

17.6.7 Flushing the Receive Buffer

The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be

emptied of its contents. Unread data will be lost. If the buffer has to be flushed during normal

operation, due to for instance an error condition, read the UDRn I/O location until the RXCn Flag

is cleared. The following code example shows how to flush the receive buffer.

Note: 1. The example code assumes that the part specific header file is included.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

17.7 Asynchronous Data Reception

The USART includes a clock recovery and a data recovery unit for handling asynchronous data

reception. The clock recovery logic is used for synchronizing the internally generated baud rate

clock to the incoming asynchronous serial frames at the RxDn pin. The data recovery logic sam-

ples and low pass filters each incoming bit, thereby improving the noise immunity of the

Receiver. The asynchronous reception operational range depends on the accuracy of the inter-

nal baud rate clock, the rate of the incoming frames, and the frame size in number of bits.

Assembly Code Example(1)

USART_Flush:

sbis UCSRnA, RXCn

ret

in r16, UDRn

rjmp USART_Flush

C Code Example(1)

void USART_Flush( void )

{

unsigned char dummy;

while ( UCSRnA & (1<<RXCn) ) dummy = UDRn;

}

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17.7.1 Asynchronous Clock Recovery

The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 17-5

illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times

the baud rate for Normal mode, and eight times the baud rate for Double Speed mode. The hor-

izontal arrows illustrate the synchronization variation due to the sampling process. Note the

larger time variation when using the Double Speed mode (U2Xn = 1) of operation. Samples

denoted zero are samples done when the RxDn line is idle (i.e., no communication activity).

Figure 17-5. Start Bit Sampling

When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the

start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in

the figure. The clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and sam-

ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the

figure), to decide if a valid start bit is received. If two or more of these three samples have logical

high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts

looking for the next high to low-transition. If however, a valid start bit is detected, the clock recov-

ery logic is synchronized and the data recovery can begin. The synchronization process is

repeated for each start bit.

17.7.2 Asynchronous Data Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data

recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight

states for each bit in Double Speed mode. Figure 17-6 shows the sampling of the data bits and

the parity bit. Each of the samples is given a number that is equal to the state of the recovery

unit.

Figure 17-6. Sampling of Data and Parity Bit

12345678 9 10 11 12 13 14 15 16 12

STARTIDLE

BIT 0

1234 5 678120

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

12345678 9 10 11 12 13 14 15 16 1

BIT n

1234 5 6781

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

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The decision of the logic level of the received bit is taken by doing a majority voting of the logic

value to the three samples in the center of the received bit. The center samples are emphasized

on the figure by having the sample number inside boxes. The majority voting process is done as

follows: If two or all three samples have high levels, the received bit is registered to be a logic 1.

If two or all three samples have low levels, the received bit is registered to be a logic 0. This

majority voting process acts as a low pass filter for the incoming signal on the RxDn pin. The

recovery process is then repeated until a complete frame is received. Including the first stop bit.

Note that the Receiver only uses the first stop bit of a frame.

Figure 17-7 shows the sampling of the stop bit and the earliest possible beginning of the start bit

of the next frame.

Figure 17-7. Stop Bit Sampling and Next Start Bit Sampling

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop

bit is registered to have a logic 0 value, the Frame Error (FEn) Flag will be set.

A new high to low transition indicating the start bit of a new frame can come right after the last of

the bits used for majority voting. For Normal Speed mode, the first low level sample can be at

point marked (A) in Figure 17-7. For Double Speed mode the first low level must be delayed to

(B). (C) marks a stop bit of full length. The early start bit detection influences the operational

range of the Receiver.

17.7.3 Asynchronous Operational Range

The operational range of the Receiver is dependent on the mismatch between the received bit

rate and the internally generated baud rate. If the Transmitter is sending frames at too fast or too

slow bit rates, or the internally generated baud rate of the Receiver does not have a similar (see

Table 17-2) base frequency, the Receiver will not be able to synchronize the frames to the start

bit.

The following equations can be used to calculate the ratio of the incoming data rate and internal

receiver baud rate.

DSum of character size and parity size (D = 5 to 10 bit)

12345678 9 10 0/1 0/1 0/1

STOP 1

1234 5 6 0/1

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

(A) (B) (C)

Table 1.

Rslow D1+()S

S1–DS⋅SF

---------------------------------------------=

Rfast D2+()S

D1+()SS

-----------------------------------=

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SSamples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed

mode.

SFFirst sample number used for majority voting. SF = 8 for normal speed and SF = 4

for Double Speed mode.

SMMiddle sample number used for majority voting. SM = 9 for normal speed and

SM= 5 for Double Speed mode.

Rslow is the ratio of the slowest incoming data rate that can be accepted in relation to the

receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be

accepted in relation to the receiver baud rate.

Table 17-2 and Table 17-3 list the maximum receiver baud rate error that can be tolerated. Note

that Normal Speed mode has higher toleration of baud rate variations.

The recommendations of the maximum receiver baud rate error was made under the assump-

tion that the Receiver and Transmitter equally divides the maximum total error.

There are two possible sources for the receivers baud rate error. The Receiver’s system clock

(XTAL) will always have some minor instability over the supply voltage range and the tempera-

ture range. When using a crystal to generate the system clock, this is rarely a problem, but for a

resonator the system clock may differ more than 2% depending of the resonators tolerance. The

second source for the error is more controllable. The baud rate generator can not always do an

exact division of the system frequency to get the baud rate wanted. In this case an UBRRn value

that gives an acceptable low error can be used if possible.

Table 17-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode

(U2Xn = 0)

# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%)

Recommended Max

Receiver Error (%)

5 93.20 106.67 +6.67/-6.8 ± 3.0

6 94.12 105.79 +5.79/-5.88 ± 2.5

7 94.81 105.11 +5.11/-5.19 ± 2.0

8 95.36 104.58 +4.58/-4.54 ± 2.0

9 95.81 104.14 +4.14/-4.19 ± 1.5

10 96.17 103.78 +3.78/-3.83 ± 1.5

Table 17-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode

(U2Xn = 1)

# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%)

Recommended Max

Receiver Error (%)

5 94.12 105.66 +5.66/-5.88 ± 2.5

6 94.92 104.92 +4.92/-5.08 ± 2.0

7 95.52 104,35 +4.35/-4.48 ± 1.5

8 96.00 103.90 +3.90/-4.00 ± 1.5

9 96.39 103.53 +3.53/-3.61 ± 1.5

10 96.70 103.23 +3.23/-3.30 ± 1.0

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17.8 Multi-processor Communication Mode

Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering

function of incoming frames received by the USART Receiver. Frames that do not contain

address information will be ignored and not put into the receive buffer. This effectively reduces

the number of incoming frames that has to be handled by the CPU, in a system with multiple

MCUs that communicate via the same serial bus. The Transmitter is unaffected by the MPCMn

setting, but has to be used differently when it is a part of a system utilizing the Multi-processor

Communication mode.

If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indi-

cates if the frame contains data or address information. If the Receiver is set up for frames with

nine data bits, then the ninth bit (RXB8n) is used for identifying address and data frames. When

the frame type bit (the first stop or the ninth bit) is one, the frame contains an address. When the

frame type bit is zero the frame is a data frame.

The Multi-processor Communication mode enables several slave MCUs to receive data from a

master MCU. This is done by first decoding an address frame to find out which MCU has been

addressed. If a particular slave MCU has been addressed, it will receive the following data

frames as normal, while the other slave MCUs will ignore the received frames until another

address frame is received.

17.8.1 Using MPCMn

For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn = 7). The

ninth bit (TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame

(TXB = 0) is being transmitted. The slave MCUs must in this case be set to use a 9-bit character

frame format.

The following procedure should be used to exchange data in Multi-processor Communication

mode:

1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in

UCSRnA is set).

2. The Master MCU sends an address frame, and all slaves receive and read this frame.

In the Slave MCUs, the RXCn Flag in UCSRnA will be set as normal.

3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If

so, it clears the MPCMn bit in UCSRnA, otherwise it waits for the next address byte and

keeps the MPCMn setting.

4. The addressed MCU will receive all data frames until a new address frame is received.

The other Slave MCUs, which still have the MPCMn bit set, will ignore the data frames.

5. When the last data frame is received by the addressed MCU, the addressed MCU sets

the MPCMn bit and waits for a new address frame from master. The process then

repeats from 2.

Using any of the 5- to 8-bit character frame formats is possible, but impractical since the

Receiver must change between using n and n+1 character frame formats. This makes

full-duplex operation difficult since the Transmitter and Receiver uses the same character size

setting. If 5- to 8-bit character frames are used, the Transmitter must be set to use two stop bit

(USBSn = 1) since the first stop bit is used for indicating the frame type.

Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCMn bit. The

MPCMn bit shares the same I/O location as the TXCn Flag and this might accidentally be

cleared when using SBI or CBI instructions.

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17.9 USART Register Description

17.9.1 USART I/O Data Register n– UDRn

The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the

same I/O address referred to as USART Data Register or UDRn. The Transmit Data Buffer Reg-

ister (TXB) will be the destination for data written to the UDRn Register location. Reading the

UDRn Register location will return the contents of the Receive Data Buffer Register (RXB).

For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to

zero by the Receiver.

The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set.

Data written to UDRn when the UDREn Flag is not set, will be ignored by the USART Transmit-

ter. When data is written to the transmit buffer, and the Transmitter is enabled, the Transmitter

will load the data into the Transmit Shift Register when the Shift Register is empty. Then the

data will be serially transmitted on the TxDn pin.

The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the

receive buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Mod-

ify-Write instructions (SBI and CBI) on this location. Be careful when using bit test instructions

(SBIC and SBIS), since these also will change the state of the FIFO.

17.9.2 USART Control and Status Register n A – UCSRnA

• Bit 7 – RXCn: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive

buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled, the receive

buffer will be flushed and consequently the RXCn bit will become zero. The RXCn Flag can be

used to generate a Receive Complete interrupt (see description of the RXCIEn bit).

• Bit 6 – TXCn: USART Transmit Complete

This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and

there are no new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is auto-

matically cleared when a transmit complete interrupt is executed, or it can be cleared by writing

a one to its bit location. The TXCn Flag can generate a Transmit Complete interrupt (see

description of the TXCIEn bit).

• Bit 5 – UDREn: USART Data Register Empty

The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn

is one, the buffer is empty, and therefore ready to be written. The UDREn Flag can generate a

Data Register Empty interrupt (see description of the UDRIEn bit).

UDREn is set after a reset to indicate that the Transmitter is ready.

Bit 76543210

RXB[7:0] UDRn (Read)

TXB[7:0] UDRn (Write)

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

RXCn TXCn UDREn FEn DORn UPEn U2Xn MPCMn UCSRnA

Read/Write R R/W R R R R R/W R/W

Initial Value 0 0 1 0 0 0 0 0

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• Bit 4 – FEn: Frame Error

This bit is set if the next character in the receive buffer had a Frame Error when received. I.e.,

when the first stop bit of the next character in the receive buffer is zero. This bit is valid until the

receive buffer (UDRn) is read. The FEn bit is zero when the stop bit of received data is one.

Always set this bit to zero when writing to UCSRnA.

• Bit 3 – DORn: Data OverRun

This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive

buffer is full (two characters), it is a new character waiting in the Receive Shift Register, and a

new start bit is detected. This bit is valid until the receive buffer (UDRn) is read. Always set this

bit to zero when writing to UCSRnA.

• Bit 2 – UPEn: USART Parity Error

This bit is set if the next character in the receive buffer had a Parity Error when received and the

Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer

(UDRn) is read. Always set this bit to zero when writing to UCSRnA.

• Bit 1 – U2Xn: Double the USART Transmission Speed

This bit only has effect for the asynchronous operation. Write this bit to zero when using syn-

chronous operation.

Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively dou-

bling the transfer rate for asynchronous communication.

• Bit 0 – MPCMn: Multi-processor Communication Mode

This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to

one, all the incoming frames received by the USART Receiver that do not contain address infor-

mation will be ignored. The Transmitter is unaffected by the MPCMn setting. For more detailed

information see “Multi-processor Communication Mode” on page 183.

17.9.3 USART Control and Status Register n B – UCSRnB

• Bit 7 – RXCIEn: RX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt

will be generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is

written to one and the RXCn bit in UCSRnA is set.

• Bit 6 – TXCIEn: TX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt

will be generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is

written to one and the TXCn bit in UCSRnA is set.

• Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable n

Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will

be generated only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written

to one and the UDREn bit in UCSRnA is set.

Bit 76543210

RXCIEn TXCIEn UDRIEn RXENn TXENn UCSZn2 RXB8n TXB8n UCSRnB

Read/Write R/W R/W R/W R/W R/W R/W R R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 4 – RXENn: Receiver Enable n

Writing this bit to one enables the USART Receiver. The Receiver will override normal port oper-

ation for the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer

invalidating the FEn, DORn, and UPEn Flags.

• Bit 3 – TXENn: Transmitter Enable n

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port

operation for the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to

zero) will not become effective until ongoing and pending transmissions are completed, i.e.,

when the Transmit Shift Register and Transmit Buffer Register do not contain data to be trans-

mitted. When disabled, the Transmitter will no longer override the TxDn port.

• Bit 2 – UCSZn2: Character Size n

The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits

(Character SiZe) in a frame the Receiver and Transmitter use.

• Bit 1 – RXB8n: Receive Data Bit 8 n

RXB8n is the ninth data bit of the received character when operating with serial frames with nine

data bits. Must be read before reading the low bits from UDRn.

• Bit 0 – TXB8n: Transmit Data Bit 8 n

TXB8n is the ninth data bit in the character to be transmitted when operating with serial frames

with nine data bits. Must be written before writing the low bits to UDRn.

17.9.4 USART Control and Status Register n C – UCSRnC

• Bits 7:6 – UMSELn1:0 USART Mode Select

These bits select the mode of operation of the USARTn as shown in Table 17-4.

Note: 1. See “USART in SPI Mode” on page 192 for full description of the Master SPI Mode (MSPIM)

operation

• Bits 5:4 – UPMn1:0: Parity Mode

These bits enable and set type of parity generation and check. If enabled, the Transmitter will

automatically generate and send the parity of the transmitted data bits within each frame. The

Bit 7 6 543 2 1 0

UMSELn1 UMSELn0 UPMn1 UPMn0 USBSn UCSZn1 UCSZn0 UCPOLn UCSRnC

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 1 1 0

Table 17-4. UMSELn Bits Settings

UMSELn1 UMSELn0 Mode

0 0 Asynchronous USART

0 1 Synchronous USART

1 0 (Reserved)

1 1 Master SPI (MSPIM)(1)

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Receiver will generate a parity value for the incoming data and compare it to the UPMn setting.

If a mismatch is detected, the UPEn Flag in UCSRnA will be set.

• Bit 3 – USBSn: Stop Bit Select

This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores

this setting.

• Bit 2:1 – UCSZn1:0: Character Size

The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits

(Character SiZe) in a frame the Receiver and Transmitter use.

• Bit 0 – UCPOLn: Clock Polarity

This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is

used. The UCPOLn bit sets the relationship between data output change and data input sample,

and the synchronous clock (XCKn).

Table 17-5. UPMn Bits Settings

UPMn1 UPMn0 Parity Mode

0 0 Disabled

01Reserved

1 0 Enabled, Even Parity

1 1 Enabled, Odd Parity

Table 17-6. USBS Bit Settings

USBSn Stop Bit(s)

01-bit

12-bit

Table 17-7. UCSZn Bits Settings

UCSZn2 UCSZn1 UCSZn0 Character Size

0 0 0 5-bit

0 0 1 6-bit

0 1 0 7-bit

0 1 1 8-bit

100Reserved

101Reserved

110Reserved

1 1 1 9-bit

Table 17-8. UCPOLn Bit Settings

UCPOLn

Transmitted Data Changed (Output of

TxDn Pin)

Received Data Sampled (Input on RxDn

Pin)

0 Rising XCKn Edge Falling XCKn Edge

1 Falling XCKn Edge Rising XCKn Edge

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17.9.5 USART Baud Rate Registers – UBRRnL and UBRRnH

• Bit 15:12 – Reserved Bits

These bits are reserved for future use. For compatibility with future devices, these bit must be

written to zero when UBRRnH is written.

• Bit 11:0 – UBRR11:0: USART Baud Rate Register

This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four

most significant bits, and the UBRRnL contains the eight least significant bits of the USART

baud rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud

rate is changed. Writing UBRRnL will trigger an immediate update of the baud rate prescaler.

17.10 Examples of Baud Rate Setting

For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-

chronous operation can be generated by using the UBRRn settings in Table 17-9. UBRRn

values which yield an actual baud rate differing less than 0.5% from the target baud rate, are

bold in the table. Higher error ratings are acceptable, but the Receiver will have less noise resis-

tance when the error ratings are high, especially for large serial frames (see “Asynchronous

Operational Range” on page 181). The error values are calculated using the following equation:

Bit 151413121110 9 8

–––– UBRRn[11:8] UBRRnH

UBRRn[7:0] UBRRnL

76543210

Read/Write RRRRR/WR/WR/WR/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

00000000

Error[%] BaudRateClosest Match

BaudRate

-------------------------------------------------------- 1–

⎝⎠

⎛⎞

100%•=

Table 17-9. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies

Baud

Rate

(bps)

fosc = 1.0000 MHz fosc = 1.8432 MHz fosc = 2.0000 MHz

U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

2400 250.2%510.2%470.0%950.0%510.2%1030.2%

4800 120.2%250.2%230.0%470.0%250.2%510.2%

9600 6-7.0%120.2%110.0%230.0%120.2%250.2%

14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%

19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%

28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%

38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%

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Note: 1. UBRRn = 0, Error = 0.0%

57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%

76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%

115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%

230.4k––––––00.0%––––

250k––––––––––00.0%

Max.(1) 62.5 kbps 125 kbps 115.2 kbps 230.4 kbps 125 kbps 250 kbps

Table 17-9. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies

Baud

Rate

(bps)

fosc = 1.0000 MHz fosc = 1.8432 MHz fosc = 2.0000 MHz

U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

Table 17-10. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 3.6864 MHz fosc = 4.0000 MHz fosc = 7.3728 MHz

U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%

4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%

9600 230.0%470.0%250.2%510.2%470.0%950.0%

14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%

19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%

28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%

38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%

57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%

76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%

115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%

230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%

250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%

0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8%

1M ––––––––––0-7.8%

Max. (1) 230.4 kbps 460.8 kbps 250 kbps 0.5 Mbps 460.8 kbps 921.6 kbps

1. UBRRn = 0, Error = 0.0%

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Table 17-11. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 8.0000 MHz fosc = 11.0592 MHz fosc = 14.7456 MHz

U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%

4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%

9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%

14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%

19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%

28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%

38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%

57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%

76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0%

115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%

230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%

250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%

0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%

1M ––00.0%––––0-7.8%1-7.8%

Max. (1) 0.5 Mbps 1 Mbps 691.2 kbps 1.3824 Mbps 921.6 kbps 1.8432 Mbps

1. UBRRn = 0, Error = 0.0%

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Table 17-12. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 16.0000 MHz fosc = 18.4320 MHz fosc = 20.0000 MHz

U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

UBRR

nError

2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%

4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%

9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%

14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%

19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%

28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%

38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%

57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%

76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%

115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%

230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%

250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%

0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%

1M 00.0%10.0%––––––––

Max. (1) 1 Mbps 2 Mbps 1.152 Mbps 2.304 Mbps 1.25 Mbps 2.5 Mbps

1. UBRRn = 0, Error = 0.0%

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18. USART in SPI Mode

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be

set to a master SPI compliant mode of operation. The Master SPI Mode (MSPIM) has the follow-

ing features:

•Full Duplex, Three-wire Synchronous Data Transfer

•Master Operation

•Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)

•LSB First or MSB First Data Transfer (Configurable Data Order)

•Queued Operation (Double Buffered)

•High Resolution Baud Rate Generator

•High Speed Operation (fXCKmax = fCK/2)

•Flexible Interrupt Generation

18.1 Overview

Setting both UMSELn1:0 bits to one enables the USART in MSPIM logic. In this mode of opera-

tion the SPI master control logic takes direct control over the USART resources. These

resources include the transmitter and receiver shift register and buffers, and the baud rate gen-

erator. The parity generator and checker, the data and clock recovery logic, and the RX and TX

control logic is disabled. The USART RX and TX control logic is replaced by a common SPI

transfer control logic. However, the pin control logic and interrupt generation logic is identical in

both modes of operation.

The I/O register locations are the same in both modes. However, some of the functionality of the

control registers changes when using MSPIM.

18.2 Clock Generation

The Clock Generation logic generates the base clock for the Transmitter and Receiver. For

USART MSPIM mode of operation only internal clock generation (i.e. master operation) is sup-

ported. The Data Direction Register for the XCKn pin (DDR_XCKn) must therefore be set to one

(i.e. as output) for the USART in MSPIM to operate correctly. Preferably the DDR_XCKn should

be set up before the USART in MSPIM is enabled (i.e. TXENn and RXENn bit set to one).

The internal clock generation used in MSPIM mode is identical to the USART synchronous mas-

ter mode. The baud rate or UBRRn setting can therefore be calculated using the same

equations, see Table 18-1:

Table 18-1. Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating Baud

Rate(1) Equation for Calculating UBRRn

Value

Synchronous Master

mode

BAUD fOSC

2UBRRn1+()

---------------------------------------=

UBRRnfOSC

2BAUD

-------------------- 1–=

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Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)

BAUD Baud rate (in bits per second, bps)

fOSC System Oscillator clock frequency

UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095)

18.3 SPI Data Modes and Timing

There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which

are determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are

shown in Figure 18-1. Data bits are shifted out and latched in on opposite edges of the XCKn

signal, ensuring sufficient time for data signals to stabilize. The UCPOLn and UCPHAn function-

ality is summarized in Table 18-2. Note that changing the setting of any of these bits will corrupt

all ongoing communication for both the Receiver and Transmitter.

Figure 18-1. UCPHAn and UCPOLn data transfer timing diagrams.

18.4 Frame Formats

A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM

mode has two valid frame formats:

• 8-bit data with MSB first

• 8-bit data with LSB first

A frame starts with the least or most significant data bit. Then the next data bits, up to a total of

eight, are succeeding, ending with the most or least significant bit accordingly. When a complete

frame is transmitted, a new frame can directly follow it, or the communication line can be set to

an idle (high) state.

Table 18-2. UCPOLn and UCPHAn Functionality-

UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge

0 0 0 Sample (Rising) Setup (Falling)

0 1 1 Setup (Rising) Sample (Falling)

1 0 2 Sample (Falling) Setup (Rising)

1 1 3 Setup (Falling) Sample (Rising)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

UCPOL=0 UCPOL=1

UCPHA=0 UCPHA=1

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The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The

Receiver and Transmitter use the same setting. Note that changing the setting of any of these

bits will corrupt all ongoing communication for both the Receiver and Transmitter.

16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit com-

plete interrupt will then signal that the 16-bit value has been shifted out.

18.4.1 USART MSPIM Initialization

The USART in MSPIM mode has to be initialized before any communication can take place. The

initialization process normally consists of setting the baud rate, setting master mode of operation

(by setting DDR_XCKn to one), setting frame format and enabling the Transmitter and the

Receiver. Only the transmitter can operate independently. For interrupt driven USART opera-

tion, the Global Interrupt Flag should be cleared (and thus interrupts globally disabled) when

doing the initialization.

Note: To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be

zero at the time the transmitter is enabled. Contrary to the normal mode USART operation the

UBRRn must then be written to the desired value after the transmitter is enabled, but before the

first transmission is started. Setting UBRRn to zero before enabling the transmitter is not neces-

sary if the initialization is done immediately after a reset since UBRRn is reset to zero.

Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that

there is no ongoing transmissions during the period the registers are changed. The TXCn Flag

can be used to check that the Transmitter has completed all transfers, and the RXCn Flag can

be used to check that there are no unread data in the receive buffer. Note that the TXCn Flag

must be cleared before each transmission (before UDRn is written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C func-

tion that are equal in functionality. The examples assume polling (no interrupts enabled). The

baud rate is given as a function parameter. For the assembly code, the baud rate parameter is

assumed to be stored in the r17:r16 registers.

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Note: 1. The example code assumes that the part specific header file is included. For I/O Registers

located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" instructions must

be replaced with instructions that allow access to extended I/O. Typically "LDS" and "STS"

combined with "SBRS", "SBRC", "SBR", and "CBR".

Assembly Code Example(1)

USART_Init:

clr r18

out UBRRnH,r18

out UBRRnL,r18

; Setting the XCKn port pin as output, enables master mode.

sbi XCKn_DDR, XCKn

; Set MSPI mode of operation and SPI data mode 0.

ldi r18, (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)

out UCSRnC,r18

; Enable receiver and transmitter.

ldi r18, (1<<RXENn)|(1<<TXENn)

out UCSRnB,r18

; Set baud rate.

; IMPORTANT: The Baud Rate must be set after the transmitter is enabled!

out UBRRnH, r17

out UBRRnL, r18

ret

C Code Example(1)

void USART_Init( unsigned int baud )

{

UBRRn = 0;

/* Setting the XCKn port pin as output, enables master mode. */

XCKn_DDR |= (1<<XCKn);

/* Set MSPI mode of operation and SPI data mode 0. */

UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);

/* Enable receiver and transmitter. */

UCSRnB = (1<<RXENn)|(1<<TXENn);

/* Set baud rate. */

/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled

UBRRn = baud;

}

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18.5 Data Transfer

Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in

the UCSRnB register is set to one. When the Transmitter is enabled, the normal port operation

of the TxDn pin is overridden and given the function as the Transmitter's serial output. Enabling

the receiver is optional and is done by setting the RXENn bit in the UCSRnB register to one.

When the receiver is enabled, the normal pin operation of the RxDn pin is overridden and given

the function as the Receiver's serial input. The XCKn will in both cases be used as the transfer

clock.

After initialization the USART is ready for doing data transfers. A data transfer is initiated by writ-

ing to the UDRn I/O location. This is the case for both sending and receiving data since the

transmitter controls the transfer clock. The data written to UDRn is moved from the transmit buf-

fer to the shift register when the shift register is ready to send a new frame.

Note: To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must

be read once for each byte transmitted. The input buffer operation is identical to normal USART

mode, i.e. if an overflow occurs the character last received will be lost, not the first data in the buf-

fer. This means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the UDRn

is not read before all transfers are completed, then byte 3 to be received will be lost, and not byte

The following code examples show a simple USART in MSPIM mode transfer function based on

polling of the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The

USART has to be initialized before the function can be used. For the assembly code, the data to

be sent is assumed to be stored in Register R16 and the data received will be available in the

same register (R16) after the function returns.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,

before loading it with new data to be transmitted. The function then waits for data to be present

in the receive buffer by checking the RXCn Flag, before reading the buffer and returning the

value.

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Note: 1. The example code assumes that the part specific header file is included. For I/O Registers

located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" instructions must

be replaced with instructions that allow access to extended I/O. Typically "LDS" and "STS"

combined with "SBRS", "SBRC", "SBR", and "CBR".

18.5.1 Transmitter and Receiver Flags and Interrupts

The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode

are identical in function to the normal USART operation. However, the receiver error status flags

(FE, DOR, and PE) are not in use and is always read as zero.

18.5.2 Disabling the Transmitter or Receiver

The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to

the normal USART operation.

Assembly Code Example(1)

USART_MSPIM_Transfer:

; Wait for empty transmit buffer

sbis UCSRnA, UDREn

rjmp USART_MSPIM_Transfer

; Put data (r16) into buffer, sends the data

out UDRn,r16

; Wait for data to be received

USART_MSPIM_Wait_RXCn:

sbis UCSRnA, RXCn

rjmp USART_MSPIM_Wait_RXCn

; Get and return received data from buffer

in r16, UDRn

ret

C Code Example(1)

unsigned char USART_Receive( void )

{

/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn)) );

/* Put data into buffer, sends the data */

UDRn = data;

/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) );

/* Get and return received data from buffer */

return UDRn;

}

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18.6 USART MSPIM Register Description

The following section describes the registers used for SPI operation using the USART.

18.6.1 USART MSPIM I/O Data Register - UDRn

The function and bit description of the USART data register (UDRn) in MSPI mode is identical to

normal USART operation. See “USART I/O Data Register n– UDRn” on page 184.

18.6.2 USART MSPIM Control and Status Register n A - UCSRnA

•

• Bit 7 - RXCn: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive

buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled, the receive

buffer will be flushed and consequently the RXCn bit will become zero. The RXCn Flag can be

used to generate a Receive Complete interrupt (see description of the RXCIEn bit).

• Bit 6 - TXCn: USART Transmit Complete

This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and

there are no new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is auto-

matically cleared when a transmit complete interrupt is executed, or it can be cleared by writing

a one to its bit location. The TXCn Flag can generate a Transmit Complete interrupt (see

description of the TXCIEn bit).

• Bit 5 - UDREn: USART Data Register Empty

The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn

is one, the buffer is empty, and therefore ready to be written. The UDREn Flag can generate a

Data Register Empty interrupt (see description of the UDRIE bit). UDREn is set after a reset to

indicate that the Transmitter is ready.

• Bit 4:0 - Reserved Bits in MSPI mode

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices,

these bits must be written to zero when UCSRnA is written.

18.6.3 USART MSPIM Control and Status Register n B - UCSRnB

• Bit 7 - RXCIEn: RX Complete Interrupt Enable

Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt

will be generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is

written to one and the RXCn bit in UCSRnA is set.

Bit 7 6 5 4 3 2 1 0

RXCn TXCn UDREn - - - - - UCSRnA

Read/Write R/W R/W R/W R R R R R

Initial Value 0 0 0 0 0 1 1 0

Bit 7 6 5 4 3 2 1 0

RXCIEn TXCIEn UDRIE RXENn TXENn - - - UCSRnB

Read/Write R/W R/W R/W R/W R/W R R R

Initial Value 0 0 0 0 0 1 1 0

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• Bit 6 - TXCIEn: TX Complete Interrupt Enable

Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt

will be generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is

written to one and the TXCn bit in UCSRnA is set.

• Bit 5 - UDRIE: USART Data Register Empty Interrupt Enable

Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will

be generated only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written

to one and the UDREn bit in UCSRnA is set.

• Bit 4 - RXENn: Receiver Enable

Writing this bit to one enables the USART Receiver in MSPIM mode. The Receiver will override

normal port operation for the RxDn pin when enabled. Disabling the Receiver will flush the

receive buffer. Only enabling the receiver in MSPI mode (i.e. setting RXENn=1 and TXENn=0)

has no meaning since it is the transmitter that controls the transfer clock and since only master

mode is supported.

• Bit 3 - TXENn: Transmitter Enable

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port

operation for the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to

zero) will not become effective until ongoing and pending transmissions are completed, i.e.,

when the Transmit Shift Register and Transmit Buffer Register do not contain data to be trans-

mitted. When disabled, the Transmitter will no longer override the TxDn port.

• Bit 2:0 - Reserved Bits in MSPI mode

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices,

these bits must be written to zero when UCSRnB is written.

18.6.4 USART MSPIM Control and Status Register n C - UCSRnC

• Bit 7:6 - UMSELn1:0: USART Mode Select

These bits select the mode of operation of the USART as shown in Table 18-3. See “USART

Control and Status Register n C – UCSRnC” on page 186 for full description of the normal

USART operation. The MSPIM is enabled when both UMSELn bits are set to one. The UDO-

RDn, UCPHAn, and UCPOLn can be set in the same write operation where the MSPIM is

enabled.

Bit 7 6 543 2 1 0

UMSELn1 UMSELn0 - - - UDORDn UCPHAn UCPOLn UCSRnC

Read/Write R/W R/W R R R R/W R/W R/W

Initial Value 0 0 0 0 0 1 1 0

Table 18-3. UMSELn Bits Settings

UMSELn1 UMSELn0 Mode

0 0 Asynchronous USART

0 1 Synchronous USART

1 0 (Reserved)

1 1 Master SPI (MSPIM)

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• Bit 5:3 - Reserved Bits in MSPI mode

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices,

these bits must be written to zero when UCSRnC is written.

• Bit 2 - UDORDn: Data Order

When set to one the LSB of the data word is transmitted first. When set to zero the MSB of the

data word is transmitted first. Refer to the Frame Formats section page 4 for details.

• Bit 1 - UCPHAn: Clock Phase

The UCPHAn bit setting determine if data is sampled on the leasing edge (first) or tailing (last)

edge of XCKn. Refer to the SPI Data Modes and Timing section page 4 for details.

• Bit 0 - UCPOLn: Clock Polarity

The UCPOLn bit sets the polarity of the XCKn clock. The combination of the UCPOLn and

UCPHAn bit settings determine the timing of the data transfer. Refer to the SPI Data Modes and

Timing section page 4 for details.

18.6.5 USART MSPIM Baud Rate Registers - UBRRnL and UBRRnH

The function and bit description of the baud rate registers in MSPI mode is identical to normal

USART operation. See “USART Baud Rate Registers – UBRRnL and UBRRnH” on page 188.

18.7 AVR USART MSPIM vs.

AVR SPI

The USART in MSPIM mode is fully compatible with the AVR SPI regarding:

• Master mode timing diagram.

• The UCPOLn bit functionality is identical to the SPI CPOL bit.

• The UCPHAn bit functionality is identical to the SPI CPHA bit.

• The UDORDn bit functionality is identical to the SPI DORD bit.

However, since the USART in MSPIM mode reuses the USART resources, the use of the

USART in MSPIM mode is somewhat different compared to the SPI. In addition to differences of

the control register bits, and that only master operation is supported by the USART in MSPIM

mode, the following features differ between the two modules:

• The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no

buffer.

• The USART in MSPIM mode receiver includes an additional buffer level.

• The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode.

• The SPI double speed mode (SPI2X) bit is not included. However, the same effect is

achieved by setting UBRRn accordingly.

• Interrupt timing is not compatible.

• Pin control differs due to the master only operation of the USART in MSPIM mode.

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A comparison of the USART in MSPIM mode and the SPI pins is shown in Table 18-4 on page

201.

19. 2-wire Serial Interface

19.1 Features •Simple Yet Powerful and Flexible Communication Interface, only two Bus Lines Needed

•Both Master and Slave Operation Supported

•Device can Operate as Transmitter or Receiver

•7-bit Address Space Allows up to 128 Different Slave Addresses

•Multi-master Arbitration Support

•Up to 400 kHz Data Transfer Speed

•Slew-rate Limited Output Drivers

•Noise Suppression Circuitry Rejects Spikes on Bus Lines

•Fully Programmable Slave Address with General Call Support

•Address Recognition Causes Wake-up When AVR is in Sleep Mode

19.2 2-wire Serial Interface Bus Definition

The 2-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The

TWI protocol allows the systems designer to interconnect up to 128 different devices using only

two bi-directional bus lines, one for clock (SCL) and one for data (SDA). The only external hard-

ware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All

devices connected to the bus have individual addresses, and mechanisms for resolving bus

contention are inherent in the TWI protocol.

Figure 19-1. TWI Bus Interconnection

Table 18-4. Comparison of USART in MSPIM mode and SPI pins.

USART_MSPIM SPI Comment

TxDn MOSI Master Out only

RxDn MISO Master In only

XCKn SCK (Functionally identical)

(N/A) SS Not supported by USART in MSPIM

Device 1 Device 2 Device 3 Device n

SDA

SCL

........

R1 R2

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19.2.1 TWI Terminology

The following definitions are frequently encountered in this section.

The PRTWI bit in “Power Reduction Register - PRR” on page 39 must be written to zero to

enable the 2-wire Serial Interface.

19.2.2 Electrical Interconnection

As depicted in Figure 19-1, both bus lines are connected to the positive supply voltage through

pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector.

This implements a wired-AND function which is essential to the operation of the interface. A low

level on a TWI bus line is generated when one or more TWI devices output a zero. A high level

is output when all TWI devices tri-state their outputs, allowing the pull-up resistors to pull the line

high. Note that all AVR devices connected to the TWI bus must be powered in order to allow any

bus operation.

The number of devices that can be connected to the bus is only limited by the bus capacitance

limit of 400 pF and the 7-bit slave address space. A detailed specification of the electrical char-

acteristics of the TWI is given in “2-wire Serial Interface Characteristics” on page 303. Two

different sets of specifications are presented there, one relevant for bus speeds below 100 kHz,

and one valid for bus speeds up to 400 kHz.

19.3 Data Transfer and Frame Format

19.3.1 Transferring Bits

Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level

of the data line must be stable when the clock line is high. The only exception to this rule is for

generating start and stop conditions.

Table 19-1. TWI Terminology

Term Description

Master The device that initiates and terminates a transmission. The Master also generates the

SCL clock.

Slave The device addressed by a Master.

Transmitter The device placing data on the bus.

Receiver The device reading data from the bus.

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Figure 19-2. Data Validity

19.3.2 START and STOP Conditions

The Master initiates and terminates a data transmission. The transmission is initiated when the

Master issues a START condition on the bus, and it is terminated when the Master issues a

STOP condition. Between a START and a STOP condition, the bus is considered busy, and no

other master should try to seize control of the bus. A special case occurs when a new START

condition is issued between a START and STOP condition. This is referred to as a REPEATED

START condition, and is used when the Master wishes to initiate a new transfer without relin-

quishing control of the bus. After a REPEATED START, the bus is considered busy until the next

STOP. This is identical to the START behavior, and therefore START is used to describe both

START and REPEATED START for the remainder of this datasheet, unless otherwise noted. As

depicted below, START and STOP conditions are signalled by changing the level of the SDA

line when the SCL line is high.

Figure 19-3. START, REPEATED START and STOP conditions

SDA

SCL

Data Stable Data Stable

Data Change

SDA

SCL

START STOPREPEATED START

STOP START

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19.3.3 Address Packet Format

All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address bits, one

READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read opera-

tion is to be performed, otherwise a write operation should be performed. When a Slave

recognizes that it is being addressed, it should acknowledge by pulling SDA low in the ninth SCL

(ACK) cycle. If the addressed Slave is busy, or for some other reason can not service the Mas-

ter’s request, the SDA line should be left high in the ACK clock cycle. The Master can then

transmit a STOP condition, or a REPEATED START condition to initiate a new transmission. An

address packet consisting of a slave address and a READ or a WRITE bit is called SLA+R or

SLA+W, respectively.

The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the

designer, but the address 0000 000 is reserved for a general call.

When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK

cycle. A general call is used when a Master wishes to transmit the same message to several

slaves in the system. When the general call address followed by a Write bit is transmitted on the

bus, all slaves set up to acknowledge the general call will pull the SDA line low in the ack cycle.

The following data packets will then be received by all the slaves that acknowledged the general

call. Note that transmitting the general call address followed by a Read bit is meaningless, as

this would cause contention if several slaves started transmitting different data.

All addresses of the format 1111 xxx should be reserved for future purposes.

Figure 19-4. Address Packet Format

19.3.4 Data Packet Format

All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and

an acknowledge bit. During a data transfer, the Master generates the clock and the START and

STOP conditions, while the Receiver is responsible for acknowledging the reception. An

Acknowledge (ACK) is signalled by the Receiver pulling the SDA line low during the ninth SCL

cycle. If the Receiver leaves the SDA line high, a NACK is signalled. When the Receiver has

received the last byte, or for some reason cannot receive any more bytes, it should inform the

Transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted first.

SDA

SCL

START

12 789

Addr MSB Addr LSB R/W ACK

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Figure 19-5. Data Packet Format

19.3.5 Combining Address and Data Packets into a Transmission

A transmission basically consists of a START condition, a SLA+R/W, one or more data packets

and a STOP condition. An empty message, consisting of a START followed by a STOP condi-

tion, is illegal. Note that the Wired-ANDing of the SCL line can be used to implement

handshaking between the Master and the Slave. The Slave can extend the SCL low period by

pulling the SCL line low. This is useful if the clock speed set up by the Master is too fast for the

Slave, or the Slave needs extra time for processing between the data transmissions. The Slave

extending the SCL low period will not affect the SCL high period, which is determined by the

Master. As a consequence, the Slave can reduce the TWI data transfer speed by prolonging the

SCL duty cycle.

Figure 19-6 shows a typical data transmission. Note that several data bytes can be transmitted

between the SLA+R/W and the STOP condition, depending on the software protocol imple-

mented by the application software.

Figure 19-6. Typical Data Transmission

12 789

Data MSB Data LSB ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

SLA+R/W Data Byte

STOP, REPEATED

START or Next

Data Byte

12 789

Data Byte

Data MSB Data LSB ACK

SDA

SCL

START

12 789

Addr MSB Addr LSB R/W ACK

SLA+R/W STOP

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19.4 Multi-master Bus Systems, Arbitration and Synchronization

The TWI protocol allows bus systems with several masters. Special concerns have been taken

in order to ensure that transmissions will proceed as normal, even if two or more masters initiate

a transmission at the same time. Two problems arise in multi-master systems:

• An algorithm must be implemented allowing only one of the masters to complete the

transmission. All other masters should cease transmission when they discover that they have

lost the selection process. This selection process is called arbitration. When a contending

master discovers that it has lost the arbitration process, it should immediately switch to Slave

mode to check whether it is being addressed by the winning master. The fact that multiple

masters have started transmission at the same time should not be detectable to the slaves,

i.e. the data being transferred on the bus must not be corrupted.

• Different masters may use different SCL frequencies. A scheme must be devised to

synchronize the serial clocks from all masters, in order to let the transmission proceed in a

lockstep fashion. This will facilitate the arbitration process.

The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from

all masters will be wired-ANDed, yielding a combined clock with a high period equal to the one

from the Master with the shortest high period. The low period of the combined clock is equal to

the low period of the Master with the longest low period. Note that all masters listen to the SCL

line, effectively starting to count their SCL high and low time-out periods when the combined

SCL line goes high or low, respectively.

Figure 19-7. SCL Synchronization Between Multiple Masters

low

high

SCL from

Master A

SCL from

Master B

SCL Bus

Line

low

high

Masters Start

Counting Low Period

Masters Start

Counting High Period

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Arbitration is carried out by all masters continuously monitoring the SDA line after outputting

data. If the value read from the SDA line does not match the value the Master had output, it has

lost the arbitration. Note that a Master can only lose arbitration when it outputs a high SDA value

while another Master outputs a low value. The losing Master should immediately go to Slave

mode, checking if it is being addressed by the winning Master. The SDA line should be left high,

but losing masters are allowed to generate a clock signal until the end of the current data or

address packet. Arbitration will continue until only one Master remains, and this may take many

bits. If several masters are trying to address the same Slave, arbitration will continue into the

data packet.

Figure 19-8. Arbitration Between Two Masters

Note that arbitration is not allowed between:

• A REPEATED START condition and a data bit.

• A STOP condition and a data bit.

• A REPEATED START and a STOP condition.

It is the user software’s responsibility to ensure that these illegal arbitration conditions never

occur. This implies that in multi-master systems, all data transfers must use the same composi-

tion of SLA+R/W and data packets. In other words: All transmissions must contain the same

number of data packets, otherwise the result of the arbitration is undefined.

SDA from

Master A

SDA from

Master B

SDA Line

Synchronized

SCL Line

START Master A Loses

Arbitration, SDAA SDA

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19.5 Overview of the TWI Module

The TWI module is comprised of several submodules, as shown in Figure 19-9. All registers

drawn in a thick line are accessible through the AVR data bus.

Figure 19-9. Overview of the TWI Module

19.5.1 SCL and SDA Pins

These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a

slew-rate limiter in order to conform to the TWI specification. The input stages contain a spike

suppression unit removing spikes shorter than 50 ns. Note that the internal pull-ups in the AVR

pads can be enabled by setting the PORT bits corresponding to the SCL and SDA pins, as

explained in the I/O Port section. The internal pull-ups can in some systems eliminate the need

for external ones.

TWI Unit

Address Register

(TWAR)

Address Match Unit

Address Comparator

Control Unit

Control Register

(TWCR)

Status Register

(TWSR)

State Machine and

Status control

SCL

Slew-rate

Control

Spike

Filter

SDA

Slew-rate

Control

Spike

Filter

Bit Rate Generator

Bit Rate Register

(TWBR)

Prescaler

Bus Interface Unit

START / STOP

Control

Arbitration detection Ack

Spike Suppression

Address/Data Shift

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19.5.2 Bit Rate Generator Unit

This unit controls the period of SCL when operating in a Master mode. The SCL period is con-

trolled by settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status

CPU clock frequency in the Slave must be at least 16 times higher than the SCL frequency. Note

that slaves may prolong the SCL low period, thereby reducing the average TWI bus clock

period. The SCL frequency is generated according to the following equation:

• TWBR = Value of the TWI Bit Rate Register.

•PrescalerValue = Value of the prescaler, see Table 19-2 on page 212.

Note: TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than 10, the

Master may produce an incorrect output on SDA and SCL for the reminder of the byte. The prob-

lem occurs when operating the TWI in Master mode, sending Start + SLA + R/W to a Slave (a

Slave does not need to be connected to the bus for the condition to happen).

19.5.3 Bus Interface Unit

This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and

Arbitration detection hardware. The TWDR contains the address or data bytes to be transmitted,

or the address or data bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also

contains a register containing the (N)ACK bit to be transmitted or received. This (N)ACK Regis-

ter is not directly accessible by the application software. However, when receiving, it can be set

or cleared by manipulating the TWI Control Register (TWCR). When in Transmitter mode, the

value of the received (N)ACK bit can be determined by the value in the TWSR.

The START/STOP Controller is responsible for generation and detection of START, REPEATED

START, and STOP conditions. The START/STOP controller is able to detect START and STOP

conditions even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up

if addressed by a Master.

If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continu-

ously monitors the transmission trying to determine if arbitration is in process. If the TWI has lost

an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate

status codes generated.

19.5.4 Address Match Unit

The Address Match unit checks if received address bytes match the seven-bit address in the

TWI Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the

TWAR is written to one, all incoming address bits will also be compared against the General Call

address. Upon an address match, the Control Unit is informed, allowing correct action to be

taken. The TWI may or may not acknowledge its address, depending on settings in the TWCR.

The Address Match unit is able to compare addresses even when the AVR MCU is in sleep

mode, enabling the MCU to wake up if addressed by a Master. If another interrupt (e.g., INT0)

occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts opera-

tion and return to it’s idle state. If this cause any problems, ensure that TWI Address Match is the

only enabled interrupt when entering Power-down.

SCL frequency CPU Clock frequency

16 2(TWBR) PrescalerValue()⋅+

--------------------------------------------------------------------------------------------=

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19.5.5 Control Unit

The Control unit monitors the TWI bus and generates responses corresponding to settings in the

TWI Control Register (TWCR). When an event requiring the attention of the application occurs

on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Sta-

tus Register (TWSR) is updated with a status code identifying the event. The TWSR only

contains relevant status information when the TWI Interrupt Flag is asserted. At all other times,

the TWSR contains a special status code indicating that no relevant status information is avail-

able. As long as the TWINT Flag is set, the SCL line is held low. This allows the application

software to complete its tasks before allowing the TWI transmission to continue.

The TWINT Flag is set in the following situations:

• After the TWI has transmitted a START/REPEATED START condition.

• After the TWI has transmitted SLA+R/W.

• After the TWI has transmitted an address byte.

• After the TWI has lost arbitration.

• After the TWI has been addressed by own slave address or general call.

• After the TWI has received a data byte.

• After a STOP or REPEATED START has been received while still addressed as a Slave.

• When a bus error has occurred due to an illegal START or STOP condition.

19.6 TWI Register Description

19.6.1 TWI Bit Rate Register – TWBR

• Bits 7..0 – TWI Bit Rate Register

TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency

divider which generates the SCL clock frequency in the Master modes. See “Bit Rate Generator

Unit” on page 209 for calculating bit rates.

19.6.2 TWI Control Register – TWCR

The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a

Master access by applying a START condition to the bus, to generate a Receiver acknowledge,

to generate a stop condition, and to control halting of the bus while the data to be written to the

bus are written to the TWDR. It also indicates a write collision if data is attempted written to

TWDR while the register is inaccessible.

Bit 76543210

TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0 TWBR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE TWCR

Read/Write R/W R/W R/W R/W R R/W R R/W

Initial Value00000000

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• Bit 7 – TWINT: TWI Interrupt Flag

This bit is set by hardware when the TWI has finished its current job and expects application

software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the

TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched. The TWINT

Flag must be cleared by software by writing a logic one to it. Note that this flag is not automati-

cally cleared by hardware when executing the interrupt routine. Also note that clearing this flag

starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Sta-

tus Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this

flag.

• Bit 6 – TWEA: TWI Enable Acknowledge Bit

The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to

one, the ACK pulse is generated on the TWI bus if the following conditions are met:

1. The device’s own slave address has been received.

2. A general call has been received, while the TWGCE bit in the TWAR is set.

3. A data byte has been received in Master Receiver or Slave Receiver mode.

By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial

Bus temporarily. Address recognition can then be resumed by writing the TWEA bit to one

again.

• Bit 5 – TWSTA: TWI START Condition Bit

The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire

Serial Bus. The TWI hardware checks if the bus is available, and generates a START condition

on the bus if it is free. However, if the bus is not free, the TWI waits until a STOP condition is

detected, and then generates a new START condition to claim the bus Master status. TWSTA

must be cleared by software when the START condition has been transmitted.

• Bit 4 – TWSTO: TWI STOP Condition Bit

Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire

Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared auto-

matically. In Slave mode, setting the TWSTO bit can be used to recover from an error condition.

This will not generate a STOP condition, but the TWI returns to a well-defined unaddressed

Slave mode and releases the SCL and SDA lines to a high impedance state.

• Bit 3 – TWWC: TWI Write Collision Flag

The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is

low. This flag is cleared by writing the TWDR Register when TWINT is high.

• Bit 2 – TWEN: TWI Enable Bit

The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to

one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the

slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI

transmissions are terminated, regardless of any ongoing operation.

• Bit 1 – Res: Reserved Bit

This bit is a reserved bit and will always read as zero.

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• Bit 0 – TWIE: TWI Interrupt Enable

When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be acti-

vated for as long as the TWINT Flag is high.

19.6.3 TWI Status Register – TWSR

• Bits 7..3 – TWS: TWI Status

These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different status

codes are described later in this section. Note that the value read from TWSR contains both the

5-bit status value and the 2-bit prescaler value. The application designer should mask the pres-

caler bits to zero when checking the Status bits. This makes status checking independent of

prescaler setting. This approach is used in this datasheet, unless otherwise noted.

• Bit 2 – Res: Reserved Bit

This bit is reserved and will always read as zero.

• Bits 1..0 – TWPS: TWI Prescaler Bits

These bits can be read and written, and control the bit rate prescaler.

To calculate bit rates, see “Bit Rate Generator Unit” on page 209. The value of TWPS1..0 is

used in the equation.

19.6.4 TWI Data Register – TWDR

In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR

contains the last byte received. It is writable while the TWI is not in the process of shifting a byte.

This occurs when the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Regis-

ter cannot be initialized by the user before the first interrupt occurs. The data in TWDR remains

stable as long as TWINT is set. While data is shifted out, data on the bus is simultaneously

shifted in. TWDR always contains the last byte present on the bus, except after a wake up from

a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined.

Bit 76543210

TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR

Read/Write RRRRRRR/WR/W

Initial Value11111000

Table 19-2. TWI Bit Rate Prescaler

TWPS1 TWPS0 Prescaler Value

001

014

1016

1164

Bit 76543210

TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value11111111

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In the case of a lost bus arbitration, no data is lost in the transition from Master to Slave. Han-

dling of the ACK bit is controlled automatically by the TWI logic, the CPU cannot access the ACK

bit directly.

• Bits 7..0 – TWD: TWI Data Register

These eight bits constitute the next data byte to be transmitted, or the latest data byte received

on the 2-wire Serial Bus.

19.6.5 TWI (Slave) Address Register – TWAR

The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of

TWAR) to which the TWI will respond when programmed as a Slave Transmitter or Receiver,

and not needed in the Master modes. In multi master systems, TWAR must be set in masters

which can be addressed as Slaves by other Masters.

The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an

associated address comparator that looks for the slave address (or general call address if

enabled) in the received serial address. If a match is found, an interrupt request is generated.

• Bits 7..1 – TWA: TWI (Slave) Address Register

These seven bits constitute the slave address of the TWI unit.

• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit

If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus.

19.6.6 TWI (Slave) Address Mask Register – TWAMR

• Bits 7..1 – TWAM: TWI Address Mask

The TWAMR can be loaded with a 7-bit Salve Address mask. Each of the bits in TWAMR can

mask (disable) the corresponding address bits in the TWI Address Register (TWAR). If the mask

bit is set to one then the address match logic ignores the compare between the incoming

address bit and the corresponding bit in TWAR. Figure 19-10 shown the address match logic in

detail.

Bit 76543210

TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value11111110

Bit 76543210

TWAM[6:0] – TWAMR

Read/Write R/W R/W R/W R/W R/W R/W R/W R

Initial Value00000000

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Figure 19-10. TWI Address Match Logic, Block Diagram

• Bit 0 – Res: Reserved Bit

This bit is an unused bit in the ATmega48/88/168, and will always read as zero.

19.7 Using the TWI

The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like

reception of a byte or transmission of a START condition. Because the TWI is interrupt-based,

the application software is free to carry on other operations during a TWI byte transfer. Note that

the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in

SREG allow the application to decide whether or not assertion of the TWINT Flag should gener-

ate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in

order to detect actions on the TWI bus.

When the TWINT Flag is asserted, the TWI has finished an operation and awaits application

response. In this case, the TWI Status Register (TWSR) contains a value indicating the current

state of the TWI bus. The application software can then decide how the TWI should behave in

the next TWI bus cycle by manipulating the TWCR and TWDR Registers.

Figure 19-11 is a simple example of how the application can interface to the TWI hardware. In

this example, a Master wishes to transmit a single data byte to a Slave. This description is quite

abstract, a more detailed explanation follows later in this section. A simple code example imple-

menting the desired behavior is also presented.

Address

Match

Address Bit Comparator 0

Address Bit Comparator 6..1

TWAR0

TWAMR0

Address

Bit 0

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Figure 19-11. Interfacing the Application to the TWI in a Typical Transmission

1. The first step in a TWI transmission is to transmit a START condition. This is done by

writing a specific value into TWCR, instructing the TWI hardware to transmit a START

condition. Which value to write is described later on. However, it is important that the

TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI

will not start any operation as long as the TWINT bit in TWCR is set. Immediately after

the application has cleared TWINT, the TWI will initiate transmission of the START

condition.

2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the START condition has success-

fully been sent.

3. The application software should now examine the value of TWSR, to make sure that the

START condition was successfully transmitted. If TWSR indicates otherwise, the appli-

cation software might take some special action, like calling an error routine. Assuming

that the status code is as expected, the application must load SLA+W into TWDR.

Remember that TWDR is used both for address and data. After TWDR has been

loaded with the desired SLA+W, a specific value must be written to TWCR, instructing

the TWI hardware to transmit the SLA+W present in TWDR. Which value to write is

described later on. However, it is important that the TWINT bit is set in the value written.

Writing a one to TWINT clears the flag. The TWI will not start any operation as long as

the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT,

the TWI will initiate transmission of the address packet.

4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the address packet has success-

fully been sent. The status code will also reflect whether a Slave acknowledged the

packet or not.

5. The application software should now examine the value of TWSR, to make sure that the

address packet was successfully transmitted, and that the value of the ACK bit was as

expected. If TWSR indicates otherwise, the application software might take some spe-

cial action, like calling an error routine. Assuming that the status code is as expected,

the application must load a data packet into TWDR. Subsequently, a specific value

must be written to TWCR, instructing the TWI hardware to transmit the data packet

present in TWDR. Which value to write is described later on. However, it is important

that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag.

START SLA+W A Data A STOP

1. Application

writes to TWCR to

initiate

transmission of

START

2. TWINT set.

Status code indicates

START condition sent

4. TWINT set.

Status code indicates

SLA+W sent, ACK

received

6. TWINT set.

Status code indicates

data sent, ACK received

3. Check TWSR to see if START was

sent. Application loads SLA+W into

TWDR, and loads appropriate control

signals into TWCR, makin sure that

TWINT is written to one,

and TWSTA is written to zero.

5. Check TWSR to see if SLA+W was

sent and ACK received.

Application loads data into TWDR, and

loads appropriate control signals into

TWCR, making sure that TWINT is

written to one

7. Check TWSR to see if data was sent

and ACK received.

Application loads appropriate control

signals to send STOP into TWCR,

making sure that TWINT is written to one

TWI bus

Indicates

TWINT set

Application

Action

TWI

Hardware

Action

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The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immedi-

ately after the application has cleared TWINT, the TWI will initiate transmission of the

data packet.

6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the data packet has successfully

been sent. The status code will also reflect whether a Slave acknowledged the packet

or not.

7. The application software should now examine the value of TWSR, to make sure that the

data packet was successfully transmitted, and that the value of the ACK bit was as

expected. If TWSR indicates otherwise, the application software might take some spe-

cial action, like calling an error routine. Assuming that the status code is as expected,

the application must write a specific value to TWCR, instructing the TWI hardware to

transmit a STOP condition. Which value to write is described later on. However, it is

important that the TWINT bit is set in the value written. Writing a one to TWINT clears

the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set.

Immediately after the application has cleared TWINT, the TWI will initiate transmission

of the STOP condition. Note that TWINT is NOT set after a STOP condition has been

sent.

Even though this example is simple, it shows the principles involved in all TWI transmissions.

These can be summarized as follows:

• When the TWI has finished an operation and expects application response, the TWINT Flag

is set. The SCL line is pulled low until TWINT is cleared.

• When the TWINT Flag is set, the user must update all TWI Registers with the value relevant

for the next TWI bus cycle. As an example, TWDR must be loaded with the value to be

transmitted in the next bus cycle.

• After all TWI Register updates and other pending application software tasks have been

completed, TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a

one to TWINT clears the flag. The TWI will then commence executing whatever operation

was specified by the TWCR setting.

In the following an assembly and C implementation of the example is given. Note that the code

below assumes that several definitions have been made, for example by using include-files.

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Assembly Code Example C Example Comments

ldi r16,

(1<<TWINT)|(1<<TWSTA)|

(1<<TWEN)

out TWCR, r16

TWCR = (1<<TWINT)|(1<<TWSTA)|

(1<<TWEN) Send START condition

wait1:

in r16,TWCR

sbrs r16,TWINT

rjmp wait1

while (!(TWCR & (1<<TWINT)))

;Wait for TWINT Flag set. This

indicates that the START

condition has been transmitted

in r16,TWSR

andi r16, 0xF8

cpi r16, START

brne ERROR

if ((TWSR & 0xF8) != START)

ERROR();

Check value of TWI Status

status different from START go to

ERROR

ldi r16, SLA_W

out TWDR, r16

ldi r16, (1<<TWINT) |

(1<<TWEN)

out TWCR, r16

TWDR = SLA_W;

TWCR = (1<<TWINT) |

(1<<TWEN);

Load SLA_W into TWDR

TWCR to start transmission of

address

wait2:

in r16,TWCR

sbrs r16,TWINT

rjmp wait2

while (!(TWCR & (1<<TWINT)))

;

Wait for TWINT Flag set. This

indicates that the SLA+W has

been transmitted, and

ACK/NACK has been received.

in r16,TWSR

andi r16, 0xF8

cpi r16, MT_SLA_ACK

brne ERROR

if ((TWSR & 0xF8) !=

MT_SLA_ACK)

ERROR();

Check value of TWI Status

status different from

MT_SLA_ACK go to ERROR

ldi r16, DATA

out TWDR, r16

ldi r16, (1<<TWINT) |

(1<<TWEN)

out TWCR, r16

TWDR = DATA;

TWCR = (1<<TWINT) |

(1<<TWEN);

Load DATA into TWDR Register.

Clear TWINT bit in TWCR to

start transmission of data

wait3:

in r16,TWCR

sbrs r16,TWINT

rjmp wait3

while (!(TWCR & (1<<TWINT)))

;

Wait for TWINT Flag set. This

indicates that the DATA has been

transmitted, and ACK/NACK has

been received.

in r16,TWSR

andi r16, 0xF8

cpi r16, MT_DATA_ACK

brne ERROR

if ((TWSR & 0xF8) !=

MT_DATA_ACK)

ERROR();

Check value of TWI Status

status different from

MT_DATA_ACK go to ERROR

ldi r16,

(1<<TWINT)|(1<<TWEN)|

(1<<TWSTO)

out TWCR, r16

TWCR = (1<<TWINT)|(1<<TWEN)|

(1<<TWSTO); Transmit STOP condition

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19.8 Transmission Modes

The TWI can operate in one of four major modes. These are named Master Transmitter (MT),

Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these

modes can be used in the same application. As an example, the TWI can use MT mode to write

data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters

are present in the system, some of these might transmit data to the TWI, and then SR mode

would be used. It is the application software that decides which modes are legal.

The following sections describe each of these modes. Possible status codes are described

along with figures detailing data transmission in each of the modes. These figures contain the

following abbreviations:

S: START condition

Rs: REPEATED START condition

R: Read bit (high level at SDA)

W: Write bit (low level at SDA)

A: Acknowledge bit (low level at SDA)

A: Not acknowledge bit (high level at SDA)

Data: 8-bit data byte

P: STOP condition

SLA: Slave Address

In Figure 19-13 to Figure 19-19, circles are used to indicate that the TWINT Flag is set. The

numbers in the circles show the status code held in TWSR, with the prescaler bits masked to

zero. At these points, actions must be taken by the application to continue or complete the TWI

transfer. The TWI transfer is suspended until the TWINT Flag is cleared by software.

When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate soft-

ware action. For each status code, the required software action and details of the following serial

transfer are given in Table 19-3 to Table 19-6. Note that the prescaler bits are masked to zero in

these tables.

19.8.1 Master Transmitter Mode

In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver

(see Figure 19-12). In order to enter a Master mode, a START condition must be transmitted.

The format of the following address packet determines whether Master Transmitter or Master

Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is trans-

mitted, MR mode is entered. All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

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Figure 19-12. Data Transfer in Master Transmitter Mode

A START condition is sent by writing the following value to TWCR:

TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to trans-

mit a START condition and TWINT must be written to one to clear the TWINT Flag. The TWI will

then test the 2-wire Serial Bus and generate a START condition as soon as the bus becomes

free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and the

status code in TWSR will be 0x08 (see Table 19-3). In order to enter MT mode, SLA+W must be

transmitted. This is done by writing SLA+W to TWDR. Thereafter the TWINT bit should be

cleared (by writing it to one) to continue the transfer. This is accomplished by writing the follow-

ing value to TWCR:

When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are 0x18, 0x20, or 0x38. The appropriate action to be taken for each of these status codes

is detailed in Table 19-3.

When SLA+W has been successfully transmitted, a data packet should be transmitted. This is

done by writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not,

the access will be discarded, and the Write Collision bit (TWWC) will be set in the TWCR Regis-

ter. After updating TWDR, the TWINT bit should be cleared (by writing it to one) to continue the

transfer. This is accomplished by writing the following value to TWCR:

This scheme is repeated until the last byte has been sent and the transfer is ended by generat-

ing a STOP condition or a repeated START condition. A STOP condition is generated by writing

the following value to TWCR:

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X01X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

Device 3 Device n

SDA

SCL

........ R1 R2

VCC

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After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same

Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables

the Master to switch between Slaves, Master Transmitter mode and Master Receiver mode with-

out losing control of the bus.

Table 19-3. Status codes for Master Transmitter Mode

Status

Code

(TWSR)

Prescaler

Bits

are 0

Status of the 2-wire Se-

rial Bus and 2-wire Seri-

al Interface Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from

TWDR

To TWCR

TWI

0x08 A START condition has

been transmitted

Load SLA+W 0 0 1 X SLA+W will be transmitted;

ACK or NOT ACK will be received

0x10 A repeated START

condition has been

transmitted

Load SLA+W

Load SLA+R

SLA+W will be transmitted;

ACK or NOT ACK will be received

SLA+R will be transmitted;

Logic will switch to Master Receiver mode

0x18 SLA+W has been

transmitted;

ACK has been received

Load data

byte or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be transmitted and ACK or

NOT ACK will be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

0x20 SLA+W has been

transmitted;

NOT ACK has been re-

ceived

Load data

byte or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be transmitted and ACK or

NOT ACK will be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

0x28 Data byte has been

transmitted;

ACK has been received

Load data

byte or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be transmitted and ACK or

NOT ACK will be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

0x30 Data byte has been

transmitted;

NOT ACK has been re-

ceived

Load data

byte or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be transmitted and ACK or

NOT ACK will be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

0x38 Arbitration lost in

SLA+W or data bytes

No TWDR ac-

tion or

No TWDR ac-

tion

2-wire Serial Bus will be released and not

addressed Slave mode entered

A START condition will be transmitted

when the bus becomes free

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Figure 19-13. Formats and States in the Master Transmitter Mode

19.8.2 Master Receiver Mode

In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter

(Slave see Figure 19-14). In order to enter a Master mode, a START condition must be transmit-

ted. The format of the following address packet determines whether Master Transmitter or

Master Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R

is transmitted, MR mode is entered. All the status codes mentioned in this section assume that

the prescaler bits are zero or are masked to zero.

S SLA W A DATA A P

$08 $18 $28

R SLA W

$10

$20

$30

A or A

$38

Other master

continues

A or A

$38

Other master

continues

$68

Other master

continues

$78 $B0

To corresponding

states in slave mode

Successfull

transmission

to a slave

receiver

Next transfer

started with a

repeated start

condition

Not acknowledge

received after the

slave address

Not acknowledge

received after a data

byte

Arbitration lost in slave

address or data byte

Arbitration lost and

addressed as slave

DATA A

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the 2-Wire Serial Bus. The

rescaler bits are zero or masked to zero

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Figure 19-14. Data Transfer in Master Receiver Mode

A START condition is sent by writing the following value to TWCR:

TWEN must be written to one to enable the 2-wire Serial Interface, TWSTA must be written to

one to transmit a START condition and TWINT must be set to clear the TWINT Flag. The TWI

will then test the 2-wire Serial Bus and generate a START condition as soon as the bus

becomes free. After a START condition has been transmitted, the TWINT Flag is set by hard-

ware, and the status code in TWSR will be 0x08 (See Table 19-3). In order to enter MR mode,

SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter the TWINT bit

should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing

the following value to TWCR:

When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are 0x38, 0x40, or 0x48. The appropriate action to be taken for each of these status codes

is detailed in Table 19-4. Received data can be read from the TWDR Register when the TWINT

Flag is set high by hardware. This scheme is repeated until the last byte has been received.

After the last byte has been received, the MR should inform the ST by sending a NACK after the

last received data byte. The transfer is ended by generating a STOP condition or a repeated

START condition. A STOP condition is generated by writing the following value to TWCR:

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X01X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3 Device n

SDA

SCL

........

R1 R2

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After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same

Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables

the Master to switch between Slaves, Master Transmitter mode and Master Receiver mode with-

out losing control over the bus.

Table 19-4. Status codes for Master Receiver Mode

Status

Code

(TWSR)

Prescaler

Bits

are 0

Status of the 2-wire Se-

rial Bus and 2-wire Seri-

al Interface Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from

TWDR

To TWCR

TWI

0x08 A START condition has

been transmitted

Load SLA+R 0 0 1 X SLA+R will be transmitted

ACK or NOT ACK will be received

0x10 A repeated START

condition has been

transmitted

Load SLA+R

Load SLA+W

SLA+R will be transmitted

ACK or NOT ACK will be received

SLA+W will be transmitted

Logic will switch to Master Transmitter

mode

0x38 Arbitration lost in

SLA+R or NOT ACK bit

No TWDR ac-

tion or

No TWDR ac-

tion

2-wire Serial Bus will be released and not

addressed Slave mode will be entered

A START condition will be transmitted

when the bus

becomes free

0x40 SLA+R has been trans-

mitted;

ACK has been received

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will be

returned

0x48 SLA+R has been trans-

mitted;

NOT ACK has been re-

ceived

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

0x50 Data byte has been re-

ceived;

ACK has been returned

Read data

byte or

Read data

byte

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will be

returned

0x58 Data byte has been re-

ceived;

NOT ACK has been re-

turned

Read data

byte or

Read data

byte or

Read data

byte

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START con-

dition will be transmitted and TWSTO Flag

will be reset

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Figure 19-15. Formats and States in the Master Receiver Mode

19.8.3 Slave Receiver Mode

In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter

(see Figure 19-16). All the status codes mentioned in this section assume that the prescaler bits

are zero or are masked to zero.

Figure 19-16. Data transfer in Slave Receiver mode

S SLA R A DATA A

$08 $40 $50

SLA R

$10

$48

A or A

$38

Other master

continues

$38

Other master

continues

$68

Other master

continues

$78 $B0

To corresponding

states in slave mode

Successfull

reception

from a slave

receiver

Next transfer

started with a

repeated start

condition

Not acknowledge

received after the

slave address

Arbitration lost in slave

address or data byte

Arbitration lost and

addressed as slave

DATA A

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the 2-Wire Serial Bus. The

rescaler bits are zero or masked to zero

PDATA A

$58

Device 3 Device n

SDA

SCL

........ R1 R2

Device 2

MASTER

TRANSMITTER

Device 1

SLAVE

RECEIVER

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To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:

The upper 7 bits are the address to which the 2-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address (0x00),

otherwise it will ignore the general call address.

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable

the acknowledgement of the device’s own slave address or the general call address. TWSTA

and TWSTO must be written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own

slave address (or the general call address if enabled) followed by the data direction bit. If the

direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode is entered. After

its own slave address and the write bit have been received, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. The appropriate action to be taken for each status code is detailed in Table 19-5.

The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master

mode (see states 0x68 and 0x78).

If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA

after the next received data byte. This can be used to indicate that the Slave is not able to

receive any more bytes. While TWEA is zero, the TWI does not acknowledge its own slave

address. However, the 2-wire Serial Bus is still monitored and address recognition may resume

at any time by setting TWEA. This implies that the TWEA bit may be used to temporarily isolate

the TWI from the 2-wire Serial Bus.

In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA

bit is set, the interface can still acknowledge its own slave address or the general call address by

using the 2-wire Serial Bus clock as a clock source. The part will then wake up from sleep and

the TWI will hold the SCL clock low during the wake up and until the TWINT Flag is cleared (by

writing it to one). Further data reception will be carried out as normal, with the AVR clocks run-

ning as normal. Observe that if the AVR is set up with a long start-up time, the SCL line may be

held low for a long time, blocking other data transmissions.

Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present

on the bus when waking up from these Sleep modes.

TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

value Device’s Own Slave Address

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 0100010 X

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Table 19-5. Status Codes for Slave Receiver Mode

Status

Code

(TWSR)

Prescaler

Bits

are 0

Status of the 2-wire Seri-

al Bus and 2-wire Serial

Interface Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from

TWDR

To TWCR

TWI

0x60 Own SLA+W has been

received;

ACK has been returned

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x68 Arbitration lost in

SLA+R/W as Master;

own SLA+W has been

received; ACK has been

returned

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x70 General call address has

been

received; ACK has been

returned

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x78 Arbitration lost in

SLA+R/W as Master;

General call address has

been received; ACK has

been

returned

No TWDR ac-

tion or

No TWDR ac-

tion

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x80 Previously addressed

with own SLA+W; data

has been received; ACK

has been returned

Read data

byte or

Read data

byte

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x88 Previously addressed

with own SLA+W; data

has been received; NOT

ACK has been returned

Read data

byte or

Read data

byte or

Read data

byte or

Read data

byte

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA;

a START condition will be transmitted

when the bus

becomes free

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted

when the bus

becomes free

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0x90 Previously addressed

with

general call; data has

been received; ACK has

been returned

Read data

byte or

Read data

byte

Data byte will be received and NOT ACK

will be

returned

Data byte will be received and ACK will

be returned

0x98 Previously addressed

with

general call; data has

been

received; NOT ACK has

been

returned

Read data

byte or

Read data

byte or

Read data

byte or

Read data

byte

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA;

a START condition will be transmitted

when the bus

becomes free

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted

when the bus

becomes free

0xA0 A STOP condition or re-

peated START condition

has been

received while still ad-

dressed as Slave

No action 0

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA;

a START condition will be transmitted

when the bus

becomes free

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted

when the bus

becomes free

Table 19-5. Status Codes for Slave Receiver Mode

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Figure 19-17. Formats and States in the Slave Receiver Mode

19.8.4 Slave Transmitter Mode

In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver

(see Figure 19-18). All the status codes mentioned in this section assume that the prescaler bits

are zero or are masked to zero.

Figure 19-18. Data Transfer in Slave Transmitter Mode

To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:

S SLA W A DATA A

$60 $80

$88

$68

Reception of the own

slave address and one or

more data bytes. All are

acknowledged

Last data byte received

is not acknowledged

Arbitration lost as master

and addressed as slave

Reception of the general call

address and one or more data

bytes

Last data byte received is

not acknowledged

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the 2-Wire Serial Bus. The

prescaler bits are zero or masked to zero

P or SDATA A

$80 $A0

P or SA

ADATAA

$70 $90

$98

$78

P or SDATA A

$90 $A0

P or SA

General Call

Arbitration lost as master and

addressed as slave by general call

DATA A

Device 3 Device n

SDA

SCL

........ R1 R2

Device 2

MASTER

RECEIVER

Device 1

SLAVE

TRANSMITTER

TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

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The upper seven bits are the address to which the 2-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address (0x00),

otherwise it will ignore the general call address.

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable

the acknowledgement of the device’s own slave address or the general call address. TWSTA

and TWSTO must be written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own

slave address (or the general call address if enabled) followed by the data direction bit. If the

direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode is entered. After

its own slave address and the write bit have been received, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. The appropriate action to be taken for each status code is detailed in Table 19-6.

The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the

Master mode (see state 0xB0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the trans-

fer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master Receiver

transmits a NACK or ACK after the final byte. The TWI is switched to the not addressed Slave

mode, and will ignore the Master if it continues the transfer. Thus the Master Receiver receives

all “1” as serial data. State 0xC8 is entered if the Master demands additional data bytes (by

transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero and expect-

ing NACK from the Master).

While TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire

Serial Bus is still monitored and address recognition may resume at any time by setting TWEA.

This implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial

Bus.

In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA

bit is set, the interface can still acknowledge its own slave address or the general call address by

using the 2-wire Serial Bus clock as a clock source. The part will then wake up from sleep and

the TWI will hold the SCL clock will low during the wake up and until the TWINT Flag is cleared

(by writing it to one). Further data transmission will be carried out as normal, with the AVR clocks

running as normal. Observe that if the AVR is set up with a long start-up time, the SCL line may

be held low for a long time, blocking other data transmissions.

Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present

on the bus when waking up from these sleep modes.

value Device’s Own Slave Address

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 0100010 X

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Table 19-6. Status Codes for Slave Transmitter Mode

Status

Code

(TWSR)

Prescal-

er Bits

are 0

Status of the 2-wire Seri-

al Bus and 2-wire Serial

Interface Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from

TWDR

To TWCR

TWI

0xA8 Own SLA+R has been

received;

ACK has been returned

Load data

byte or

Load data

byte

Last data byte will be transmitted and

NOT ACK should be received

Data byte will be transmitted and ACK

should be received

0xB0 Arbitration lost in

SLA+R/W as Master;

own SLA+R has been

received; ACK has been

returned

Load data

byte or

Load data

byte

Last data byte will be transmitted and

NOT ACK should be received

Data byte will be transmitted and ACK

should be received

0xB8 Data byte in TWDR has

been

transmitted; ACK has

been

received

Load data

byte or

Load data

byte

Last data byte will be transmitted and

NOT ACK should be received

Data byte will be transmitted and ACK

should be received

0xC0 Data byte in TWDR has

been

transmitted; NOT ACK

has been

received

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA;

a START condition will be transmitted

when the bus

becomes free

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted

when the bus

becomes free

0xC8 Last data byte in TWDR

has been transmitted

(TWEA = “0”); ACK has

been received

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion or

No TWDR ac-

tion

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave

mode;

no recognition of own SLA or GCA;

a START condition will be transmitted

when the bus

becomes free

Switched to the not addressed Slave

mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted

when the bus

becomes free

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Figure 19-19. Formats and States in the Slave Transmitter Mode

19.8.5 Miscellaneous States

There are two status codes that do not correspond to a defined TWI state, see Table 19-7.

Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not

set. This occurs between other states, and when the TWI is not involved in a serial transfer.

Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus

error occurs when a START or STOP condition occurs at an illegal position in the format frame.

Examples of such illegal positions are during the serial transfer of an address byte, a data byte,

or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the

TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the

TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no other bits in

TWCR are affected). The SDA and SCL lines are released, and no STOP condition is

transmitted.

S SLA R A DATA A

$A8 $B8

$B0

Reception of the own

slave address and one or

more data bytes

Last data byte transmitted.

Switched to not addressed

slave (TWEA = '0')

Arbitration lost as master

and addressed as slave

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the 2-Wire Serial Bus. The

prescaler bits are zero or masked to zero

P or SDATA

$C0

DATA A

$C8

P or SAll 1's

Table 19-7. Miscellaneous States

Status

Code

(TWSR)

Prescaler

Bits

are 0

Status of the 2-wire Se-

rial Bus and 2-wire Seri-

al Interface Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from

TWDR

To TWCR

TWI

0xF8 No relevant state infor-

mation available;

TWINT = “0”

No TWDR ac-

tion

No TWCR action Wait or proceed current transfer

0x00 Bus error due to an ille-

gal START or STOP

condition

No TWDR ac-

tion

0 1 1 X Only the internal hardware is affected, no

STOP condition is sent on the bus. In all

cases, the bus is released and TWSTO is

cleared.

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19.8.6 Combining Several TWI Modes

In some cases, several TWI modes must be combined in order to complete the desired action.

Consider for example reading data from a serial EEPROM. Typically, such a transfer involves

the following steps:

1. The transfer must be initiated.

2. The EEPROM must be instructed what location should be read.

3. The reading must be performed.

4. The transfer must be finished.

Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct

the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data

must be read from the Slave, implying the use of the MR mode. Thus, the transfer direction must

be changed. The Master must keep control of the bus during all these steps, and the steps

should be carried out as an atomical operation. If this principle is violated in a multi master sys-

tem, another Master can alter the data pointer in the EEPROM between steps 2 and 3, and the

Master will read the wrong data location. Such a change in transfer direction is accomplished by

transmitting a REPEATED START between the transmission of the address byte and reception

of the data. After a REPEATED START, the Master keeps ownership of the bus. The following

figure shows the flow in this transfer.

Figure 19-20. Combining Several TWI Modes to Access a Serial EEPROM

19.9 Multi-master Systems and Arbitration

If multiple masters are connected to the same bus, transmissions may be initiated simultane-

ously by one or more of them. The TWI standard ensures that such situations are handled in

such a way that one of the masters will be allowed to proceed with the transfer, and that no data

will be lost in the process. An example of an arbitration situation is depicted below, where two

masters are trying to transmit data to a Slave Receiver.

Figure 19-21. An Arbitration Example

Master Transmitter Master Receiver

S = START Rs = REPEATED START P = STOP

Transmitted from master to slave Transmitted from slave to master

S SLA+W A ADDRESS A Rs SLA+R A DATA A P

Device 1

MASTER

TRANSMITTER

Device 2

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device n

SDA

SCL

........ R1 R2

VCC

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Several different scenarios may arise during arbitration, as described below:

• Two or more masters are performing identical communication with the same Slave. In this

case, neither the Slave nor any of the masters will know about the bus contention.

• Two or more masters are accessing the same Slave with different data or direction bit. In this

case, arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters

trying to output a one on SDA while another Master outputs a zero will lose the arbitration.

Losing masters will switch to not addressed Slave mode or wait until the bus is free and

transmit a new START condition, depending on application software action.

• Two or more masters are accessing different slaves. In this case, arbitration will occur in the

SLA bits. Masters trying to output a one on SDA while another Master outputs a zero will lose

the arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are

being addressed by the winning Master. If addressed, they will switch to SR or ST mode,

depending on the value of the READ/WRITE bit. If they are not being addressed, they will

switch to not addressed Slave mode or wait until the bus is free and transmit a new START

condition, depending on application software action.

This is summarized in Figure 19-22. Possible status values are given in circles.

Figure 19-22. Possible Status Codes Caused by Arbitration

20. Analog Comparator

The Analog Comparator compares the input values on the positive pin AIN0 and negative pin

AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin

AIN1, the Analog Comparator output, ACO, is set. The comparator’s output can be set to trigger

the Timer/Counter1 Input Capture function. In addition, the comparator can trigger a separate

interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on com-

parator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is

shown in Figure 20-1.

The Power Reduction ADC bit, PRADC, in “Power Reduction Register - PRR” on page 39 must

be disabled by writing a logical zero to be able to use the ADC input MUX.

Own

Address / General Call

received

Arbitration lost in SLA

TWI bus will be released and not addressed slave mode will be entered

A START condition will be transmitted when the bus becomes free

Arbitration lost in Data

Direction

Yes

Write

Data byte will be received and NOT ACK will be returned

Data byte will be received and ACK will be returned

Last data byte will be transmitted and NOT ACK should be received

Data byte will be transmitted and ACK should be received

Read

68/78

SLASTART Data STOP

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Figure 20-1. Analog Comparator Block Diagram(2)

Notes: 1. See Table 20-2 on page 236.

2. Refer to Figure 1-1 on page 2 and Table 10-9 on page 77 for Analog Comparator pin

placement.

20.0.1 ADC Control and Status Register B – ADCSRB

• Bit 6 – ACME: Analog Comparator Multiplexer Enable

When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the

ADC multiplexer selects the negative input to the Analog Comparator. When this bit is written

logic zero, AIN1 is applied to the negative input of the Analog Comparator. For a detailed

description of this bit, see “Analog Comparator Multiplexed Input” on page 236.

20.0.2 Analog Comparator Control and Status Register – ACSR

• Bit 7 – ACD: Analog Comparator Disable

When this bit is written logic one, the power to the Analog Comparator is switched off. This bit

can be set at any time to turn off the Analog Comparator. This will reduce power consumption in

Active and Idle mode. When changing the ACD bit, the Analog Comparator Interrupt must be

disabled by clearing the ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is

changed.

• Bit 6 – ACBG: Analog Comparator Bandgap Select

When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog

Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Compar-

ator. See “Internal Voltage Reference” on page 46.

ACBG

BANDGAP

REFERENCE

ADC MULTIPLEXER

OUTPUT

ACME

ADEN

(1)

Bit 7 6543210

– ACME –––ADTS2 ADTS1 ADTS0 ADCSRB

Read/Write R R/W R R R R/W R/W R/W

Initial Value0 0000000

Bit 76543210

ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 ACSR

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value00N/A00000

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• Bit 5 – ACO: Analog Comparator Output

The output of the Analog Comparator is synchronized and then directly connected to ACO. The

synchronization introduces a delay of 1 - 2 clock cycles.

• Bit 4 – ACI: Analog Comparator Interrupt Flag

This bit is set by hardware when a comparator output event triggers the interrupt mode defined

by ACIS1 and ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set

and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding inter-

rupt handling vector. Alternatively, ACI is cleared by writing a logic one to the flag.

• Bit 3 – ACIE: Analog Comparator Interrupt Enable

When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Com-

parator interrupt is activated. When written logic zero, the interrupt is disabled.

• Bit 2 – ACIC: Analog Comparator Input Capture Enable

When written logic one, this bit enables the input capture function in Timer/Counter1 to be trig-

gered by the Analog Comparator. The comparator output is in this case directly connected to the

input capture front-end logic, making the comparator utilize the noise canceler and edge select

features of the Timer/Counter1 Input Capture interrupt. When written logic zero, no connection

between the Analog Comparator and the input capture function exists. To make the comparator

trigger the Timer/Counter1 Input Capture interrupt, the ICIE1 bit in the Timer Interrupt Mask

• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select

These bits determine which comparator events that trigger the Analog Comparator interrupt. The

different settings are shown in Table 20-1.

When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by

clearing its Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the

bits are changed.

Table 20-1. ACIS1/ACIS0 Settings

ACIS1 ACIS0 Interrupt Mode

0 0 Comparator Interrupt on Output Toggle.

01Reserved

1 0 Comparator Interrupt on Falling Output Edge.

1 1 Comparator Interrupt on Rising Output Edge.

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20.1 Analog Comparator Multiplexed Input

It is possible to select any of the ADC7..0 pins to replace the negative input to the Analog Com-

parator. The ADC multiplexer is used to select this input, and consequently, the ADC must be

switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in

ADCSRB) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2..0 in ADMUX

select the input pin to replace the negative input to the Analog Comparator, as shown in Table

20-2. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog

Comparator.

20.1.1 Digital Input Disable Register 1 – DIDR1

• Bit 7..2 – Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bit 1, 0 – AIN1D, AIN0D: AIN1, AIN0 Digital Input Disable

When this bit is written logic one, the digital input buffer on the AIN1/0 pin is disabled. The corre-

sponding PIN Register bit will always read as zero when this bit is set. When an analog signal is

applied to the AIN1/0 pin and the digital input from this pin is not needed, this bit should be writ-

ten logic one to reduce power consumption in the digital input buffer.

Table 20-2. Analog Comparator Multiplexed Input

ACME ADEN MUX2..0 Analog Comparator Negative Input

0 x xxx AIN1

1 1 xxx AIN1

10000ADC0

10001ADC1

10010ADC2

10011ADC3

10100ADC4

10101ADC5

10110ADC6

10111ADC7

Bit 76543210

––––––AIN1DAIN0DDIDR1

Read/Write RRRRRRR/WR/W

Initial Value00000000

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21. Analog-to-Digital Converter

21.1 Features •10-bit Resolution

•0.5 LSB Integral Non-linearity

•± 2 LSB Absolute Accuracy

•13 - 260 µs Conversion Time

•Up to 15 kSPS at Maximum Resolution

•6 Multiplexed Single Ended Input Channels

•2 Additional Multiplexed Single Ended Input Channels (TQFP and QFN Package only)

•Optional Left Adjustment for ADC Result Readout

•0 - VCC ADC Input Voltage Range

•Selectable 1.1V ADC Reference Voltage

•Free Running or Single Conversion Mode

•Interrupt on ADC Conversion Complete

•Sleep Mode Noise Canceler

The ATmega48/88/168 features a 10-bit successive approximation ADC. The ADC is connected

to an 8-channel Analog Multiplexer which allows eight single-ended voltage inputs constructed

from the pins of Port A. The single-ended voltage inputs refer to 0V (GND).

The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is

held at a constant level during conversion. A block diagram of the ADC is shown in Figure 21-1.

The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ±

0.3V from VCC. See the paragraph “ADC Noise Canceler” on page 244 on how to connect this

pin.

Internal reference voltages of nominally 1.1V or AVCC are provided On-chip. The voltage refer-

ence may be externally decoupled at the AREF pin by a capacitor for better noise performance.

The Power Reduction ADC bit, PRADC, in “Power Reduction Register - PRR” on page 39 must

be disabled by writing a logical zero to enable the ADC.

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Figure 21-1. Analog to Digital Converter Block Schematic Operation

The ADC converts an analog input voltage to a 10-bit digital value through successive approxi-

mation. The minimum value represents GND and the maximum value represents the voltage on

the AREF pin minus 1 LSB. Optionally, AVCC or an internal 1.1V reference voltage may be con-

nected to the AREF pin by writing to the REFSn bits in the ADMUX Register. The internal

voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve

noise immunity.

The analog input channel is selected by writing to the MUX bits in ADMUX. Any of the ADC input

pins, as well as GND and a fixed bandgap voltage reference, can be selected as single ended

inputs to the ADC. The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Volt-

age reference and input channel selections will not go into effect until ADEN is set. The ADC

does not consume power when ADEN is cleared, so it is recommended to switch off the ADC

before entering power saving sleep modes.

ADC CONVERSION

COMPLETE IRQ

8-BIT DATA BUS

15 0

ADC MULTIPLEXER

SELECT (ADMUX) ADC CTRL. & STATUS

(ADCH/ADCL)

MUX2

ADIE

ADFR

ADSC

ADEN

ADIF ADIF

MUX1

MUX0

ADPS0

ADPS1

ADPS2

MUX3

CONVERSION LOGIC

10-BIT DAC

SAMPLE & HOLD

COMPARATOR

INTERNAL 1.1V

REFERENCE

MUX DECODER

AVCC

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADC1

ADC0

REFS0

REFS1

ADLAR

CHANNEL SELECTION

ADC[9:0]

ADC MULTIPLEXER

OUTPUT

AREF

BANDGAP

REFERENCE

PRESCALER

GND

INPUT

MUX

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The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and

ADCL. By default, the result is presented right adjusted, but can optionally be presented left

adjusted by setting the ADLAR bit in ADMUX.

If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read

ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data

Registers belongs to the same conversion. Once ADCL is read, ADC access to Data Registers

is blocked. This means that if ADCL has been read, and a conversion completes before ADCH is

read, neither register is updated and the result from the conversion is lost. When ADCH is read,

ADC access to the ADCH and ADCL Registers is re-enabled.

The ADC has its own interrupt which can be triggered when a conversion completes. When ADC

access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt

will trigger even if the result is lost.

21.2 Starting a Conversion

A single conversion is started by disabling the Power Reduction ADC bit, PRADC, in “Power

Reduction Register - PRR” on page 39 by writing a logical zero to it and writing a logical one to

the ADC Start Conversion bit, ADSC. This bit stays high as long as the conversion is in progress

and will be cleared by hardware when the conversion is completed. If a different data channel is

selected while a conversion is in progress, the ADC will finish the current conversion before per-

forming the channel change.

Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is

enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is

selected by setting the ADC Trigger Select bits, ADTS in ADCSRB (See description of the ADTS

bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal,

the ADC prescaler is reset and a conversion is started. This provides a method of starting con-

versions at fixed intervals. If the trigger signal still is set when the conversion completes, a new

conversion will not be started. If another positive edge occurs on the trigger signal during con-

version, the edge will be ignored. Note that an Interrupt Flag will be set even if the specific

interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus

be triggered without causing an interrupt. However, the Interrupt Flag must be cleared in order to

trigger a new conversion at the next interrupt event.

Figure 21-2. ADC Auto Trigger Logic

ADSC

ADIF

SOURCE 1

SOURCE n

ADTS[2:0]

CONVERSION

LOGIC

PRESCALER

START CLKADC

.EDGE

DETECTOR

ADATE

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Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon

as the ongoing conversion has finished. The ADC then operates in Free Running mode, con-

stantly sampling and updating the ADC Data Register. The first conversion must be started by

writing a logical one to the ADSC bit in ADCSRA. In this mode the ADC will perform successive

conversions independently of whether the ADC Interrupt Flag, ADIF is cleared or not.

If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to

one. ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be

read as one during a conversion, independently of how the conversion was started.

21.3 Prescaling and Conversion Timing

Figure 21-3. ADC Prescaler

By default, the successive approximation circuitry requires an input clock frequency between 50

kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the

input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.

The ADC module contains a prescaler, which generates an acceptable ADC clock frequency

from any CPU frequency above 100 kHz. The prescaling is set by the ADPS bits in ADCSRA.

The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit

in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously

reset when ADEN is low.

When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion

starts at the following rising edge of the ADC clock cycle.

A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched

on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.

The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conver-

sion and 13.5 ADC clock cycles after the start of an first conversion. When a conversion is

complete, the result is written to the ADC Data Registers, and ADIF is set. In Single Conversion

mode, ADSC is cleared simultaneously. The software may then set ADSC again, and a new

conversion will be initiated on the first rising ADC clock edge.

7-BIT ADC PRESCALER

ADC CLOCK SOURCE

ADPS0

ADPS1

ADPS2

CK/128

CK/2

CK/4

CK/8

CK/16

CK/32

CK/64

Reset

ADEN

START

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When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures

a fixed delay from the trigger event to the start of conversion. In this mode, the sample-and-hold

takes place two ADC clock cycles after the rising edge on the trigger source signal. Three addi-

tional CPU clock cycles are used for synchronization logic.

In Free Running mode, a new conversion will be started immediately after the conversion com-

pletes, while ADSC remains high. For a summary of conversion times, see Table 21-1.

Figure 21-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)

Figure 21-5. ADC Timing Diagram, Single Conversion

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

Sample & Hold

ADIF

ADCH

ADCL

Cycle Number

ADEN

1212

13 14 15 16 17 18 19 20 21 22 23 24 25 1 2

First Conversion Next

Conversion

MUX and REFS

Update

MUX and REFS

Update

Conversion

Complete

123456789 10 11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Sample & Hold

MUX and REFS

Update

Conversion

Complete MUX and REFS

Update

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Figure 21-6. ADC Timing Diagram, Auto Triggered Conversion

Figure 21-7. ADC Timing Diagram, Free Running Conversion

Table 21-1. ADC Conversion Time

Condition

Sample & Hold (Cycles from

Start of Conversion) Conversion Time (Cycles)

First conversion 13.5 25

Normal conversions, single ended 1.5 13

Auto Triggered conversions 2 13.5

1 2 3 4 5 6 7 8 910 11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

Trigger

Source

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Conversion

Complete

Prescaler

Reset

ADATE

Prescaler

Reset

Sample &

Hold

MUX and REFS

Update

11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Conversion

Complete

Sample & Hold

MUX and REFS

Update

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21.4 Changing Channel or Reference Selection

The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary

selection only takes place at a safe point during the conversion. The channel and reference

selection is continuously updated until a conversion is started. Once the conversion starts, the

channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Con-

tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in

ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after

ADSC is written. The user is thus advised not to write new channel or reference selection values

to ADMUX until one ADC clock cycle after ADSC is written.

If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special

care must be taken when updating the ADMUX Register, in order to control which conversion

will be affected by the new settings.

If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the

ADMUX Register is changed in this period, the user cannot tell if the next conversion is based

on the old or the new settings. ADMUX can be safely updated in the following ways:

a. When ADATE or ADEN is cleared.

b. During conversion, minimum one ADC clock cycle after the trigger event.

c. After a conversion, before the Interrupt Flag used as trigger source is cleared.

When updating ADMUX in one of these conditions, the new settings will affect the next ADC

conversion.

21.4.1 ADC Input Channels

When changing channel selections, the user should observe the following guidelines to ensure

that the correct channel is selected:

In Single Conversion mode, always select the channel before starting the conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the conversion to complete before changing the channel selection.

In Free Running mode, always select the channel before starting the first conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the first conversion to complete, and then change the channel

selection. Since the next conversion has already started automatically, the next result will reflect

the previous channel selection. Subsequent conversions will reflect the new channel selection.

21.4.2 ADC Voltage Reference

The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single

ended channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as

either AVCC, internal 1.1V reference, or external AREF pin.

AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is gener-

ated from the internal bandgap reference (VBG) through an internal amplifier. In either case, the

external AREF pin is directly connected to the ADC, and the reference voltage can be made

more immune to noise by connecting a capacitor between the AREF pin and ground. VREF can

also be measured at the AREF pin with a high impedant voltmeter. Note that VREF is a high

impedant source, and only a capacitive load should be connected in a system.

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If the user has a fixed voltage source connected to the AREF pin, the user may not use the other

reference voltage options in the application, as they will be shorted to the external voltage. If no

external voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as ref-

erence selection. The first ADC conversion result after switching reference voltage source may

be inaccurate, and the user is advised to discard this result.

21.5 ADC Noise Canceler

The ADC features a noise canceler that enables conversion during sleep mode to reduce noise

induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC

Noise Reduction and Idle mode. To make use of this feature, the following procedure should be

used:

a. Make sure that the ADC is enabled and is not busy converting. Single Conversion

mode must be selected and the ADC conversion complete interrupt must be

enabled.

b. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion

once the CPU has been halted.

c. If no other interrupts occur before the ADC conversion completes, the ADC inter-

rupt will wake up the CPU and execute the ADC Conversion Complete interrupt

routine. If another interrupt wakes up the CPU before the ADC conversion is com-

plete, that interrupt will be executed, and an ADC Conversion Complete interrupt

request will be generated when the ADC conversion completes. The CPU will

remain in active mode until a new sleep command is executed.

Note that the ADC will not be automatically turned off when entering other sleep modes than Idle

mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-

ing such sleep modes to avoid excessive power consumption.

21.5.1 Analog Input Circuitry

The analog input circuitry for single ended channels is illustrated in Figure 21-8 An analog

source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard-

less of whether that channel is selected as input for the ADC. When the channel is selected, the

source must drive the S/H capacitor through the series resistance (combined resistance in the

input path).

The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or

less. If such a source is used, the sampling time will be negligible. If a source with higher imped-

ance is used, the sampling time will depend on how long time the source needs to charge the

S/H capacitor, with can vary widely. The user is recommended to only use low impedant sources

with slowly varying signals, since this minimizes the required charge transfer to the S/H

capacitor.

Signal components higher than the Nyquist frequency (fADC/2) should not be present for either

kind of channels, to avoid distortion from unpredictable signal convolution. The user is advised

to remove high frequency components with a low-pass filter before applying the signals as

inputs to the ADC.

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Figure 21-8. Analog Input Circuitry

21.5.2 Analog Noise Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of

analog measurements. If conversion accuracy is critical, the noise level can be reduced by

applying the following techniques:

a. Keep analog signal paths as short as possible. Make sure analog tracks run over

the analog ground plane, and keep them well away from high-speed switching digi-

tal tracks.

b. The AVCC pin on the device should be connected to the digital VCC supply voltage

via an LC network as shown in Figure 21-9.

c. Use the ADC noise canceler function to reduce induced noise from the CPU.

d. If any ADC [3..0] port pins are used as digital outputs, it is essential that these do

not switch while a conversion is in progress. However, using the 2-wire Interface

ADCn

IIH

1..100 kΩ

CS/H= 14 pF

VCC/2

IIL

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(ADC4 and ADC5) will only affect the conversion on ADC4 and ADC5 and not the

other ADC channels.

Figure 21-9. ADC Power Connections

21.5.3 ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps

(LSBs). The lowest code is read as 0, and the highest code is read as 2n-1.

Several parameters describe the deviation from the ideal behavior:

• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition

(at 0.5 LSB). Ideal value: 0 LSB.

GND

VCC

PC5 (ADC5/SCL)

PC4 (ADC4/SDA)

PC3 (ADC3)

PC2 (ADC2)

PC1 (ADC1)

PC0 (ADC0)

ADC7

GND

AREF

AVCC

ADC6

PB5

10µH

100nF Analog Ground Plane

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Figure 21-10. Offset Error

• Gain error: After adjusting for offset, the gain error is found as the deviation of the last

transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum).

Ideal value: 0 LSB

Figure 21-11. Gain Error

• Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum

deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0

LSB.

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

Offset

Error

Output Code

VREF Input Voltage

Ideal ADC

Actual ADC

Gain

Error

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Figure 21-12. Integral Non-linearity (INL)

• Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval

between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.

Figure 21-13. Differential Non-linearity (DNL)

• Quantization Error: Due to the quantization of the input voltage into a finite number of codes,

a range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.

• Absolute accuracy: The maximum deviation of an actual (unadjusted) transition compared to

an ideal transition for any code. This is the compound effect of offset, gain error, differential

error, non-linearity, and quantization error. Ideal value: ±0.5 LSB.

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

INL

Output Code

0x3FF

0x000

0VREF Input Voltage

DNL

1 LSB

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21.6 ADC Conversion Result

After the conversion is complete (ADIF is high), the conversion result can be found in the ADC

Result Registers (ADCL, ADCH).

For single ended conversion, the result is

where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see

Table 21-2 on page 249 and Table 21-3 on page 250). 0x000 represents analog ground, and

0x3FF represents the selected reference voltage minus one LSB.

21.6.1 ADC Multiplexer Selection Register – ADMUX

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 21-2. If these bits are

changed during a conversion, the change will not go in effect until this conversion is complete

(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external

reference voltage is being applied to the AREF pin.

• Bit 5 – ADLAR: ADC Left Adjust Result

The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.

Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the

ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-

sions. For a complete description of this bit, see “The ADC Data Register – ADCL and ADCH” on

page 251.

• Bit 4 – Res: Reserved Bit

This bit is an unused bit in the ATmega48/88/168, and will always read as zero.

• Bits 3:0 – MUX3:0: Analog Channel Selection Bits

ADC VIN 1024⋅

VREF

-----------------------------=

Bit 76543210

REFS1 REFS0 ADLAR – MUX3 MUX2 MUX1 MUX0 ADMUX

Read/Write R/W R/W R/W R R/W R/W R/W R/W

Initial Value00000000

Table 21-2. Voltage Reference Selections for ADC

REFS1 REFS0 Voltage Reference Selection

0 0 AREF, Internal Vref turned off

AVCC with external capacitor at AREF pin

10Reserved

1 1 Internal 1.1V Voltage Reference with external capacitor at AREF pin

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The value of these bits selects which analog inputs are connected to the ADC. See Table 21-3

for details. If these bits are changed during a conversion, the change will not go in effect until this

conversion is complete (ADIF in ADCSRA is set).

21.6.2 ADC Control and Status Register A – ADCSRA

• Bit 7 – ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the

ADC off while a conversion is in progress, will terminate this conversion.

• Bit 6 – ADSC: ADC Start Conversion

In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode,

write this bit to one to start the first conversion. The first conversion after ADSC has been written

after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled,

will take 25 ADC clock cycles instead of the normal 13. This first conversion performs initializa-

tion of the ADC.

ADSC will read as one as long as a conversion is in progress. When the conversion is complete,

it returns to zero. Writing zero to this bit has no effect.

Table 21-3. Input Channel Selections

MUX3..0 Single Ended Input

0000 ADC0

0001 ADC1

0010 ADC2

0011 ADC3

0100 ADC4

0101 ADC5

0110 ADC6

0111 ADC7

1000 (reserved)

1001 (reserved)

1010 (reserved)

1011 (reserved)

1100 (reserved)

1101 (reserved)

1110 1.1V (VBG)

1111 0V (GND)

Bit 76543210

ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 ADCSRA

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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• Bit 5 – ADATE: ADC Auto Trigger Enable

When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a con-

version on a positive edge of the selected trigger signal. The trigger source is selected by setting

the ADC Trigger Select bits, ADTS in ADCSRB.

• Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the Data Registers are updated. The

ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set.

ADIF is cleared by hardware when executing the corresponding interrupt handling vector. Alter-

natively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a

Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also applies if the SBI

and CBI instructions are used.

• Bit 3 – ADIE: ADC Interrupt Enable

When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Inter-

rupt is activated.

• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the system clock frequency and the input clock

to the ADC.

21.6.3 The ADC Data Register – ADCL and ADCH

21.6.3.1 ADLAR = 0

Table 21-4. ADC Prescaler Selections

ADPS2 ADPS1 ADPS0 Division Factor

000 2

001 2

010 4

011 8

100 16

101 32

110 64

111 128

Bit 151413121110 9 8

––––––ADC9ADC8ADCH

ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 ADCL

76543210

Read/Write RRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

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21.6.3.2 ADLAR = 1

When an ADC conversion is complete, the result is found in these two registers.

When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if

the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read

ADCH. Otherwise, ADCL must be read first, then ADCH.

The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from

the registers. If ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result

is right adjusted.

• ADC9:0: ADC Conversion Result

These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on

page 249.

21.6.4 ADC Control and Status Register B – ADCSRB

• Bit 7, 5:3 – Res: Reserved Bits

These bits are reserved for future use. To ensure compatibility with future devices, these bist

must be written to zero when ADCSRB is written.

• Bit 2:0 – ADTS2:0: ADC Auto Trigger Source

If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger

an ADC conversion. If ADATE is cleared, the ADTS2:0 settings will have no effect. A conversion

will be triggered by the rising edge of the selected Interrupt Flag. Note that switching from a trig-

ger source that is cleared to a trigger source that is set, will generate a positive edge on the

trigger signal. If ADEN in ADCSRA is set, this will start a conversion. Switching to Free Running

mode (ADTS[2:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set.

Bit 151413121110 9 8

ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADCH

ADC1 ADC0 ––––––ADCL

76543210

Read/Write RRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

Bit 76543210

– ACME – – – ADTS2 ADTS1 ADTS0 ADCSRB

Read/Write R R/W R R R R/W R/W R/W

Initial Value00000000

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21.6.5 Digital Input Disable Register 0 – DIDR0

• Bits 7:6 – Res: Reserved Bits

These bits are reserved for future use. To ensure compatibility with future devices, these bits

must be written to zero when DIDR0 is written.

• Bit 5..0 – ADC5D..ADC0D: ADC5..0 Digital Input Disable

When this bit is written logic one, the digital input buffer on the corresponding ADC pin is dis-

abled. The corresponding PIN Register bit will always read as zero when this bit is set. When an

analog signal is applied to the ADC5..0 pin and the digital input from this pin is not needed, this

bit should be written logic one to reduce power consumption in the digital input buffer.

Note that ADC pins ADC7 and ADC6 do not have digital input buffers, and therefore do not

require Digital Input Disable bits.

Table 21-5. ADC Auto Trigger Source Selections

ADTS2 ADTS1 ADTS0 Trigger Source

0 0 0 Free Running mode

0 0 1 Analog Comparator

0 1 0 External Interrupt Request 0

0 1 1 Timer/Counter0 Compare Match A

1 0 0 Timer/Counter0 Overflow

1 0 1 Timer/Counter1 Compare Match B

1 1 0 Timer/Counter1 Overflow

1 1 1 Timer/Counter1 Capture Event

Bit 76543210

– – ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D DIDR0

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial Value00000000

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22. debugWIRE On-chip Debug System

22.1 Features •Complete Program Flow Control

•Emulates All On-chip Functions, Both Digital and Analog, except RESET Pin

•Real-time Operation

•Symbolic Debugging Support (Both at C and Assembler Source Level, or for Other HLLs)

•Unlimited Number of Program Break Points (Using Software Break Points)

•Non-intrusive Operation

•Electrical Characteristics Identical to Real Device

•Automatic Configuration System

•High-Speed Operation

•Programming of Non-volatile Memories

22.2 Overview

The debugWIRE On-chip debug system uses a One-wire, bi-directional interface to control the

program flow, execute AVR instructions in the CPU and to program the different non-volatile

memories.

22.3 Physical Interface

When the debugWIRE Enable (DWEN) Fuse is programmed and Lock bits are unprogrammed,

the debugWIRE system within the target device is activated. The RESET port pin is configured

as a wire-AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the commu-

nication gateway between target and emulator.

Figure 22-1. The debugWIRE Setup

Figure 22-1 shows the schematic of a target MCU, with debugWIRE enabled, and the emulator

connector. The system clock is not affected by debugWIRE and will always be the clock source

selected by the CKSEL Fuses.

GND

dW(RESET)

VCC

1.8 - 5.5V

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When designing a system where debugWIRE will be used, the following observations must be

made for correct operation:

• Pull-up resistors on the dW/(RESET) line must not be smaller than 10kΩ. The pull-up resistor

is not required for debugWIRE functionality.

• Connecting the RESET pin directly to VCC will not work.

• Capacitors connected to the RESET pin must be disconnected when using debugWire.

• All external reset sources must be disconnected.

22.4 Software Break Points

debugWIRE supports Program memory Break Points by the AVR Break instruction. Setting a

Break Point in AVR Studio® will insert a BREAK instruction in the Program memory. The instruc-

tion replaced by the BREAK instruction will be stored. When program execution is continued, the

stored instruction will be executed before continuing from the Program memory. A break can be

inserted manually by putting the BREAK instruction in the program.

The Flash must be re-programmed each time a Break Point is changed. This is automatically

handled by AVR Studio through the debugWIRE interface. The use of Break Points will therefore

reduce the Flash Data retention. Devices used for debugging purposes should not be shipped to

end customers.

22.5 Limitations of debugWIRE

The debugWIRE communication pin (dW) is physically located on the same pin as External

Reset (RESET). An External Reset source is therefore not supported when the debugWIRE is

enabled.

The debugWIRE system accurately emulates all I/O functions when running at full speed, i.e.,

when the program in the CPU is running. When the CPU is stopped, care must be taken while

accessing some of the I/O Registers via the debugger (AVR Studio).

A programmed DWEN Fuse enables some parts of the clock system to be running in all sleep

modes. This will increase the power consumption while in sleep. Thus, the DWEN Fuse should

be disabled when debugWire is not used.

22.6 debugWIRE Related Register in I/O Memory

The following section describes the registers used with the debugWire.

22.6.1 debugWire Data Register – DWDR

The DWDR Register provides a communication channel from the running program in the MCU

to the debugger. This register is only accessible by the debugWIRE and can therefore not be

used as a general purpose register in the normal operations.

Bit 76543210

DWDR[7:0] DWDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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23. Self-Programming the Flash, ATmega48

In ATmega48, there is no Read-While-Write support, and no separate Boot Loader Section. The

SPM instruction can be executed from the entire Flash.

The device provides a Self-Programming mechanism for downloading and uploading program

code by the MCU itself. The Self-Programming can use any available data interface and associ-

ated protocol to read code and write (program) that code into the Program memory.

The Program memory is updated in a page by page fashion. Before programming a page with

the data stored in the temporary page buffer, the page must be erased. The temporary page buf-

fer is filled one word at a time using SPM and the buffer can be filled either before the Page

Erase command or between a Page Erase and a Page Write operation:

Alternative 1, fill the buffer before a Page Erase

• Fill temporary page buffer

• Perform a Page Erase

• Perform a Page Write

Alternative 2, fill the buffer after Page Erase

• Perform a Page Erase

• Fill temporary page buffer

• Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example

in the temporary page buffer) before the erase, and then be re-written. When using alternative 1,

the Boot Loader provides an effective Read-Modify-Write feature which allows the user software

to first read the page, do the necessary changes, and then write back the modified data. If alter-

native 2 is used, it is not possible to read the old data while loading since the page is already

erased. The temporary page buffer can be accessed in a random sequence. It is essential that

the page address used in both the Page Erase and Page Write operation is addressing the same

page.

23.0.1 Performing Page Erase by SPM

To execute Page Erase, set up the address in the Z-pointer, write “00000011” to SPMCSR and

execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE in the Z-register. Other bits in the Z-pointer will

be ignored during this operation.

• The CPU is halted during the Page Erase operation.

23.0.2 Filling the Temporary Buffer (Page Loading)

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write

“00000001” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The

content of PCWORD in the Z-register is used to address the data in the temporary buffer. The

temporary buffer will auto-erase after a Page Write operation or by writing the RWWSRE bit in

SPMCSR. It is also erased after a system reset. Note that it is not possible to write more than

one time to each address without erasing the temporary buffer.

If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be

lost.

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23.0.3 Performing a Page Write

To execute Page Write, set up the address in the Z-pointer, write “00000101” to SPMCSR and

execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE. Other bits in the Z-pointer must be written to

zero during this operation.

• The CPU is halted during the Page Write operation.

23.1 Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands.

Since the Flash is organized in pages (see Table 25-12 on page 284), the Program Counter can

be treated as having two different sections. One section, consisting of the least significant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is shown in Figure 24-3. Note that the Page Erase and Page Write operations are

addressed independently. Therefore it is of major importance that the software addresses the

same page in both the Page Erase and Page Write operation.

The LPM instruction uses the Z-pointer to store the address. Since this instruction addresses the

Flash byte-by-byte, also the LSB (bit Z0) of the Z-pointer is used.

Figure 23-1. Addressing the Flash During SPM(1)

Note: 1. The different variables used in Figure 24-3 are listed in Table 25-12 on page 284.

Bit 151413121110 9 8

ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8

ZL (R30) Z7Z6Z5Z4Z3Z2Z1Z0

76543210

PROGRAM MEMORY

0115

Z - REGISTER

BIT

ZPAGEMSB

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

ZPCMSB

INSTRUCTION WORD

PAGE PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM

COUNTER

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23.1.1 Store Program Memory Control and Status Register – SPMCSR

The Store Program Memory Control and Status Register contains the control bits needed to con-

trol the Program memory operations.

• Bit 7 – SPMIE: SPM Interrupt Enable

When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM

ready interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SELF-

PRGEN bit in the SPMCSR Register is cleared.

• Bit 6 – RWWSB: Read-While-Write Section Busy

This bit is for compatibility with devices supporting Read-While-Write. It will always read as zero

in ATmega48.

• Bit 5 – Res: Reserved Bit

This bit is a reserved bit in the ATmega48/88/168 and will always read as zero.

• Bit 4 – RWWSRE: Read-While-Write Section Read Enable

The functionality of this bit in ATmega48 is a subset of the functionality in ATmega88/168. If the

RWWSRE bit is written while filling the temporary page buffer, the temporary page buffer will be

cleared and the data will be lost.

• Bit 3 – BLBSET: Boot Lock Bit Set

The functionality of this bit in ATmega48 is a subset of the functionality in ATmega88/168. An

LPM instruction within three cycles after BLBSET and SELFPRGEN are set in the SPMCSR

destination register. See “Reading the Fuse and Lock Bits from Software” on page 259 for

details.

• Bit 2 – PGWRT: Page Write

If this bit is written to one at the same time as SELFPRGEN, the next SPM instruction within four

clock cycles executes Page Write, with the data stored in the temporary buffer. The page

address is taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The

PGWRT bit will auto-clear upon completion of a Page Write, or if no SPM instruction is executed

within four clock cycles. The CPU is halted during the entire Page Write operation.

• Bit 1 – PGERS: Page Erase

If this bit is written to one at the same time as SELFPRGEN, the next SPM instruction within four

clock cycles executes Page Erase. The page address is taken from the high part of the

Z-pointer. The data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of

a Page Erase, or if no SPM instruction is executed within four clock cycles. The CPU is halted

during the entire Page Write operation.

Bit 7 6 5 4 3 2 1 0

SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SELFPRGEN SPMCSR

Read/Write R/W R R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 0 – SELFPRGEN: Self Programming Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with

either RWWSRE, BLBSET, PGWRT, or PGERS, the following SPM instruction will have a spe-

cial meaning, see description above. If only SELFPRGEN is written, the following SPM

instruction will store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer.

The LSB of the Z-pointer is ignored. The SELFPRGEN bit will auto-clear upon completion of an

SPM instruction, or if no SPM instruction is executed within four clock cycles. During Page Erase

and Page Write, the SELFPRGEN bit remains high until the operation is completed.

Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the lower

five bits will have no effect.

23.1.2 EEPROM Write Prevents Writing to SPMCSR

Note that an EEPROM write operation will block all software programming to Flash. Reading the

Fuses and Lock bits from software will also be prevented during the EEPROM write operation. It

is recommended that the user checks the status bit (EEPE) in the EECR Register and verifies

that the bit is cleared before writing to the SPMCSR Register.

23.1.3 Reading the Fuse and Lock Bits from Software

It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the

Z-pointer with 0x0001 and set the BLBSET and SELFPRGEN bits in SPMCSR. When an LPM

instruction is executed within three CPU cycles after the BLBSET and SELFPRGEN bits are set

in SPMCSR, the value of the Lock bits will be loaded in the destination register. The BLBSET

and SELFPRGEN bits will auto-clear upon completion of reading the Lock bits or if no LPM

instruction is executed within three CPU cycles or no SPM instruction is executed within four

CPU cycles. When BLBSET and SELFPRGEN are cleared, LPM will work as described in the

Instruction set Manual.

The algorithm for reading the Fuse Low byte is similar to the one described above for reading

the Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET

and SELFPRGEN bits in SPMCSR. When an LPM instruction is executed within three cycles

after the BLBSET and SELFPRGEN bits are set in the SPMCSR, the value of the Fuse Low byte

(FLB) will be loaded in the destination register as shown below.See Table 25-5 on page 280 for

a detailed description and mapping of the Fuse Low byte.

Similarly, when reading the Fuse High byte (FHB), load 0x0003 in the Z-pointer. When an LPM

instruction is executed within three cycles after the BLBSET and SELFPRGEN bits are set in the

SPMCSR, the value of the Fuse High byte will be loaded in the destination register as shown

below. See Table 25-4 on page 280 for detailed description and mapping of the Extended Fuse

byte.

Bit 76543210

Rd ––––––LB2LB1

Bit 76543210

Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0

Bit 76543210

Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0

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Similarly, when reading the Extended Fuse byte (EFB), load 0x0002 in the Z-pointer. When an

LPM instruction is executed within three cycles after the BLBSET and SELFPRGEN bits are set

in the SPMCSR, the value of the Extended Fuse byte will be loaded in the destination register as

shown below. See Table 25-5 on page 280 for detailed description and mapping of the Extended

Fuse byte.

Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are

unprogrammed, will be read as one.

23.1.4 Preventing Flash Corruption

During periods of low VCC, the Flash program can be corrupted because the supply voltage is

too low for the CPU and the Flash to operate properly. These issues are the same as for board

level systems using the Flash, and the same design solutions should be applied.

A Flash program corruption can be caused by two situations when the voltage is too low. First, a

regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,

the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions

is too low.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

1. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.

This can be done by enabling the internal Brown-out Detector (BOD) if the operating

voltage matches the detection level. If not, an external low VCC reset protection circuit

can be used. If a reset occurs while a write operation is in progress, the write operation

will be completed provided that the power supply voltage is sufficient.

2. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will pre-

vent the CPU from attempting to decode and execute instructions, effectively protecting

the SPMCSR Register and thus the Flash from unintentional writes.

23.1.5 Programming Time for Flash when Using SPM

The calibrated RC Oscillator is used to time Flash accesses. Table 24-5 shows the typical pro-

gramming time for Flash accesses from the CPU.

Bit 76543210

Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0

Table 23-1. SPM Programming Time

Symbol Min Programming Time Max Programming Time

Flash write (Page Erase, Page Write, and

write Lock bits by SPM) 3.7 ms 4.5 ms

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23.1.6 Simple Assembly Code Example for a Boot Loader

Note that the RWWSB bit will always be read as zero in ATmega48. Nevertheless, it is recom-

mended to check this bit as shown in the code example, to ensure compatibility with devices

supporting Read-While-Write.

;-the routine writes one page of data from RAM to Flash

; the first data location in RAM is pointed to by the Y pointer

; the first data location in Flash is pointed to by the Z-pointer

;-error handling is not included

;-the routine must be placed inside the Boot space

; (at least the Do_spm sub routine). Only code inside NRWW section can

; be read during Self-Programming (Page Erase and Page Write).

;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),

; loophi (r25), spmcrval (r20)

; storing and restoring of registers is not included in the routine

; register usage can be optimized at the expense of code size

;-It is assumed that either the interrupt table is moved to the Boot

; loader section or that the interrupts are disabled.

.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not

words

.org SMALLBOOTSTART

Write_page:

; Page Erase

ldi spmcrval, (1<<PGERS) | (1<<SELFPRGEN)

rcall Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

rcall Do_spm

; transfer data from RAM to Flash page buffer

ldi looplo, low(PAGESIZEB);init loop variable

ldi loophi, high(PAGESIZEB);not required for PAGESIZEB<=256

Wrloop:

ld r0, Y+

ld r1, Y+

ldi spmcrval, (1<<SELFPRGEN)

rcall Do_spm

adiw ZH:ZL, 2

sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256

brne Wrloop

; execute Page Write

subi ZL, low(PAGESIZEB) ;restore pointer

sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256

ldi spmcrval, (1<<PGWRT) | (1<<SELFPRGEN)

rcall Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

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rcall Do_spm

; read back and check, optional

ldi looplo, low(PAGESIZEB);init loop variable

ldi loophi, high(PAGESIZEB);not required for PAGESIZEB<=256

subi YL, low(PAGESIZEB) ;restore pointer

sbci YH, high(PAGESIZEB)

Rdloop:

lpm r0, Z+

ld r1, Y+

cpse r0, r1

rjmp Error

sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256

brne Rdloop

; return to RWW section

; verify that RWW section is safe to read

Return:

in temp1, SPMCSR

sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is

not ready yet

ret

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

rcall Do_spm

rjmp Return

Do_spm:

; check for previous SPM complete

Wait_spm:

in temp1, SPMCSR

sbrc temp1, SELFPRGEN

rjmp Wait_spm

; input: spmcrval determines SPM action

; disable interrupts if enabled, store status

in temp2, SREG

cli

; check that no EEPROM write access is present

Wait_ee:

sbic EECR, EEPE

rjmp Wait_ee

; SPM timed sequence

out SPMCSR, spmcrval

spm

; restore SREG (to enable interrupts if originally enabled)

out SREG, temp2

ret

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24. Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and

ATmega168

In ATmega88 and ATmega168, the Boot Loader Support provides a real Read-While-Write

Self-Programming mechanism for downloading and uploading program code by the MCU itself.

This feature allows flexible application software updates controlled by the MCU using a

Flash-resident Boot Loader program. The Boot Loader program can use any available data

interface and associated protocol to read code and write (program) that code into the Flash

memory, or read the code from the program memory. The program code within the Boot Loader

section has the capability to write into the entire Flash, including the Boot Loader memory. The

Boot Loader can thus even modify itself, and it can also erase itself from the code if the feature

is not needed anymore. The size of the Boot Loader memory is configurable with fuses and the

Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives

the user a unique flexibility to select different levels of protection.

24.1 Boot Loader Features

•Read-While-Write Self-Programming

•Flexible Boot Memory Size

•High Security (Separate Boot Lock Bits for a Flexible Protection)

•Separate Fuse to Select Reset Vector

•Optimized Page(1) Size

•Code Efficient Algorithm

•Efficient Read-Modify-Write Support

Note: 1. A page is a section in the Flash consisting of several bytes (see Table 25-12 on page 284)

used during programming. The page organization does not affect normal operation.

24.2 Application and Boot Loader Flash Sections

The Flash memory is organized in two main sections, the Application section and the Boot

Loader section (see Figure 24-2). The size of the different sections is configured by the

BOOTSZ Fuses as shown in Table 24-6 on page 276 and Figure 24-2. These two sections can

have different level of protection since they have different sets of Lock bits.

24.2.1 Application Section

The Application section is the section of the Flash that is used for storing the application code.

The protection level for the Application section can be selected by the application Boot Lock bits

(Boot Lock bits 0), see Table 24-2 on page 267. The Application section can never store any

Boot Loader code since the SPM instruction is disabled when executed from the Application

section.

24.2.2 BLS – Boot Loader Section

While the Application section is used for storing the application code, the The Boot Loader soft-

ware must be located in the BLS since the SPM instruction can initiate a programming when

executing from the BLS only. The SPM instruction can access the entire Flash, including the

BLS itself. The protection level for the Boot Loader section can be selected by the Boot Loader

Lock bits (Boot Lock bits 1), see Table 24-3 on page 267.

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24.3 Read-While-Write and No Read-While-Write Flash Sections

Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-

ware update is dependent on which address that is being programmed. In addition to the two

sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also

divided into two fixed sections, the Read-While-Write (RWW) section and the No

Read-While-Write (NRWW) section. The limit between the RWW- and NRWW sections is given

in Table 24-7 on page 276 and Figure 24-2 on page 266. The main difference between the two

sections is:

• When erasing or writing a page located inside the RWW section, the NRWW section can be

read during the operation.

• When erasing or writing a page located inside the NRWW section, the CPU is halted during

the entire operation.

Note that the user software can never read any code that is located inside the RWW section dur-

ing a Boot Loader software operation. The syntax “Read-While-Write section” refers to which

section that is being programmed (erased or written), not which section that actually is being

read during a Boot Loader software update.

24.3.1 RWW – Read-While-Write Section

If a Boot Loader software update is programming a page inside the RWW section, it is possible

to read code from the Flash, but only code that is located in the NRWW section. During an

on-going programming, the software must ensure that the RWW section never is being read. If

the user software is trying to read code that is located inside the RWW section (i.e., by a

call/jmp/lpm or an interrupt) during programming, the software might end up in an unknown

state. To avoid this, the interrupts should either be disabled or moved to the Boot Loader sec-

tion. The Boot Loader section is always located in the NRWW section. The RWW Section Busy

bit (RWWSB) in the Store Program Memory Control and Status Register (SPMCSR) will be read

as logical one as long as the RWW section is blocked for reading. After a programming is com-

pleted, the RWWSB must be cleared by software before reading code located in the RWW

section. See “Store Program Memory Control and Status Register – SPMCSR” on page 268. for

details on how to clear RWWSB.

24.3.2 NRWW – No Read-While-Write Section

The code located in the NRWW section can be read when the Boot Loader software is updating

a page in the RWW section. When the Boot Loader code updates the NRWW section, the CPU

is halted during the entire Page Erase or Page Write operation.

Table 24-1. Read-While-Write Features

Which Section does the Z-pointer

Address During the Programming?

Which Section Can

be Read During

Programming?

Is the CPU

Halted?

Read-While-Write

Supported?

RWW Section NRWW Section No Yes

NRWW Section None Yes No

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Figure 24-1. Read-While-Write vs. No Read-While-Write

Read-While-Write

(RWW) Section

No Read-While-Write

(NRWW) Section

Z-pointer

Addresses RWW

Section

Z-pointer

Addresses NRWW

Section

CPU is Halted

During the Operation

Code Located in

NRWW Section

Can be Read During

the Operation

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Figure 24-2. Memory Sections

Note: 1. The parameters in the figure above are given in Table 24-6 on page 276.

24.4 Boot Loader Lock Bits

If no Boot Loader capability is needed, the entire Flash is available for application code. The

Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives

the user a unique flexibility to select different levels of protection.

The user can select:

• To protect the entire Flash from a software update by the MCU.

• To protect only the Boot Loader Flash section from a software update by the MCU.

• To protect only the Application Flash section from a software update by the MCU.

• Allow software update in the entire Flash.

See Table 24-2 and Table 24-3 for further details. The Boot Lock bits can be set in software and

in Serial or Parallel Programming mode, but they can be cleared by a Chip Erase command

only. The general Write Lock (Lock Bit mode 2) does not control the programming of the Flash

memory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit mode 1) does not

control reading nor writing by LPM/SPM, if it is attempted.

0x0000

Flashend

Program Memory

BOOTSZ = '11'

Application Flash Section

Boot Loader Flash Section Flashend

Program Memory

BOOTSZ = '10'

0x0000

Program Memory

BOOTSZ = '01'

Program Memory

BOOTSZ = '00'

Application Flash Section

Boot Loader Flash Section

0x0000

Flashend

Application Flash Section

Flashend

End RWW

Start NRWW

Application Flash Section

Boot Loader Flash Section

End RWW

Start NRWW

End RWW

Start NRWW

0x0000

End RWW, End Application

Start NRWW, Start Boot Loader

Application Flash SectionApplication Flash Section

Application Flash Section

Read-While-Write SectionNo Read-While-Write Section Read-While-Write SectionNo Read-While-Write Section

Read-While-Write SectionNo Read-While-Write SectionRead-While-Write SectionNo Read-While-Write Section

End Application

Start Boot Loader

End Application

Start Boot Loader

End Application

Start Boot Loader

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Note: 1. “1” means unprogrammed, “0” means programmed

24.5 Entering the Boot Loader Program

Entering the Boot Loader takes place by a jump or call from the application program. This may

be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively,

the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash

start address after a reset. In this case, the Boot Loader is started after a reset. After the applica-

tion code is loaded, the program can start executing the application code. Note that the fuses

cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is pro-

grammed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be

changed through the serial or parallel programming interface.

Note: 1. “1” means unprogrammed, “0” means programmed

Table 24-2. Boot Lock Bit0 Protection Modes (Application Section)(1)

BLB0 Mode BLB02 BLB01 Protection

111

No restrictions for SPM or LPM accessing the Application

section.

2 1 0 SPM is not allowed to write to the Application section.

300

SPM is not allowed to write to the Application section, and LPM

executing from the Boot Loader section is not allowed to read

from the Application section. If Interrupt Vectors are placed in

the Boot Loader section, interrupts are disabled while executing

from the Application section.

401

LPM executing from the Boot Loader section is not allowed to

read from the Application section. If Interrupt Vectors are placed

in the Boot Loader section, interrupts are disabled while

executing from the Application section.

Table 24-3. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)

BLB1 Mode BLB12 BLB11 Protection

111

No restrictions for SPM or LPM accessing the Boot Loader

section.

2 1 0 SPM is not allowed to write to the Boot Loader section.

300

SPM is not allowed to write to the Boot Loader section, and LPM

executing from the Application section is not allowed to read

from the Boot Loader section. If Interrupt Vectors are placed in

the Application section, interrupts are disabled while executing

from the Boot Loader section.

401

LPM executing from the Application section is not allowed to

read from the Boot Loader section. If Interrupt Vectors are

placed in the Application section, interrupts are disabled while

executing from the Boot Loader section.

Table 24-4. Boot Reset Fuse(1)

BOOTRST Reset Address

1 Reset Vector = Application Reset (address 0x0000)

0 Reset Vector = Boot Loader Reset (see Table 24-6 on page 276)

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24.5.1 Store Program Memory Control and Status Register – SPMCSR

The Store Program Memory Control and Status Register contains the control bits needed to con-

trol the Boot Loader operations.

• Bit 7 – SPMIE: SPM Interrupt Enable

When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM

ready interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SELF-

PRGEN bit in the SPMCSR Register is cleared.

• Bit 6 – RWWSB: Read-While-Write Section Busy

When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initi-

ated, the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section

cannot be accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a

Self-Programming operation is completed. Alternatively the RWWSB bit will automatically be

cleared if a page load operation is initiated.

• Bit 5 – Res: Reserved Bit

This bit is a reserved bit in the ATmega48/88/168 and always read as zero.

• Bit 4 – RWWSRE: Read-While-Write Section Read Enable

When programming (Page Erase or Page Write) to the RWW section, the RWW section is

blocked for reading (the RWWSB will be set by hardware). To re-enable the RWW section, the

user software must wait until the programming is completed (SELFPRGEN will be cleared).

Then, if the RWWSRE bit is written to one at the same time as SELFPRGEN, the next SPM

instruction within four clock cycles re-enables the RWW section. The RWW section cannot be

re-enabled while the Flash is busy with a Page Erase or a Page Write (SELFPRGEN is set). If

the RWWSRE bit is written while the Flash is being loaded, the Flash load operation will abort

and the data loaded will be lost.

• Bit 3 – BLBSET: Boot Lock Bit Set

If this bit is written to one at the same time as SELFPRGEN, the next SPM instruction within four

clock cycles sets Boot Lock bits and Memory Lock bits, according to the data in R0. The data in

R1 and the address in the Z-pointer are ignored. The BLBSET bit will automatically be cleared

upon completion of the Lock bit set, or if no SPM instruction is executed within four clock cycles.

An LPM instruction within three cycles after BLBSET and SELFPRGEN are set in the SPMCSR

destination register. See “Reading the Fuse and Lock Bits from Software” on page 272 for

details.

Bit 7 6 5 4 3 2 1 0

SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SELFPRGEN SPMCSR

Read/Write R/W R R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 2 – PGWRT: Page Write

If this bit is written to one at the same time as SELFPRGEN, the next SPM instruction within four

clock cycles executes Page Write, with the data stored in the temporary buffer. The page

address is taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The

PGWRT bit will auto-clear upon completion of a Page Write, or if no SPM instruction is executed

within four clock cycles. The CPU is halted during the entire Page Write operation if the NRWW

section is addressed.

• Bit 1 – PGERS: Page Erase

If this bit is written to one at the same time as SELFPRGEN, the next SPM instruction within four

clock cycles executes Page Erase. The page address is taken from the high part of the

Z-pointer. The data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of

a Page Erase, or if no SPM instruction is executed within four clock cycles. The CPU is halted

during the entire Page Write operation if the NRWW section is addressed.

• Bit 0 – SELFPRGEN: Self Programming Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with

either RWWSRE, BLBSET, PGWRT or PGERS, the following SPM instruction will have a spe-

cial meaning, see description above. If only SELFPRGEN is written, the following SPM

instruction will store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer.

The LSB of the Z-pointer is ignored. The SELFPRGEN bit will auto-clear upon completion of an

SPM instruction, or if no SPM instruction is executed within four clock cycles. During Page Erase

and Page Write, the SELFPRGEN bit remains high until the operation is completed.

Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the lower

five bits will have no effect.

24.6 Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands.

Since the Flash is organized in pages (see Table 25-12 on page 284), the Program Counter can

be treated as having two different sections. One section, consisting of the least significant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is1 shown in Figure 24-3. Note that the Page Erase and Page Write operations are

addressed independently. Therefore it is of major importance that the Boot Loader software

addresses the same page in both the Page Erase and Page Write operation. Once a program-

ming operation is initiated, the address is latched and the Z-pointer can be used for other

operations.

The only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits.

The content of the Z-pointer is ignored and will have no effect on the operation. The LPM

instruction does also use the Z-pointer to store the address. Since this instruction addresses the

Flash byte-by-byte, also the LSB (bit Z0) of the Z-pointer is used.

Bit 151413121110 9 8

ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8

ZL (R30) Z7Z6Z5Z4Z3Z2Z1Z0

76543210

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Figure 24-3. Addressing the Flash During SPM(1)

Note: 1. The different variables used in Figure 24-3 are listed in Table 24-8 on page 276.

24.7 Self-Programming the Flash

The program memory is updated in a page by page fashion. Before programming a page with

the data stored in the temporary page buffer, the page must be erased. The temporary page buf-

fer is filled one word at a time using SPM and the buffer can be filled either before the Page

Erase command or between a Page Erase and a Page Write operation:

Alternative 1, fill the buffer before a Page Erase

• Fill temporary page buffer

• Perform a Page Erase

• Perform a Page Write

Alternative 2, fill the buffer after Page Erase

• Perform a Page Erase

• Fill temporary page buffer

• Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example

in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1,

the Boot Loader provides an effective Read-Modify-Write feature which allows the user software

to first read the page, do the necessary changes, and then write back the modified data. If alter-

native 2 is used, it is not possible to read the old data while loading since the page is already

erased. The temporary page buffer can be accessed in a random sequence. It is essential that

the page address used in both the Page Erase and Page Write operation is addressing the same

page. See “Simple Assembly Code Example for a Boot Loader” on page 274 for an assembly

code example.

PROGRAM MEMORY

0115

Z - REGISTER

BIT

ZPAGEMSB

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

ZPCMSB

INSTRUCTION WORD

PAGE PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM

COUNTER

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24.7.1 Performing Page Erase by SPM

To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and

execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE in the Z-register. Other bits in the Z-pointer will

be ignored during this operation.

• Page Erase to the RWW section: The NRWW section can be read during the Page Erase.

• Page Erase to the NRWW section: The CPU is halted during the operation.

24.7.2 Filling the Temporary Buffer (Page Loading)

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write

“00000001” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The

content of PCWORD in the Z-register is used to address the data in the temporary buffer. The

temporary buffer will auto-erase after a Page Write operation or by writing the RWWSRE bit in

SPMCSR. It is also erased after a system reset. Note that it is not possible to write more than

one time to each address without erasing the temporary buffer.

If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be

lost.

24.7.3 Performing a Page Write

To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and

execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE. Other bits in the Z-pointer must be written to

zero during this operation.

• Page Write to the RWW section: The NRWW section can be read during the Page Write.

• Page Write to the NRWW section: The CPU is halted during the operation.

24.7.4 Using the SPM Interrupt

If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the

SELFPRGEN bit in SPMCSR is cleared. This means that the interrupt can be used instead of

polling the SPMCSR Register in software. When using the SPM interrupt, the Interrupt Vectors

should be moved to the BLS section to avoid that an interrupt is accessing the RWW section

when it is blocked for reading. How to move the interrupts is described in “Watchdog Timer” on

page 47.

24.7.5 Consideration While Updating BLS

Special care must be taken if the user allows the Boot Loader section to be updated by leaving

Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the

entire Boot Loader, and further software updates might be impossible. If it is not necessary to

change the Boot Loader software itself, it is recommended to program the Boot Lock bit11 to

protect the Boot Loader software from any internal software changes.

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24.7.6 Prevent Reading the RWW Section During Self-Programming

During Self-Programming (either Page Erase or Page Write), the RWW section is always

blocked for reading. The user software itself must prevent that this section is addressed during

the self programming operation. The RWWSB in the SPMCSR will be set as long as the RWW

section is busy. During Self-Programming the Interrupt Vector table should be moved to the BLS

as described in “Watchdog Timer” on page 47, or the interrupts must be disabled. Before

addressing the RWW section after the programming is completed, the user software must clear

the RWWSB by writing the RWWSRE. See “Simple Assembly Code Example for a Boot Loader”

on page 274 for an example.

24.7.7 Setting the Boot Loader Lock Bits by SPM

To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCSR

and execute SPM within four clock cycles after writing SPMCSR. The only accessible Lock bits

are the Boot Lock bits that may prevent the Application and Boot Loader section from any soft-

ware update by the MCU.

See Table 24-2 and Table 24-3 for how the different settings of the Boot Loader bits affect the

Flash access.

If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an

SPM instruction is executed within four cycles after BLBSET and SELFPRGEN are set in

SPMCSR. The Z-pointer is don’t care during this operation, but for future compatibility it is rec-

ommended to load the Z-pointer with 0x0001 (same as used for reading the lOck bits). For future

compatibility it is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when writing the Lock

bits. When programming the Lock bits the entire Flash can be read during the operation.

24.7.8 EEPROM Write Prevents Writing to SPMCSR

Note that an EEPROM write operation will block all software programming to Flash. Reading the

Fuses and Lock bits from software will also be prevented during the EEPROM write operation. It

is recommended that the user checks the status bit (EEPE) in the EECR Register and verifies

that the bit is cleared before writing to the SPMCSR Register.

24.7.9 Reading the Fuse and Lock Bits from Software

It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the

Z-pointer with 0x0001 and set the BLBSET and SELFPRGEN bits in SPMCSR. When an LPM

instruction is executed within three CPU cycles after the BLBSET and SELFPRGEN bits are set

in SPMCSR, the value of the Lock bits will be loaded in the destination register. The BLBSET

and SELFPRGEN bits will auto-clear upon completion of reading the Lock bits or if no LPM

instruction is executed within three CPU cycles or no SPM instruction is executed within four

CPU cycles. When BLBSET and SELFPRGEN are cleared, LPM will work as described in the

Instruction set Manual.

Bit 76543210

R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1

Bit 76543210

Rd – – BLB12 BLB11 BLB02 BLB01 LB2 LB1

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The algorithm for reading the Fuse Low byte is similar to the one described above for reading

the Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET

and SELFPRGEN bits in SPMCSR. When an LPM instruction is executed within three cycles

after the BLBSET and SELFPRGEN bits are set in the SPMCSR, the value of the Fuse Low byte

(FLB) will be loaded in the destination register as shown below. Refer to Table 25-5 on page 280

for a detailed description and mapping of the Fuse Low byte.

Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an LPM instruc-

tion is executed within three cycles after the BLBSET and SELFPRGEN bits are set in the

SPMCSR, the value of the Fuse High byte (FHB) will be loaded in the destination register as

shown below. Refer to Table 25-6 on page 281 for detailed description and mapping of the Fuse

High byte.

When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an LPM instruction

is executed within three cycles after the BLBSET and SELFPRGEN bits are set in the SPMCSR,

the value of the Extended Fuse byte (EFB) will be loaded in the destination register as shown

below. Refer to Table 25-4 on page 280 for detailed description and mapping of the Extended

Fuse byte.

Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are

unprogrammed, will be read as one.

24.7.10 Preventing Flash Corruption

During periods of low VCC, the Flash program can be corrupted because the supply voltage is

too low for the CPU and the Flash to operate properly. These issues are the same as for board

level systems using the Flash, and the same design solutions should be applied.

A Flash program corruption can be caused by two situations when the voltage is too low. First, a

regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,

the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions

is too low.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

1. If there is no need for a Boot Loader update in the system, program the Boot Loader

Lock bits to prevent any Boot Loader software updates.

2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.

This can be done by enabling the internal Brown-out Detector (BOD) if the operating

voltage matches the detection level. If not, an external low VCC reset protection circuit

can be used. If a reset occurs while a write operation is in progress, the write operation

will be completed provided that the power supply voltage is sufficient.

3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will pre-

vent the CPU from attempting to decode and execute instructions, effectively protecting

the SPMCSR Register and thus the Flash from unintentional writes.

Bit 76543210

Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0

Bit 76543210

Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0

Bit 76543210

Rd – – – – EFB3 EFB2 EFB1 EFB0

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24.7.11 Programming Time for Flash when Using SPM

The calibrated RC Oscillator is used to time Flash accesses. Table 24-5 shows the typical pro-

gramming time for Flash accesses from the CPU.

24.7.12 Simple Assembly Code Example for a Boot Loader

;-the routine writes one page of data from RAM to Flash

; the first data location in RAM is pointed to by the Y pointer

; the first data location in Flash is pointed to by the Z-pointer

;-error handling is not included

;-the routine must be placed inside the Boot space

; (at least the Do_spm sub routine). Only code inside NRWW section can

; be read during Self-Programming (Page Erase and Page Write).

;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),

; loophi (r25), spmcrval (r20)

; storing and restoring of registers is not included in the routine

; register usage can be optimized at the expense of code size

;-It is assumed that either the interrupt table is moved to the Boot

; loader section or that the interrupts are disabled.

.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words

.org SMALLBOOTSTART

Write_page:

; Page Erase

ldi spmcrval, (1<<PGERS) | (1<<SELFPRGEN)

call Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

call Do_spm

; transfer data from RAM to Flash page buffer

ldi looplo, low(PAGESIZEB) ;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

Wrloop:

ld r0, Y+

ld r1, Y+

ldi spmcrval, (1<<SELFPRGEN)

call Do_spm

adiw ZH:ZL, 2

sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256

brne Wrloop

; execute Page Write

subi ZL, low(PAGESIZEB) ;restore pointer

sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256

ldi spmcrval, (1<<PGWRT) | (1<<SELFPRGEN)

call Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

call Do_spm

; read back and check, optional

Table 24-5. SPM Programming Time

Symbol Min Programming Time Max Programming Time

Flash write (Page Erase, Page Write, and

write Lock bits by SPM) 3.7 ms 4.5 ms

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ldi looplo, low(PAGESIZEB) ;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

subi YL, low(PAGESIZEB) ;restore pointer

sbci YH, high(PAGESIZEB)

Rdloop:

lpm r0, Z+

ld r1, Y+

cpse r0, r1

jmp Error

sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256

brne Rdloop

; return to RWW section

; verify that RWW section is safe to read

Return:

in temp1, SPMCSR

sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is not ready yet

ret

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)

call Do_spm

rjmp Return

Do_spm:

; check for previous SPM complete

Wait_spm:

in temp1, SPMCSR

sbrc temp1, SELFPRGEN

rjmp Wait_spm

; input: spmcrval determines SPM action

; disable interrupts if enabled, store status

in temp2, SREG

cli

; check that no EEPROM write access is present

Wait_ee:

sbic EECR, EEPE

rjmp Wait_ee

; SPM timed sequence

out SPMCSR, spmcrval

spm

; restore SREG (to enable interrupts if originally enabled)

out SREG, temp2

ret

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24.7.13 ATmega88 Boot Loader Parameters

In Table 24-6 through Table 24-8, the parameters used in the description of the self program-

ming are given.

Note: The different BOOTSZ Fuse configurations are shown in Figure 24-2.

For details about these two section, see “NRWW – No Read-While-Write Section” on page 264

and “RWW – Read-While-Write Section” on page 264

Table 24-6. Boot Size Configuration, ATmega88

BOOTSZ1 BOOTSZ0

Boot

Size Pages

Application

Flash

Section

Boot

Loader

Flash

Section

End

Application

Section

Boot Reset

Address

(Start Boot

Loader

Section)

128

words 40x000 -

0xF7F

0xF80 -

0xFFF 0xF7F 0xF80

256

words 80x000 -

0xEFF

0xF00 -

0xFFF 0xEFF 0xF00

512

words 16 0x000 -

0xDFF

0xE00 -

0xFFF 0xDFF 0xE00

1024

words 32 0x000 -

0xBFF

0xC00 -

0xFFF 0xBFF 0xC00

Table 24-7. Read-While-Write Limit, ATmega88

Section Pages Address

Read-While-Write section (RWW) 96 0x000 - 0xBFF

No Read-While-Write section (NRWW) 32 0xC00 - 0xFFF

Table 24-8. Explanation of Different Variables used in Figure 24-3 and the Mapping to the

Z-pointer, ATmega88

Variable

Corresponding

Z-value(1) Description

PCMSB 11 Most significant bit in the Program Counter. (The

Program Counter is 12 bits PC[11:0])

PAG EMSB 4

Most significant bit which is used to address the

words within one page (32 words in a page requires

5 bits PC [4:0]).

ZPCMSB Z12 Bit in Z-register that is mapped to PCMSB. Because

Z0 is not used, the ZPCMSB equals PCMSB + 1.

ZPAGEMSB Z5

Bit in Z-register that is mapped to PAGEMSB.

Because Z0 is not used, the ZPAGEMSB equals

PAGEMSB + 1.

PCPAGE PC[11:5] Z12:Z6 Program counter page address: Page select, for

page erase and page write

PCWORD PC[4:0] Z5:Z1

Program counter word address: Word select, for

filling temporary buffer (must be zero during page

write operation)

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Note: 1. Z15:Z13: always ignored

Z0: should be zero for all SPM commands, byte select for the LPM instruction.

See “Addressing the Flash During Self-Programming” on page 269 for details about the use of

Z-pointer during Self-Programming.

24.7.14 ATmega168 Boot Loader Parameters

In Table 24-9 through Table 24-11, the parameters used in the description of the self program-

ming are given.

Note: The different BOOTSZ Fuse configurations are shown in Figure 24-2.

For details about these two section, see “NRWW – No Read-While-Write Section” on page 264

and “RWW – Read-While-Write Section” on page 264

Table 24-9. Boot Size Configuration, ATmega168

BOOTSZ1 BOOTSZ0

Boot

Size Pages

Application

Flash

Section

Boot

Loader

Flash

Section

End

Application

Section

Boot Reset

Address

(Start Boot

Loader

Section)

128

words 20x0000 -

0x1F7F

0x1F80 -

0x1FFF 0x1F7F 0x1F80

256

words 40x0000 -

0x1EFF

0x1F00 -

0x1FFF 0x1EFF 0x1F00

512

words 80x0000 -

0x1DFF

0x1E00 -

0x1FFF 0x1DFF 0x1E00

1024

words 16 0x0000 -

0x1BFF

0x1C00 -

0x1FFF 0x1BFF 0x1C00

Table 24-10. Read-While-Write Limit, ATmega168

Section Pages Address

Read-While-Write section (RWW) 112 0x0000 - 0x1BFF

No Read-While-Write section (NRWW) 16 0x1C00 - 0x1FFF

Table 24-11. Explanation of Different Variables used in Figure 24-3 and the Mapping to the

Z-pointer, ATmega168

Variable

Corresponding

Z-value(1) Description

PCMSB 12 Most significant bit in the Program Counter. (The

Program Counter is 12 bits PC[11:0])

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Note: 1. Z15:Z14: always ignored

Z0: should be zero for all SPM commands, byte select for the LPM instruction.

See “Addressing the Flash During Self-Programming” on page 269 for details about the use of

Z-pointer during Self-Programming.

25. Memory Programming

25.1 Program And Data Memory Lock Bits

The ATmega88/168 provides six Lock bits which can be left unprogrammed (“1”) or can be pro-

grammed (“0”) to obtain the additional features listed in Table 25-2. The Lock bits can only be

erased to “1” with the Chip Erase command.The ATmega48 has no separate Boot Loader sec-

tion. The SPM instruction is enabled for the whole Flash if the SELFPRGEN fuse is programmed

(“0”), otherwise it is disabled.

Notes: 1. “1” means unprogrammed, “0” means programmed

2. Only on ATmega88/168.

PAG EMSB 5

Most significant bit which is used to address

the words within one page (64 words in a page

requires 6 bits PC [5:0])

ZPCMSB Z13 Bit in Z-register that is mapped to PCMSB. Because

Z0 is not used, the ZPCMSB equals PCMSB + 1.

ZPAGEMSB Z6

Bit in Z-register that is mapped to PAGEMSB.

Because Z0 is not used, the ZPAGEMSB equals

PAGEMSB + 1.

PCPAGE PC[12:6] Z13:Z7 Program counter page address: Page select, for

page erase and page write

PCWORD PC[5:0] Z6:Z1

Program counter word address: Word select, for

filling temporary buffer (must be zero during page

write operation)

Table 24-11. Explanation of Different Variables used in Figure 24-3 and the Mapping to the

Z-pointer, ATmega168

Variable

Corresponding

Z-value(1) Description

Table 25-1. Lock Bit Byte(1)

Lock Bit Byte Bit No Description Default Value

7 – 1 (unprogrammed)

6 – 1 (unprogrammed)

BLB12() 5 Boot Lock bit 1 (unprogrammed)

BLB11() 4 Boot Lock bit 1 (unprogrammed)

BLB02() 3 Boot Lock bit 1 (unprogrammed)

BLB01() 2 Boot Lock bit 1 (unprogrammed)

LB2 1 Lock bit 1 (unprogrammed)

LB1 0 Lock bit 1 (unprogrammed)

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Notes: 1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.

2. “1” means unprogrammed, “0” means programmed

Notes: 1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.

2. “1” means unprogrammed, “0” means programmed

Table 25-2. Lock Bit Protection Modes(1)(2)

Memory Lock Bits Protection Type

LB Mode LB2 LB1

1 1 1 No memory lock features enabled.

210

Further programming of the Flash and EEPROM is disabled in

Parallel and Serial Programming mode. The Fuse bits are

locked in both Serial and Parallel Programming mode.(1)

300

Further programming and verification of the Flash and

EEPROM is disabled in Parallel and Serial Programming mode.

The Boot Lock bits and Fuse bits are locked in both Serial and

Parallel Programming mode.(1)

Table 25-3. Lock Bit Protection Modes(1)(2). Only ATmega88/168.

BLB0 Mode BLB02 BLB01

111

No restrictions for SPM or LPM accessing the Application

section.

2 1 0 SPM is not allowed to write to the Application section.

300

SPM is not allowed to write to the Application section, and LPM

executing from the Boot Loader section is not allowed to read

from the Application section. If Interrupt Vectors are placed in

the Boot Loader section, interrupts are disabled while executing

from the Application section.

401

LPM executing from the Boot Loader section is not allowed to

read from the Application section. If Interrupt Vectors are placed

in the Boot Loader section, interrupts are disabled while

executing from the Application section.

BLB1 Mode BLB12 BLB11

111

No restrictions for SPM or LPM accessing the Boot Loader

section.

2 1 0 SPM is not allowed to write to the Boot Loader section.

300

SPM is not allowed to write to the Boot Loader section, and LPM

executing from the Application section is not allowed to read

from the Boot Loader section. If Interrupt Vectors are placed in

the Application section, interrupts are disabled while executing

from the Boot Loader section.

401

LPM executing from the Application section is not allowed to

read from the Boot Loader section. If Interrupt Vectors are

placed in the Application section, interrupts are disabled while

executing from the Boot Loader section.

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25.2 Fuse Bits

The ATmega48/88/168 has three Fuse bytes. Table 25-4 - Table 25-7 describe briefly the func-

tionality of all the fuses and how they are mapped into the Fuse bytes. Note that the fuses are

read as logical zero, “0”, if they are programmed.

Note: 1. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 25-8 on page 283

for details.

Table 25-4. Extended Fuse Byte for mega48

Extended Fuse Byte Bit No Description Default Value

–7– 1

–6– 1

–5– 1

–4– 1

–3– 1

–2– 1

–1– 1

SELFPRGEN 0 Self Programming Enable 1 (unprogrammed)

Table 25-5. Extended Fuse Byte for mega88/168

Extended Fuse Byte Bit No Description Default Value

–7– 1

–6– 1

–5– 1

–4– 1

–3– 1

BOOTSZ1 2 Select Boot Size

(see Table 113 for details) 0 (programmed)(1)

BOOTSZ0 1 Select Boot Size

(see Table 113 for details) 0 (programmed)(1)

BOOTRST 0 Select Reset Vector 1 (unprogrammed)

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Notes: 1. See “Alternate Functions of Port C” on page 74 for description of RSTDISBL Fuse.
2. The SPIEN Fuse is not accessible in serial programming mode.
3. See “Watchdog Timer Control Register - WDTCSR” on page 50 for details.
4. See Table 8-2 on page 44 for BODLEVEL Fuse decoding.
Note: 1. The default value of SUT1..0 results in maximum start-up time for the default clock source. 
See Table 6-9 on page 31 for details.
2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 8 MHz. See Table 6-8 on 
page 30 for details.
3. The CKOUT Fuse allows the system clock to be output on PORTB0. See “Clock Output Buffer” 
on page 33 for details.
4. See “System Clock Prescaler” on page 33 for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if
Lock bit1 (LB1) is programmed. Program the Fuse bits before programming the Lock bits.
Table 25-6. Fuse High Byte
High Fuse Byte Bit No Description Default Value
RSTDISBL(1) 7 External Reset Disable 1 (unprogrammed)
DWEN 6 debugWIRE Enable 1 (unprogrammed)
SPIEN(2) 5Enable Serial Program and 
Data Downloading
0 (programmed, SPI 
programming enabled)
WDTON(3) 4 Watchdog Timer Always On 1 (unprogrammed)
EESAVE 3
EEPROM memory is 
preserved through the Chip 
Erase
1 (unprogrammed), EEPROM 
not reserved
BODLEVEL2(4) 2Brown-out Detector trigger 
level 1 (unprogrammed)
BODLEVEL1(4) 1Brown-out Detector trigger 
level 1 (unprogrammed)
BODLEVEL0(4) 0Brown-out Detector trigger 
level 1 (unprogrammed)
Table 25-7. Fuse Low Byte
Low Fuse Byte Bit No Description Default Value
CKDIV8(4) 7 Divide clock by 8 0 (programmed)
CKOUT(3) 6 Clock output 1 (unprogrammed)
SUT1 5 Select start-up time 1 (unprogrammed)(1)
SUT0 4 Select start-up time 0 (programmed)(1)
CKSEL3 3 Select Clock source 0 (programmed)(2)
CKSEL2 2 Select Clock source 0 (programmed)(2)
CKSEL1 1 Select Clock source 1 (unprogrammed)(2)
CKSEL0 0 Select Clock source 0 (programmed)(2)

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25.2.1 Latching of Fuses

The fuse values are latched when the device enters programming mode and changes of the

fuse values will have no effect until the part leaves Programming mode. This does not apply to

the EESAVE Fuse which will take effect once it is programmed. The fuses are also latched on

Power-up in Normal mode.

25.3 Signature Bytes

All Atmel microcontrollers have a three-byte signature code which identifies the device. This

code can be read in both serial and parallel mode, also when the device is locked. The three

bytes reside in a separate address space.

25.3.1 ATmega48 Signature Bytes

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x92 (indicates 4KB Flash memory).

3. 0x002: 0x05 (indicates ATmega48 device when 0x001 is 0x92).

25.3.2 ATmega88 Signature Bytes

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x93 (indicates 8KB Flash memory).

3. 0x002: 0x0A (indicates ATmega88 device when 0x001 is 0x93).

25.3.3 ATmega168 Signature Bytes

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x94 (indicates 16KB Flash memory).

3. 0x002: 0x06 (indicates ATmega168 device when 0x001 is 0x94).

25.4 Calibration Byte

The ATmega48/88/168 has a byte calibration value for the internal RC Oscillator. This byte

resides in the high byte of address 0x000 in the signature address space. During reset, this byte

is automatically written into the OSCCAL Register to ensure correct frequency of the calibrated

RC Oscillator.

25.5 Parallel Programming Parameters, Pin Mapping, and Commands

This section describes how to parallel program and verify Flash Program memory, EEPROM

Data memory, Memory Lock bits, and Fuse bits in the ATmega48/88/168. Pulses are assumed

to be at least 250 ns unless otherwise noted.

25.5.1 Signal Names

In this section, some pins of the ATmega48/88/168 are referenced by signal names describing

their functionality during parallel programming, see Figure 25-1 and Table 25-8. Pins not

described in the following table are referenced by pin names.

The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse.

The bit coding is shown in Table 25-10.

When pulsing WR or OE, the command loaded determines the action executed. The different

Commands are shown in Table 25-11.

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Figure 25-1. Parallel Programming

Table 25-8. Pin Name Mapping

Signal Name in

Programming Mode Pin Name I/O Function

RDY/BSY PD1 O 0: Device is busy programming, 1: Device is

ready for new command

OE PD2 I Output Enable (Active low)

WR PD3 I Write Pulse (Active low)

BS1 PD4 I Byte Select 1 (“0” selects Low byte, “1” selects

High byte)

XA0 PD5 I XTAL Action Bit 0

XA1 PD6 I XTAL Action Bit 1

PAG EL PD7 I Program memory and EEPROM Data Page

Load

BS2 PC2 I Byte Select 2 (“0” selects Low byte, “1” selects

2’nd High byte)

DATA {PC[1:0]: PB[5:0]} I/O Bi-directional Data bus (Output when OE is

low)

Table 25-9. Pin Values Used to Enter Programming Mode

Pin Symbol Value

PAGEL Prog_enable[3] 0

XA1 Prog_enable[2] 0

XA0 Prog_enable[1] 0

BS1 Prog_enable[0] 0

VCC

+5V

GND

XTAL1

PD1

PD2

PD3

PD4

PD5

PD6

PC[1:0]:PB[5:0]

DATA

RESET

PD7

+12 V

BS1

XA0

XA1

RDY/BSY

PAGEL

PC2

BS2

AVCC

+5V

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Table 25-10. XA1 and XA0 Coding

XA1 XA0 Action when XTAL1 is Pulsed

Load Flash or EEPROM Address (High or low address byte determined by

BS1).

0 1 Load Data (High or Low data byte for Flash determined by BS1).

1 0 Load Command

1 1 No Action, Idle

Table 25-11. Command Byte Bit Coding

Command Byte Command Executed

1000 0000 Chip Erase

0100 0000 Write Fuse bits

0010 0000 Write Lock bits

0001 0000 Write Flash

0001 0001 Write EEPROM

0000 1000 Read Signature Bytes and Calibration byte

0000 0100 Read Fuse and Lock bits

0000 0010 Read Flash

0000 0011 Read EEPROM

Table 25-12. No. of Words in a Page and No. of Pages in the Flash

Device Flash Size Page Size PCWORD

No. of

Pages PCPAGE PCMSB

ATmega48 2K words

(4K bytes) 32 words PC[4:0] 64 PC[10:5] 10

ATmega88 4K words

(8K bytes) 32 words PC[4:0] 128 PC[11:5] 11

ATmega168 8K words

(16K bytes) 64 words PC[5:0] 128 PC[12:6] 12

Table 25-13. No. of Words in a Page and No. of Pages in the EEPROM

Device

EEPROM

Size

Page

Size PCWORD

No. of

Pages PCPAGE EEAMSB

ATmega48 256 bytes 4 bytes EEA[1:0] 64 EEA[7:2] 7

ATmega88 512 bytes 4 bytes EEA[1:0] 128 EEA[8:2] 8

ATmega168 512 bytes 4 bytes EEA[1:0] 128 EEA[8:2] 8

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25.6 Serial Programming Pin Mapping

25.7 Parallel Programming

25.7.1 Enter Programming Mode

The following algorithm puts the device in parallel programming mode:

1. Apply 4.5 - 5.5V between VCC and GND.

2. Set RESET to “0” and toggle XTAL1 at least six times.

3. Set the Prog_enable pins listed in Table 25-9 on page 283 to “0000” and wait at least

100 ns.

4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100 ns after

+12V has been applied to RESET, will cause the device to fail entering programming

mode.

5. Wait at least 50 µs before sending a new command.

25.7.2 Considerations for Efficient Programming

The loaded command and address are retained in the device during programming. For efficient

programming, the following should be considered.

• The command needs only be loaded once when writing or reading multiple memory

locations.

• Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the

EESAVE Fuse is programmed) and Flash after a Chip Erase.

• Address high byte needs only be loaded before programming or reading a new 256 word

window in Flash or 256 byte EEPROM. This consideration also applies to Signature bytes

reading.

25.7.3 Chip Erase

The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock bits. The Lock bits are

not reset until the program memory has been completely erased. The Fuse bits are not

changed. A Chip Erase must be performed before the Flash and/or EEPROM are

reprogrammed.

Note: 1. The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed.

Load Command “Chip Erase”

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “1000 0000”. This is the command for Chip Erase.

4. Give XTAL1 a positive pulse. This loads the command.

5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.

6. Wait until RDY/BSY goes high before loading a new command.

Table 25-14. Pin Mapping Serial Programming

Symbol Pins I/O Description

MOSI PB3 I Serial Data in

MISO PB4 O Serial Data out

SCK PB5 I Serial Clock

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25.7.4 Programming the Flash

The Flash is organized in pages, see Table 25-12 on page 284. When programming the Flash,

the program data is latched into a page buffer. This allows one page of program data to be pro-

grammed simultaneously. The following procedure describes how to program the entire Flash

memory:

A. Load Command “Write Flash”

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “0001 0000”. This is the command for Write Flash.

4. Give XTAL1 a positive pulse. This loads the command.

B. Load Address Low byte

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “0”. This selects low address.

3. Set DATA = Address low byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the address low byte.

C. Load Data Low Byte

1. Set XA1, XA0 to “01”. This enables data loading.

2. Set DATA = Data low byte (0x00 - 0xFF).

3. Give XTAL1 a positive pulse. This loads the data byte.

D. Load Data High Byte

1. Set BS1 to “1”. This selects high data byte.

2. Set XA1, XA0 to “01”. This enables data loading.

3. Set DATA = Data high byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the data byte.

E. Latch Data

1. Set BS1 to “1”. This selects high data byte.

2. Give PAGEL a positive pulse. This latches the data bytes. (See Figure 25-3 for signal

waveforms)

F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.

While the lower bits in the address are mapped to words within the page, the higher bits address

the pages within the FLASH. This is illustrated in Figure 25-2 on page 287. Note that if less than

eight bits are required to address words in the page (pagesize < 256), the most significant bit(s)

in the address low byte are used to address the page when performing a Page Write.

G. Load Address High byte

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “1”. This selects high address.

3. Set DATA = Address high byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the address high byte.

H. Program Page

1. Give WR a negative pulse. This starts programming of the entire page of data.

RDY/BSY goes low.

2. Wait until RDY/BSY goes high (See Figure 25-3 for signal waveforms).

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I. Repeat B through H until the entire Flash is programmed or until all data has been

programmed.

J. End Page Programming

1. 1. Set XA1, XA0 to “10”. This enables command loading.

2. Set DATA to “0000 0000”. This is the command for No Operation.

3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals

are reset.

Figure 25-2. Addressing the Flash Which is Organized in Pages(1)

Note: 1. PCPAGE and PCWORD are listed in Table 25-12 on page 284.

Figure 25-3. Programming the Flash Waveforms(1)

Note: 1. “XX” is don’t care. The letters refer to the programming description above.

PROGRAM MEMORY

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

INSTRUCTION WORD

PAGE PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM

COUNTER

RDY/BSY

RESET +12V

PAGEL

BS2

0x10 ADDR. LOW ADDR. HIGH

DATA DATA L OW DATA H IG H ADDR. LOW DATA LOW DATA HIGH

XA1

XA0

BS1

XTAL1

XX XX XX

ABCDEBCDEGH

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25.7.5 Programming the EEPROM

The EEPROM is organized in pages, see Table 25-13 on page 284. When programming the

EEPROM, the program data is latched into a page buffer. This allows one page of data to be

programmed simultaneously. The programming algorithm for the EEPROM data memory is as

follows (refer to “Programming the Flash” on page 286 for details on Command, Address and

Data loading):

1. A: Load Command “0001 0001”.

2. G: Load Address High Byte (0x00 - 0xFF).

3. B: Load Address Low Byte (0x00 - 0xFF).

4. C: Load Data (0x00 - 0xFF).

5. E: Latch data (give PAGEL a positive pulse).

K: Repeat 3 through 5 until the entire buffer is filled.

L: Program EEPROM page

1. Set BS1 to “0”.

2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY

goes low.

3. Wait until to RDY/BSY goes high before programming the next page (See Figure 25-4

for signal waveforms).

Figure 25-4. Programming the EEPROM Waveforms

25.7.6 Reading the Flash

The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on

page 286 for details on Command and Address loading):

1. A: Load Command “0000 0010”.

2. G: Load Address High Byte (0x00 - 0xFF).

3. B: Load Address Low Byte (0x00 - 0xFF).

4. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.

5. Set BS1 to “1”. The Flash word high byte can now be read at DATA.

6. Set OE to “1”.

RDY/BSY

RESET +12V

PAGEL

BS2

0x11 ADDR. HIGH

DATA ADDR. LOW DATA ADDR. LOW DATA XX

XA1

XA0

BS1

XTAL1

AGBCEB C EL

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25.7.7 Reading the EEPROM

The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash”

on page 286 for details on Command and Address loading):

1. A: Load Command “0000 0011”.

2. G: Load Address High Byte (0x00 - 0xFF).

3. B: Load Address Low Byte (0x00 - 0xFF).

4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.

5. Set OE to “1”.

25.7.8 Programming the Fuse Low Bits

The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash”

on page 286 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Give WR a negative pulse and wait for RDY/BSY to go high.

25.7.9 Programming the Fuse High Bits

The algorithm for programming the Fuse High bits is as follows (refer to “Programming the

Flash” on page 286 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. Set BS1 to “0”. This selects low data byte.

25.7.10 Programming the Extended Fuse Bits

The algorithm for programming the Extended Fuse bits is as follows (refer to “Programming the

Flash” on page 286 for details on Command and Data loading):

1. 1. A: Load Command “0100 0000”.

2. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. 3. Set BS1 to “0” and BS2 to “1”. This selects extended data byte.

4. 4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. 5. Set BS2 to “0”. This selects low data byte.

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Figure 25-5. Programming the FUSES Waveforms

25.7.11 Programming the Lock Bits

The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on

page 286 for details on Command and Data loading):

1. A: Load Command “0010 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed

(LB1 and LB2 is programmed), it is not possible to program the Boot Lock bits by any

External Programming mode.

3. Give WR a negative pulse and wait for RDY/BSY to go high.

The Lock bits can only be cleared by executing Chip Erase.

25.7.12 Reading the Fuse and Lock Bits

The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programming the Flash”

on page 286 for details on Command loading):

1. A: Load Command “0000 0100”.

2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be

read at DATA (“0” means programmed).

3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be

read at DATA (“0” means programmed).

4. Set OE to “0”, BS2 to “1”, and BS1 to “0”. The status of the Extended Fuse bits can now

be read at DATA (“0” means programmed).

5. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at

DATA (“0” means programmed).

6. Set OE to “1”.

RDY/BSY

RESET +12V

PAGEL

0x40

DATA

DATA XX

XA1

XA0

BS1

XTAL1

0x40 DATA XX

Write Fuse Low byte Write Fuse high byte

0x40 DATA XX

Write Extended Fuse byte

BS2

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Figure 25-6. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read

25.7.13 Reading the Signature Bytes

The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on

page 286 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte (0x00 - 0x02).

3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.

4. Set OE to “1”.

25.7.14 Reading the Calibration Byte

The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on

page 286 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte, 0x00.

3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.

4. Set OE to “1”.

25.7.15 Parallel Programming Characteristics

Figure 25-7. Parallel Programming Timing, Including some General Timing Requirements

Lock Bits 0

BS2

Fuse High Byte

BS1

DATA

Fuse Low Byte 0

BS2

Extended Fuse Byte

Data & Contol

(DATA, XA0/1, BS1, BS2)

XTAL1

XHXL

WLWH

DVXH

XLDX

PLWL

WLRH

RDY/BSY

PAGEL

PHPL

PLBX

BVPH

XLWL

WLBX

BVWL

WLRL

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Figure 25-8. Parallel Programming Timing, Loading Sequence with Timing Requirements(1)

Note: 1. The timing requirements shown in Figure 25-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to load-

ing operation.

Figure 25-9. Parallel Programming Timing, Reading Sequence (within the Same Page) with

Timing Requirements(1)

Note: 1. The timing requirements shown in Figure 25-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to read-

ing operation.

Table 25-15. Parallel Programming Characteristics, VCC = 5V ± 10%

Symbol Parameter Min Typ Max Units

VPP Programming Enable Voltage 11.5 12.5 V

IPP Programming Enable Current 250 µA

tDVXH Data and Control Valid before XTAL1 High 67 ns

tXLXH XTAL1 Low to XTAL1 High 200 ns

tXHXL XTAL1 Pulse Width High 150 ns

tXLDX Data and Control Hold after XTAL1 Low 67 ns

tXLWL XTAL1 Low to WR Low 0 ns

XTAL1

PAGEL

PLXH

XLXH

XLPH

ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)

DATA

BS1

XA0

XA1

LOAD ADDRESS

(LOW BYTE)

LOAD DATA

(LOW BYTE)

LOAD DATA

(HIGH BYTE)

LOAD DATA

LOAD ADDRESS

(LOW BYTE)

XTAL1

ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)

DATA

BS1

XA0

XA1

LOAD ADDRESS

(LOW BYTE)

READ DATA

(LOW BYTE)

READ DATA

(HIGH BYTE)

LOAD ADDRESS

(LOW BYTE)

BVDV

OLDV

XLOL

OHDZ

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Notes: 1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits

commands.

2. tWLRH_CE is valid for the Chip Erase command.

25.8 Serial Downloading

Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while

RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (out-

put). After RESET is set low, the Programming Enable instruction needs to be executed first

before program/erase operations can be executed. NOTE, in Table 25-14 on page 285, the pin

mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the internal

SPI interface.

tXLPH XTAL1 Low to PAGEL high 0 ns

tPLXH PAGEL low to XTAL1 high 150 ns

tBVPH BS1 Valid before PAGEL High 67 ns

tPHPL PAGEL Pulse Width High 150 ns

tPLBX BS1 Hold after PAGEL Low 67 ns

tWLBX BS2/1 Hold after WR Low 67 ns

tPLWL PAGEL Low to WR Low 67 ns

tBVWL BS1 Valid to WR Low 67 ns

tWLWH WR Pulse Width Low 150 ns

tWLRL WR Low to RDY/BSY Low 0 1 µs

tWLRH WR Low to RDY/BSY High(1) 3.7 4.5 ms

tWLRH_CE WR Low to RDY/BSY High for Chip Erase(2) 7.5 9 ms

tXLOL XTAL1 Low to OE Low 0 ns

tBVDV BS1 Valid to DATA valid 0 250 ns

tOLDV OE Low to DATA Valid 250 ns

tOHDZ OE High to DATA Tri-stated 250 ns

Table 25-15. Parallel Programming Characteristics, VCC = 5V ± 10%

Symbol Parameter Min Typ Max Units

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Figure 25-10. Serial Programming and Verify(1)

Notes: 1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the

XTAL1 pin.

2. VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 1.8 - 5.5V

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming

operation (in the Serial mode ONLY) and there is no need to first execute the Chip Erase

instruction. The Chip Erase operation turns the content of every memory location in both the

Program and EEPROM arrays into 0xFF.

Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods

for the serial clock (SCK) input are defined as follows:

Low: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck >= 12 MHz

High: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck >= 12 MHz

25.8.1 Serial Programming Algorithm

When writing serial data to the ATmega48/88/168, data is clocked on the rising edge of SCK.

When reading data from the ATmega48/88/168, data is clocked on the falling edge of SCK. See

Figure 25-11 for timing details.

To program and verify the ATmega48/88/168 in the serial programming mode, the following

sequence is recommended (See four byte instruction formats in Table 25-17):

1. Power-up sequence:

Apply power between VCC and GND while RESET and SCK are set to “0”. In some sys-

tems, the programmer can not guarantee that SCK is held low during power-up. In this

case, RESET must be given a positive pulse of at least two CPU clock cycles duration

after SCK has been set to “0”.

2. Wait for at least 20 ms and enable serial programming by sending the Programming

Enable serial instruction to pin MOSI.

3. The serial programming instructions will not work if the communication is out of syn-

chronization. When in sync. the second byte (0x53), will echo back when issuing the

third byte of the Programming Enable instruction. Whether the echo is correct or not, all

four bytes of the instruction must be transmitted. If the 0x53 did not echo back, give

RESET a positive pulse and issue a new Programming Enable command.

VCC

GND

XTAL1

SCK

MISO

MOSI

RESET

+1.8 - 5.5V

AVCC

+1.8 - 5.5V

(2)

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4. The Flash is programmed one page at a time. The memory page is loaded one byte at

a time by supplying the 6 LSB of the address and data together with the Load Program

Memory Page instruction. To ensure correct loading of the page, the data low byte must

be loaded before data high byte is applied for a given address. The Program Memory

Page is stored by loading the Write Program Memory Page instruction with the 8 MSB

of the address. If polling is not used, the user must wait at least tWD_FLASH before issuing

the next page. (See Table 25-16.) Accessing the serial programming interface before

the Flash write operation completes can result in incorrect programming.

5. The EEPROM array is programmed one byte at a time by supplying the address and

data together with the appropriate Write instruction. An EEPROM memory location is

first automatically erased before new data is written. If polling is not used, the user must

wait at least tWD_EEPROM before issuing the next byte. (See Table 25-16.) In a chip

erased device, no 0xFFs in the data file(s) need to be programmed.

6. Any memory location can be verified by using the Read instruction which returns the

content at the selected address at serial output MISO.

7. At the end of the programming session, RESET can be set high to commence normal

operation.

8. Power-off sequence (if needed):

Set RESET to “1”.

Turn VCC power off.

25.8.2 Data Polling Flash

When a page is being programmed into the Flash, reading an address location within the page

being programmed will give the value 0xFF. At the time the device is ready for a new page, the

programmed value will read correctly. This is used to determine when the next page can be writ-

ten. Note that the entire page is written simultaneously and any address within the page can be

used for polling. Data polling of the Flash will not work for the value 0xFF, so when programming

this value, the user will have to wait for at least tWD_FLASH before programming the next page. As

a chip-erased device contains 0xFF in all locations, programming of addresses that are meant to

contain 0xFF, can be skipped. See Table 25-16 for tWD_FLASH value.

25.8.3 Data Polling EEPROM

When a new byte has been written and is being programmed into EEPROM, reading the

address location being programmed will give the value 0xFF. At the time the device is ready for

a new byte, the programmed value will read correctly. This is used to determine when the next

byte can be written. This will not work for the value 0xFF, but the user should have the following

in mind: As a chip-erased device contains 0xFF in all locations, programming of addresses that

are meant to contain 0xFF, can be skipped. This does not apply if the EEPROM is re-pro-

grammed without chip erasing the device. In this case, data polling cannot be used for the value

0xFF, and the user will have to wait at least tWD_EEPROM before programming the next byte. See

Table 25-16 for tWD_EEPROM value.

Table 25-16. Minimum Wait Delay Before Writing the Next Flash or EEPROM Location

Symbol Minimum Wait Delay

tWD_FLASH 4.5 ms

tWD_EEPROM 3.6 ms

tWD_ERASE 9.0 ms

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Figure 25-11. Serial Programming Waveforms

Table 25-17. Serial Programming Instruction Set

Instruction

Instruction Format

OperationByte 1 Byte 2 Byte 3 Byte4

Programming Enable 1010

1100

0101

0011

xxxx

Enable Serial Programming

after RESET goes low.

Chip Erase 1010

1100

100x

xxxx

Chip Erase EEPROM and

Flash.

Read Program

Memory

0010

H000

000a

aaaa

bbbb

oooo

Read H (high or low) data o

from Program memory at

word address a:b.

Load Program

Memory Page

0100

H000

000x

xxxx

xxbb

bbbb

iiii

Write H (high or low) data i to

Program Memory page at

word address b. Data low

byte must be loaded before

Data high byte is applied

within the same address.

Write Program

Memory Page

0100

1100

000a

aaaa

bbxx

xxxx

Write Program Memory Page

at address a:b.

Read EEPROM

Memory

1010

0000

000x

xxaa

bbbb

oooo

Read data o from EEPROM

memory at address a:b.

Write EEPROM

Memory

1100

0000

000x

xxaa

bbbb

iiii

Write data i to EEPROM

memory at address a:b.

Load EEPROM

Memory Page (page

access)

1100

0001

0000

00bb

iiii

Load data i to EEPROM

memory page buffer. After

data is loaded, program

EEPROM page.

Write EEPROM

Memory Page (page

access)

1100

0010

00xx

xxaa

bbbb

bb00

xxxx

xxxx Write EEPROM page at

address a:b.

Read Lock bits

0101

1000

0000

xxxx

xxoo

oooo

Read Lock bits. “0” =

programmed, “1” =

unprogrammed. See Table

25-1 on page 278 for details.

Write Lock bits

1010

1100

111x

xxxx

11ii

iiii

Write Lock bits. Set bits = “0”

to program Lock bits. See

Table 25-1 on page 278 for

details.

MSB

LSB

SERIAL CLOCK INPUT

(SCK)

SERIAL DATA INPUT

(MOSI)

(MISO)

SAMPLE

SERIAL DATA OUTPUT

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Note: a = address high bits, b = address low bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care
25.8.4 SPI Serial Programming Characteristics
For characteristics of the SPI module see “SPI Timing Characteristics” on page 305.
Read Signature Byte 0011 
0000
000x 
xxxx
xxxx 
xxbb
oooo 
oooo
Read Signature Byte o at 
address b.
Write Fuse bits
1010 
1100
1010 
0000
xxxx 
xxxx
iiii 
iiii
Set bits = “0” to program, “1” 
to unprogram. See Table 
XXX on page XXX for details.
Write Fuse High bits
1010 
1100
1010 
1000
xxxx 
xxxx
iiii 
iiii
Set bits = “0” to program, “1” 
to unprogram. See Table 21-1 
on page 242 for details.
Write Extended Fuse 
Bits
1010 
1100
1010 
0100
xxxx 
xxxx
xxxx 
xxii
Set bits = “0” to program, “1” 
to unprogram. See Table 
25-4 on page 280 for 
details.
Read Fuse bits
0101 
0000
0000 
0000
xxxx 
xxxx
oooo 
oooo
Read Fuse bits. “0” = 
programmed, “1” = 
unprogrammed. See Table 
XXX on page XXX for details.
Read Fuse High bits
0101 
1000
0000 
1000
xxxx 
xxxx
oooo 
oooo
Read Fuse High bits. “0” = 
pro-grammed, “1” = 
unprogrammed. See Table 
21-1 on page 242 for details.
Read Extended Fuse 
Bits
0101 
0000
0000 
1000
xxxx 
xxxx
oooo 
oooo
Read Extended Fuse bits. “0” 
= pro-grammed, “1” = 
unprogrammed. See Table 
25-4 on page 280 for details.
Read Calibration Byte 0011 
1000
000x 
xxxx
0000 
0000
oooo 
oooo Read Calibration Byte
Poll RDY/BSY
1111 
0000
0000 
0000
xxxx 
xxxx
xxxx 
xxxo
If o = “1”, a programming 
operation is still busy. Wait 
until this bit returns to “0” 
before applying another 
command.
Table 25-17. Serial Programming Instruction Set
Instruction
Instruction Format
OperationByte 1 Byte 2 Byte 3 Byte4

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26. Electrical Characteristics
26.1 Absolute Maximum Ratings*
26.2 DC Characteristics
Operating Temperature.................................. -55°C to +125°C *NOTICE: Stresses beyond those listed under “Absolute 
Maximum Ratings” may cause permanent dam-
age to the device. This is a stress rating only and 
functional operation of the device at these or 
other conditions beyond those indicated in the 
operational sections of this specification is not 
implied. Exposure to absolute maximum rating 
conditions for extended periods may affect 
device reliability.
Storage Temperature ..................................... -65°C to +150°C
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V
Voltage on RESET with respect to Ground......-0.5V to +13.0V
Maximum Operating Voltage ............................................ 6.0V
DC Current per I/O Pin ............................................... 40.0 mA
DC Current VCC and GND Pins................................ 200.0 mA
TA = -40°C to 125°C, VCC = 2.7V to 5.5V (unless otherwise noted) 
Symbol Parameter Condition Min.(5) Typ. Max.(5) Units
VIL Input Low Voltage,Except 
XTAL1 and Reset pin VCC = 2.7V - 5.5V -0.5 0.3VCC(1) V
VIL1 Input Low Voltage,
XTAL1 pin VCC = 2.7V - 5.5V -0.5 0.1VCC(1) V
VIL2 Input Low Voltage, 
RESET pin VCC = 2.7V - 5.5V -0.5 0.1VCC(1) V
VIH
Input High Voltage, 
Except XTAL1 and 
RESET pins
VCC = 2.7V - 5.5V 0.6VCC(2) VCC + 0.5 V
VIH1 Input High Voltage, 
XTAL1 pin VCC = 2.7V - 5.5V 0.7VCC(2) VCC + 0.5 V
VIH2 Input High Voltage, 
RESET pin VCC = 2.7V - 5.5V 0.9VCC(2) VCC + 0.5 V
VOL Output Low Voltage(3) IOL = 20mA, VCC = 5V
IOL = 5mA, VCC = 3V
0.8
0.5 V
VOH Output High Voltage(4) IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
4.1
2.3 V
IIL Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value) 50 nA
IIH Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value) 50 nA
RRST Reset Pull-up Resistor Vcc = 5.0V, Vin = 0V 30 60 kΩ
RPU I/O Pin Pull-up Resistor 20 50 kΩ

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Notes: 1. “Max” means the highest value where the pin is guaranteed to be read as low

2. “Min” means the lowest value where the pin is guaranteed to be read as high

3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state

conditions (non-transient), the following must be observed:

ATmega48:

1] The sum of all IOL, for ports C0 - C5, should not exceed 70 mA.

2] The sum of all IOL, for ports C6, D0 - D4, should not exceed 70 mA.

3] The sum of all IOL, for ports B0 - B7, D5 - D7, should not exceed 70 mA.

ATmega88/168:

1] The sum of all IOL, for ports C0 - C5, should not exceed 100 mA.

2] The sum of all IOL, for ports C6, D0 - D4, should not exceed 100 mA.

3] The sum of all IOL, for ports B0 - B7, D5 - D7, should not exceed 100 mA.

If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater

than the listed test condition.

4. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state

conditions (non-transient), the following must be observed:

ATmega48:

1] The sum of all IOH, for ports C0 - C5, should not exceed 70 mA.

2] The sum of all IOH, for ports C6, D0 - D4, should not exceed 70 mA.

3] The sum of all IOH, for ports B0 - B7, D5 - D7, should not exceed 70 mA.

ATmega88/168:

1] The sum of all IOH, for ports C0 - C5, should not exceed 100 mA.

2] The sum of all IOH, for ports C6, D0 - D4, should not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B7, D5 - D7, should not exceed 100 mA.

ICC

Power Supply Current(6)

Active 4MHz, VCC = 3V

(ATmega48/88/168L) 1.8 3.0 mA

Active 8MHz, VCC = 5V

(ATmega48/88/168) 6.0 10 mA

Active 15MHz, VCC = 5V

(ATmega48/88/168) 10.0 16 mA

Idle 4MHz, VCC = 3V

(ATmega48/88/168V) 0.4 1 mA

Idle 8MHz, VCC = 5V

(ATmega48/88/168L) 1.4 2.4 mA

Idle 15MHz, VCC = 5V

(ATmega48/88/168) 2.8 4 mA

Power-down mode

WDT enabled, VCC = 3V 8 30 µA

WDT enabled, VCC = 5V 12.6 50 µA

WDT disabled, VCC = 3V 5 24 µA

WDT disabled, VCC = 5V 6.6 36 µA

VACIO Analog Comparator

Input Offset Voltage

VCC = 5V

Vin = VCC/2 10 40 mV

IACLK Analog Comparator

Input Leakage Current

VCC = 5V

Vin = VCC/2 -50 50 nA

tACID Analog Comparator

Propagation Delay VCC = 4.5V 140 ns

TA = -40°C to 125°C, VCC = 2.7V to 5.5V (unless otherwise noted)

Symbol Parameter Condition Min.(5) Typ. Max.(5) Units

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If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current

greater than the listed test condition.

5. All DC Characteristics contained in this datasheet are based on actual ATmega88 microcontrollers characterization.

6. Values with “Power Reduction Register - PRR” enabled (0xEF).

26.3 External Clock Drive Waveforms

Figure 26-1. External Clock Drive Waveforms

26.4 External Clock Drive

IL1

IH1

Table 26-1. External Clock Drive

Symbol Parameter

VCC=2.7-5.5V VCC=4.5-5.5V

UnitsMin. Max. Min. Max.

1/tCLCL Oscillator Frequency 0 8 0 16 MHz

tCLCL Clock Period 125 62.5 ns

tCHCX High Time 50 25 ns

tCLCX Low Time 50 25 ns

tCLCH Rise Time 1.6 0.5 µs

tCHCL Fall Time 1.6 0.5 µs

ΔtCLCL

Change in period from

one clock cycle to the

22%

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26.5 Maximum Speed vs. VCC

Maximum frequency is dependent on VCC. As shown in Figure 26-2, the Maximum Frequency vs.

VCC curve is linear between 2.7V < VCC < 4.5V.

Figure 26-2. Maximum Frequency vs. VCC, ATmega48/88/168

26.6 LIN Re-synchronization algorithm

26.7 Synchronization Algorithm

The possibility to change the value of OSCCAL during the Oscillator operation allows for in-situ

calibration of the slave node to entering Master frames. The principle of operation is to measure

the TBit during the SYNCH Byte and to change the calibration value of OSCCAL to recover from

local frequency drifts due to local voltage or temperature deviation. The algorithm used for the

synchronization of the internal RC oscillator is depicted in Figure 26-3

16 MHz

8 MHz

2.7V 4.5V 5.5V

Safe Operating Area

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Figure 26-3. Dichotomic Algorithm Used for LIN Slave Clock Re-synchronization

26.8 Precaution Against OSCCAL Discontinuity

The Figure 28-23 illustrates the on-purpose discontinuity of RC Frequency versus OSCCAL

value. For one correct re-synchronization, the frequency change must be kept on the same side

of the discontinuity (no change of OSCCAL[7]). Since there will be no device having frequency

changed by more than 10% (see Figure 28-21), thus no reason to change the frequency value

by more than 10%. Therefore, when calibration tries to cross the border because of subsequent

increase (or decrease) in OSCCAL values, then the routine must be stopped.

example: For parts operating in the lower part of the curve, if New_OSCCAL >127 then

New_OSCCAL = 127. Similar for parts operating on the high side of the discontinuity.

Measuring

actual TBit

Increment

OSCCAL

Decrement

OSCCAL

-2% < Delta(TBit) < 2%

Delta(TBit) > 2%

Delta(TBit) < -2%

STOP:

Oscillator

Calibrated

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26.8.1 RC Oscillator Precision for LIN Slave implementation

For LIN slave devices, the precision of the RC oscillator before and after re-synchronization are

described in the Figure 26-2

27. 2-wire Serial Interface Characteristics

Table 27-1 describes the requirements for devices connected to the 2-wire Serial Bus. The ATmega48/88/168 2-wire Serial

Interface meets or exceeds these requirements under the noted conditions.

Timing symbols refer to Figure 27-1.

Table 26-2. Oscillator Tolerance Before and After re-Synchronization Algorithm

(2.7V<VCC<5.5V, -40°C to +125°C)

Parameter Clock Tolerance ΔF/FMaster

FTOL_UNSYNCH

Deviation of slave node clock from the nominal clock rate before

synchronization; relevant for nodes making use of

synchronization and direct SYNCH BREAK detection.

±14.0 %

FTOL_SYNCH

Deviation of slave node clock relative to the master node clock

after synchronization; relevant for nodes making use of

synchronization; any slave node must stay within this tolerance

for all fields of a frame which follow the SYNCH FIELD.

Note: For communication between any two nodes their bit rate

must not differ by more than ±2%.

±2.0 %

Table 27-1. 2-wire Serial Bus Requirements

Symbol Parameter Condition Min Max Units

VIL Input Low-voltage -0.5 0.3 VCC V

VIH Input High-voltage 0.7 VCC VCC + 0.5 V

Vhys(1) Hysteresis of Schmitt Trigger Inputs 0.05 VCC(2) –V

VOL(1) Output Low-voltage 3 mA sink current 0 0.4 V

tr(1) Rise Time for both SDA and SCL 20 + 0.1Cb(3)(2) 300 ns

tof(1) Output Fall Time from VIHmin to VILmax 10 pF < Cb < 400 pF(3) 20 + 0.1Cb(3)(2) 250 ns

tSP(1) Spikes Suppressed by Input Filter 0 50(2) ns

IiInput Current each I/O Pin 0.1VCC < Vi < 0.9VCC -10 10 µA

Ci(1) Capacitance for each I/O Pin – 10 pF

fSCL SCL Clock Frequency fCK(4) > max(16fSCL, 250kHz)(5) 0 400 kHz

Rp Value of Pull-up resistor

fSCL ≤ 100 kHz

fSCL > 100 kHz

tHD;STA Hold Time (repeated) START Condition fSCL ≤ 100 kHz 4.0 – µs

fSCL > 100 kHz 0.6 – µs

tLOW Low Period of the SCL Clock fSCL ≤ 100 kHz(6) 4.7 – µs

fSCL > 100 kHz(7) 1.3 – µs

VCC 0,4V–

3mA

----------------------------

1000ns

-------------------

VCC 0,4V–

3mA

----------------------------

300ns

----------------

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Notes: 1. In ATmega48/88/168, this parameter is characterized and not 100% tested.

2. Required only for fSCL > 100 kHz.

3. Cb = capacitance of one bus line in pF.

4. fCK = CPU clock frequency

5. This requirement applies to all ATmega48/88/168 2-wire Serial Interface operation. Other devices connected to the 2-wire

Serial Bus need only obey the general fSCL requirement.

6. The actual low period generated by the ATmega48/88/168 2-wire Serial Interface is (1/fSCL - 2/fCK), thus fCK must be greater

than 6 MHz for the low time requirement to be strictly met at fSCL = 100 kHz.

7. The actual low period generated by the ATmega48/88/168 2-wire Serial Interface is (1/fSCL - 2/fCK), thus the low time require-

ment will not be strictly met for fSCL > 308 kHz when fCK = 8 MHz. Still, ATmega48/88/168 devices connected to the bus may

communicate at full speed (400 kHz) with other ATmega48/88/168 devices, as well as any other device with a proper tLOW

acceptance margin.

Figure 27-1. 2-wire Serial Bus Timing

tHIGH High period of the SCL clock fSCL ≤ 100 kHz 4.0 – µs

fSCL > 100 kHz 0.6 – µs

tSU;STA Set-up time for a repeated START condition fSCL ≤ 100 kHz 4.7 – µs

fSCL > 100 kHz 0.6 – µs

tHD;DAT Data hold time fSCL ≤ 100 kHz 0 3.45 µs

fSCL > 100 kHz 0 0.9 µs

tSU;DAT Data setup time fSCL ≤ 100 kHz 250 – ns

fSCL > 100 kHz 100 – ns

tSU;STO Setup time for STOP condition fSCL ≤ 100 kHz 4.0 – µs

fSCL > 100 kHz 0.6 – µs

tBUF Bus free time between a STOP and START

condition

fSCL ≤ 100 kHz 4.7 – µs

fSCL > 100 kHz 1.3 – µs

Table 27-1. 2-wire Serial Bus Requirements

Symbol Parameter Condition Min Max Units

SU;STA

LOW

HIGH

LOW

HD;STA

HD;DAT

SU;DAT

SU;STO

BUF

SCL

SDA

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27.1 SPI Timing Characteristics

See Figure 27-2 and Figure 27-3 for details.

Note: 1. In SPI Programming mode the minimum SCK high/low period is:

- 2 tCLCL for fCK < 12 MHz

- 3 tCLCL for fCK > 12 MHz

Figure 27-2. SPI Interface Timing Requirements (Master Mode)

Table 27-2. SPI Timing Parameters

Description Mode Min Typ Max

1 SCK period Master See Table 16-4

2 SCK high/low Master 50% duty cycle

3 Rise/Fall time Master 3.6

4 Setup Master 10

5 Hold Master 10

6 Out to SCK Master 0.5 • tsck

7 SCK to out Master 10

8 SCK to out high Master 10

9SS

low to out Slave 15

10 SCK period Slave 4 • tck

11 SCK high/low(1) Slave 2 • tck

12 Rise/Fall time Slave 1600

13 Setup Slave 10

14 Hold Slave tck

15 SCK to out Slave 15

16 SCK to SS high Slave 20

17 SS high to tri-state Slave 10

18 SS low to SCK Slave 20

MOSI

(Data Output)

SCK

(CPOL = 1)

MISO

(Data Input)

SCK

(CPOL = 0)

MSB LSB

LSBMSB

...

345

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Figure 27-3. SPI Interface Timing Requirements (Slave Mode)

27.2 ADC Characteristics

MISO

(Data Output)

SCK

(CPOL = 1)

MOSI

(Data Input)

SCK

(CPOL = 0)

MSB LSB

LSBMSB

...

11 11

1213 14

Table 27-3. ADC Characteristics

Symbol Parameter Condition Min Typ Max Units

Resolution 10 Bits

Absolute accuracy (Including

INL, DNL, quantization error,

gain and offset error)

VREF = 4V, VCC = 4V,

ADC clock = 200 kHz 23.5LSB

VREF = 4V, VCC = 4V,

ADC clock = 200 kHz

Noise Reduction Mode

23.5LSB

Integral Non-Linearity (INL) VREF = 4V, VCC = 4V,

ADC clock = 200 kHz 0.6 2.5 LSB

Differential Non-Linearity

(DNL)

VREF = 4V, VCC = 4V,

ADC clock = 200 kHz 0.30 1.0 LSB

Gain Error VREF = 4V, VCC = 4V,

ADC clock = 200 kHz -3.5 -1.3 3.5 LSB

Offset Error VREF = 4V, VCC = 4V,

ADC clock = 200 kHz 1.8 3.5 LSB

Conversion Time Free Running Conversion 13 cycles µs

Clock Frequency 50 200 kHz

AVCC Analog Supply Voltage VCC - 0.3 VCC + 0.3 V

VREF Reference Voltage 1.0 AVCC V

VIN Input Voltage GND VREF V

Input Bandwidth 38.5 kHz

VINT Internal Voltage Reference 1.0 1.1 1.2 V

RREF Reference Input Resistance 25.6 32 38.4 kΩ

RAIN Analog Input Resistance 100 MΩ

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28. ATmega48/88/168 Typical Characteristics

28.1 Active Supply Current

Figure 28-1. Active Supply Current vs. Frequency (1 - 20 MHz)

Figure 28-2. Idle Supply Current vs. Frequency (1 - 20 MHz)

ACTIVE SUPPLY CURRENT vs. FREQUENCY

Temp = 125c

5.5 V

5.0 V

4.5 V

3.3 V

3.0 V

2.7 V

02468101214161820

Frequency (MHz)

(mA)

IDLE SUPPLY CURRENT vs. FREQUENCY

Temp = 125˚c

5.5 V

5.0 V

4.5 V

3.3 V

3.0 V

2.7 V

0 2 4 6 8 10 12 14 16 18 20

Frequency (MHz)

(mA)

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28.1.1 Power-Down Supply Current

Figure 28-3. Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)

Figure 28-4. Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)

POWER-DOWN SUPPLY CURRENT vs. V

WATCHDOG TIMER DISABLED / Vt Fast corners excluded

3 3.5 4 4.5 5 5.5

(V)

(uA)

125

-40

POWER-DOWN SUPPLY CURRENT vs. VCC

WATCHDOG TIMER DISABLED / Vt Fast corners excluded

2.5 3 3.5 4 4.5 5 5.5

(V)

(uA)

125

-40

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28.1.2 Pin Pull-up

Figure 28-5. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)

Figure 28-6. Output Low Voltage vs. Output Low Current (VCC = 5V)

Figure 28-7. Output Low Voltage vs. Output Low Current (VCC = 3V)

100

120

140

160

0123456

(V)

(uA)

125

-40

I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE

V = 5V

I/O PIN OUTPUT VOLTAGE vs. SINK CURRENT

Vcc = 5.00v

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14 16 18 20

(mA)

Vol (V)

I/O PIN OUTPUT VOLTAGE vs. SINK CURRENT

Vcc = 3V

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.2

0.4

0.6

0.8

1.2

0 2 4 6 8 10 12 14 16 18 20

VOL (V)

IOL (mA)

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Figure 28-8. Output High Voltage vs. Output High Current (VCC = 5V)

Figure 28-9. Output High Voltage vs. Output High Current (VCC = 3V)

Figure 28-10. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (VCC = 5V)

I/O PIN OUTPUT VOLTAGE vs. SOURCE CURRENT

Vcc = 5.00v

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

4.2

4.4

4.6

4.8

5.2

0 2 4 6 8 10 12 14 16 18 20

(mA)

Voh (V)

I/O PIN OUTPUT VOLTAGE vs. SOURCE CURRENT

= 3V

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.5

1.5

2.5

3.5

0 2 4 6 8 10 12 14 16 18 20

(mA)

Current (V)

100

120

140

160

0123456

(V)

(uA)

125

-40

I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE

V = 5V

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28.1.3 Pin Driver Strength

Figure 28-11. Output Low Voltage vs. Ouput Low Current (VCC = 5.0V)

Figure 28-12. Output Low Voltage vs. Ouput Low Current (VCC = 3.0V)

Figure 28-13. Output High Voltage vs. Ouput High Current (VCC = 5.0V)

I/O PIN OUTPUT VOLTAGE vs. SINK CURRENT

Vcc = 5.00v

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14 16 18 20

(mA)

Vol (V)

I/O PIN OUTPUT VOLTAGE vs. SINK CURRENT

Vcc = 3V

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.2

0.4

0.6

0.8

1.2

0 2 4 6 8 10 12 14 16 18 20

VOL (V)

IOL (mA)

I/O PIN OUTPUT VOLTAGE vs. SOURCE CURRENT

Vcc = 5.00v

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

4.2

4.4

4.6

4.8

5.2

0 2 4 6 8 10 12 14 16 18 20

(mA)

Voh (V)

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Figure 28-14. Output High Voltage vs. Ouput High Current (VCC = 3.0V)

28.1.4 Pin Thresholds and Hysteresis

Figure 28-15. I/O Pin Input Threshold vs. VCC (VIH, I/O Pin Read as ‘1

Figure 28-16. I/O Pin Input Threshold vs. VCC (VIL, I/O Pin Read as ‘0’)

I/O PIN OUTPUT VOLTAGE vs. SOURCE CURRENT

= 3V

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.5

1.5

2.5

3.5

0 2 4 6 8 10 12 14 16 18 20

(mA)

Current (V)

IO INPUT THRESHOLD VOLTAGE vs. V

V IH, IO PIN REA D A S '1'

1.5

2.5

3.5

22.533.544.555.5

(V)

Vih (V)

125

-40

IO INPUT THRESHOLD VOLTAGE vs. V

VIL, IO PIN READ AS '0'

125 ˚C

-40 ˚C

0.5

1.5

2.5

2 2.5 3 3.5 4 4.5 5 5.5 6

(V)

Vil (V)

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Figure 28-17. Reset Input Threshold Voltage vs. VCC (VIH, Reset Pin Read as ‘1’)

Figure 28-18. Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read as ‘0’)

28.1.5 Internal Oscillator Speed

Figure 28-19. Watchdog Oscillator Frequency vs. VCC

RESET INPUT THRESHOLD VOLTAGE vs. V

V IH, IO PIN REA D A S '1'

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.5

1.5

2.5

3.5

22.533.544.555.5

(V)

Threshold (V)

RESET INPUT THRESHOLD VOLTAGE vs. V

VIL, IO PIN READ AS '0'

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0.5

1.5

2.5

22.533.544.555.5

(V)

Threshold (V)

WATCHDOG OSCILLATOR FREQUENCY vs. OPERATING VOLTAGE

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

110

112

114

116

118

120

122

124

126

128

130

2.5 3 3.5 4 4.5 5 5.5

(V)

(kHz)

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Figure 28-20. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature

Figure 28-21. Calibrated 8 MHz RC Oscillator Frequency vs. VCC

Figure 28-22. Calibrated 8 MHz RC Oscillator Frequency vs. OSCCAL Value (for

ATmega48-15AZ and ATmega168-15AZ)

CALIBRATED XXXMHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE

5.0 V

2.7 V

7.6

7.7

7.8

7.9

8.1

8.2

8.3

8.4

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120

Temperature

(MHz)

CALIBRATED XXXMHz RC OSCILLATOR FREQUENCY vs. OPERATING VOLTAGE

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

7.2

7.4

7.6

7.8

8.2

8.4

2.533.544.555.5

(V)

(MHz)

CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

Vcc = 3.0v

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

OSCCA L (X1)

(MHz)

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Atmel ATmega48/88/168 Automotive

Figure 28-23. Calibrated 8 MHz RC Oscillator Frequency vs. OSCCAL Value (for

ATmega88-15AZ only)

28.1.6 BOD Thresholds and Analog Comparator Offset

Figure 28-24. BOD Threshold vs. Temperature (BODLEVEL is 4.0V)

CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

OSCCAL (X1)

(MHz)

BOD THRESHOLDS vs. TEMPERATURE

BOD setting = 4.30v

Rising VCC

Falling VCC

4.1

4.2

4.3

4.4

4.5

4.6

-55 -45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125

Temperature (C)

Threshold (V)

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Figure 28-25. BOD Threshold vs. Temperature (BODLEVEL is 2.7V)

Figure 28-26. Bandgap Voltage vs. VCC

28.1.7 Peripheral Units

Figure 28-27. Analog to Digital Converter GAIN vs. VCC

BOD THRESHOLDS vs. TEMPERATURE

BOD setting = 2.70v

Rising VCC

Falling VCC

2.4

2.5

2.6

2.7

2.8

2.9

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120

Temperature (C)

Threshold (V)

BANDGAP VOLTAGE vs. V

125 ˚C

85 ˚C

25 ˚C

-40 ˚C

1.075

1.08

1.085

1.09

1.095

1.1

2.5 3 3.5 4 4.5 5 5.5

Vcc (V)

Bandgap Voltage (V)

Gain vs. Temperature

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

-50 0 50 100 150

Temperature

Error (LSB)

4V IDLE

4V STD

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Figure 28-28. Analog to Digital Converter OFFSET vs. VCC

Figure 28-29. Analog to Digital Converter DNL vs. VCC

Figure 28-30. Analog to Digital Converter INL vs. VCC

Offset vs. Temperature

0.00

0.50

1.00

1.50

2.00

2.50

-50 0 50 100 150

Temperature

Error (LSB)

4V IDLE

4V STD

DNL vs. Temperature

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

-50 0 50 100 150

Temperature

Error (LSB)

4V IDLE

4V STD

INL vs. Temperature

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

-50 0 50 100 150

Temperature

Error (LSB)

4V IDLE

4V STD

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29. Register Summary

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

(0xFF) Reserved – – – – – – – –

(0xFE) Reserved – – – – – – – –

(0xFD) Reserved – – – – – – – –

(0xFC) Reserved – – – – – – – –

(0xFB) Reserved – – – – – – – –

(0xFA) Reserved – – – – – – – –

(0xF9) Reserved – – – – – – – –

(0xF8) Reserved – – – – – – – –

(0xF7) Reserved – – – – – – – –

(0xF6) Reserved – – – – – – – –

(0xF5) Reserved – – – – – – – –

(0xF4) Reserved – – – – – – – –

(0xF3) Reserved – – – – – – – –

(0xF2) Reserved – – – – – – – –

(0xF1) Reserved – – – – – – – –

(0xF0) Reserved – – – – – – – –

(0xEF) Reserved – – – – – – – –

(0xEE) Reserved – – – – – – – –

(0xED) Reserved – – – – – – – –

(0xEC) Reserved – – – – – – – –

(0xEB) Reserved – – – – – – – –

(0xEA) Reserved – – – – – – – –

(0xE9) Reserved – – – – – – – –

(0xE8) Reserved – – – – – – – –

(0xE7) Reserved – – – – – – – –

(0xE6) Reserved – – – – – – – –

(0xE5) Reserved – – – – – – – –

(0xE4) Reserved – – – – – – – –

(0xE3) Reserved – – – – – – – –

(0xE2) Reserved – – – – – – – –

(0xE1) Reserved – – – – – – – –

(0xE0) Reserved – – – – – – – –

(0xDF) Reserved – – – – – – – –

(0xDE) Reserved – – – – – – – –

(0xDD) Reserved – – – – – – – –

(0xDC) Reserved – – – – – – – –

(0xDB) Reserved – – – – – – – –

(0xDA) Reserved – – – – – – – –

(0xD9) Reserved – – – – – – – –

(0xD8) Reserved – – – – – – – –

(0xD7) Reserved – – – – – – – –

(0xD6) Reserved – – – – – – – –

(0xD5) Reserved – – – – – – – –

(0xD4) Reserved – – – – – – – –

(0xD3) Reserved – – – – – – – –

(0xD2) Reserved – – – – – – – –

(0xD1) Reserved – – – – – – – –

(0xD0) Reserved – – – – – – – –

(0xCF) Reserved – – – – – – – –

(0xCE) Reserved – – – – – – – –

(0xCD) Reserved – – – – – – – –

(0xCC) Reserved – – – – – – – –

(0xCB) Reserved – – – – – – – –

(0xCA) Reserved – – – – – – – –

(0xC9) Reserved – – – – – – – –

(0xC8) Reserved – – – – – – – –

(0xC7) Reserved – – – – – – – –

(0xC6) UDR0 USART I/O Data Register 184

(0xC5) UBRR0H USART Baud Rate Register High 188

(0xC4) UBRR0L USART Baud Rate Register Low 188

(0xC3) Reserved – – – – – – – –

(0xC2) UCSR0C UMSEL01 UMSEL00 UPM01 UPM00 USBS0 UCSZ01 /UDORD0 UCSZ00 / UCPHA0 UCPOL0 186/199

(0xC1) UCSR0B RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80 185

(0xC0) UCSR0A RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0 184

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(0xBF) Reserved – – – – – – – –
(0xBE) Reserved – – – – – – – –
(0xBD) TWAMR TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0 –213
(0xBC) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE 210
(0xBB) TWDR  2-wire Serial Interface Data Register 212
(0xBA) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE 213
(0xB9) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 –TWPS1TWPS0 212
(0xB8) TWBR  2-wire Serial Interface Bit Rate Register 210
(0xB7) Reserved – – – – – – –
(0xB6) ASSR – EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB 154
(0xB5) Reserved – – – – – – – –
(0xB4) OCR2B Timer/Counter2 Output Compare Register B 150
(0xB3) OCR2A  Timer/Counter2 Output Compare Register A 150
(0xB2) TCNT2  Timer/Counter2 (8-bit) 150
(0xB1) TCCR2B FOC2A FOC2B –– WGM22 CS22 CS21 CS20 149
(0xB0) TCCR2A COM2A1 COM2A0 COM2B1 COM2B0 –– WGM21 WGM20 146
(0xAF) Reserved – – – – – – – –
(0xAE) Reserved – – – – – – – –
(0xAD) Reserved – – – – – – – –
(0xAC) Reserved – – – – – – – –
(0xAB) Reserved – – – – – – – –
(0xAA) Reserved – – – – – – – –
(0xA9) Reserved – – – – – – – –
(0xA8) Reserved – – – – – – – –
(0xA7) Reserved – – – – – – – –
(0xA6) Reserved – – – – – – – –
(0xA5) Reserved – – – – – – – –
(0xA4) Reserved – – – – – – – –
(0xA3) Reserved – – – – – – – –
(0xA2) Reserved – – – – – – – –
(0xA1) Reserved – – – – – – – –
(0xA0) Reserved – – – – – – – –
(0x9F) Reserved – – – – – – – –
(0x9E) Reserved – – – – – – – –
(0x9D) Reserved – – – – – – – –
(0x9C) Reserved – – – – – – – –
(0x9B) Reserved – – – – – – – –
(0x9A) Reserved – – – – – – – –
(0x99) Reserved – – – – – – – –
(0x98) Reserved – – – – – – – –
(0x97) Reserved – – – – – – – –
(0x96) Reserved – – – – – – – –
(0x95) Reserved – – – – – – – –
(0x94) Reserved – – – – – – – –
(0x93) Reserved – – – – – – – –
(0x92) Reserved – – – – – – – –
(0x91) Reserved – – – – – – – –
(0x90) Reserved – – – – – – – –
(0x8F) Reserved – – – – – – – –
(0x8E) Reserved – – – – – – – –
(0x8D) Reserved – – – – – – – –
(0x8C) Reserved – – – – – – – –
(0x8B) OCR1BH Timer/Counter1 - Output Compare Register B High Byte 132
 (0x8A) OCR1BL Timer/Counter1 - Output Compare Register B Low Byte 132
(0x89) OCR1AH Timer/Counter1 - Output Compare Register A High Byte 132
(0x88) OCR1AL Timer/Counter1 - Output Compare Register A Low Byte 132
(0x87) ICR1H Timer/Counter1 - Input Capture Register High Byte 132
(0x86) ICR1L Timer/Counter1 - Input Capture Register Low Byte 132
(0x85) TCNT1H Timer/Counter1 - Counter Register High Byte 131
(0x84) TCNT1L Timer/Counter1 - Counter Register Low Byte 131
(0x83) Reserved – – – – – – – –
(0x82) TCCR1C FOC1A FOC1B – – – – – –131
(0x81) TCCR1B ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 130
(0x80) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 –– WGM11 WGM10 127
(0x7F) DIDR1 – – – – – – AIN1D AIN0D 236
(0x7E) DIDR0 – – ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D 253
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

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(0x7D) Reserved – – – – – – – –
(0x7C) ADMUX REFS1 REFS0 ADLAR – MUX3 MUX2 MUX1 MUX0 249
(0x7B) ADCSRB –ACME – – – ADTS2 ADTS1 ADTS0 252
(0x7A) ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 250
(0x79) ADCH ADC Data Register High byte 251
(0x78) ADCL ADC Data Register Low byte 251
(0x77) Reserved – – – – – – – –
(0x76) Reserved – – – – – – – –
(0x75) Reserved – – – – – – – –
(0x74) Reserved – – – – – – – –
(0x73) Reserved – – – – – – – –
(0x72) Reserved – – – – – – – –
(0x71) Reserved – – – – – – – –
(0x70) TIMSK2 – – – – – OCIE2B OCIE2A TOIE2 151
(0x6F) TIMSK1 ––ICIE1 –– OCIE1B OCIE1A TOIE1 132
(0x6E) TIMSK0 – – – – – OCIE0B OCIE0A TOIE0 102
(0x6D) PCMSK2 PCINT23 PCINT22 PCINT21 PCINT20 PCINT19 PCINT18 PCINT17 PCINT16 85
(0x6C) PCMSK1 – PCINT14 PCINT13 PCINT12 PCINT11 PCINT10 PCINT9 PCINT8 85
(0x6B) PCMSK0 PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 85
(0x6A) Reserved – – – – – – – –
(0x69) EICRA – – – – ISC11 ISC10 ISC01 ISC00 81
(0x68) PCICR – – – – – PCIE2 PCIE1 PCIE0
(0x67) Reserved – – – – – – – –
(0x66) OSCCAL Oscillator Calibration Register 31
(0x65) Reserved – – – – – – – –
(0x64) PRR PRTWI PRTIM2 PRTIM0 – PRTIM1 PRSPI PRUSART0 PRADC 39
(0x63) Reserved – – – – – – – –
(0x62) Reserved – – – – – – – –
(0x61) CLKPR CLKPCE – – – CLKPS3 CLKPS2 CLKPS1 CLKPS0 35
(0x60) WDTCSR WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 50
0x3F (0x5F) SREG I T H S V N Z C 9
0x3E (0x5E) SPH – – – – – (SP10)  SP9 SP8 11
0x3D (0x5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 11
0x3C (0x5C) Reserved – – – – – – – –
0x3B (0x5B) Reserved – – – – – – – –
0x3A (0x5A) Reserved – – – – – – – –
0x39 (0x59) Reserved – – – – – – – –
0x38 (0x58) Reserved – – – – – – – –
0x37 (0x57) SPMCSR SPMIE (RWWSB)Fig –(RWWSRE)Fig BLBSET PGWRT PGERS SELFPRGEN 268
0x36 (0x56) Reserved – – – – – – – –
0x35 (0x55) MCUCR – – –PUD–– IVSEL IVCE
0x34 (0x54) MCUSR – – – – WDRF BORF EXTRF PORF
0x33 (0x53) SMCR – – – – SM2 SM1 SM0 SE 36
0x32 (0x52) Reserved – – – – – – – –
0x31 (0x51) Reserved – – – – – – – –
0x30 (0x50) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 234
0x2F (0x4F) Reserved – – – – – – – –
0x2E (0x4E) SPDR  SPI Data Register 163
0x2D (0x4D) SPSR SPIF WCOL – – – – –SPI2X 163
0x2C (0x4C) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 161
0x2B (0x4B) GPIOR2 General Purpose I/O Register 2 23
0x2A (0x4A) GPIOR1 General Purpose I/O Register 1 23
0x29 (0x49) Reserved – – – – – – – –
0x28 (0x48) OCR0B  Timer/Counter0 Output Compare Register B
0x27 (0x47) OCR0A  Timer/Counter0 Output Compare Register A
0x26 (0x46) TCNT0  Timer/Counter0 (8-bit)
0x25 (0x45) TCCR0B FOC0A FOC0B –– WGM02 CS02 CS01 CS00
0x24 (0x44) TCCR0A COM0A1 COM0A0 COM0B1 COM0B0 ––WGM01WGM00
0x23 (0x43) GTCCR TSM – – – – – PSRASY PSRSYNC 105/155 
0x22 (0x42) EEARH (EEPROM Address Register High Byte) Figure 5 18
0x21 (0x41) EEARL EEPROM Address Register Low Byte 18
0x20 (0x40) EEDR EEPROM Data Register 18
0x1F (0x3F) EECR –– EEPM1 EEPM0 EERIE EEMPE EEPE EERE 19
0x1E (0x3E) GPIOR0 General Purpose I/O Register 0 23
0x1D (0x3D) EIMSK – – – – – –INT1INT0 82
0x1C (0x3C) EIFR – – – – – – INTF1 INTF0 83
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

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Note: 1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses 
should never be written.
2. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these 
registers, the value of single bits can be checked by using the SBIS and SBIC instructions.
3. Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the CBI and SBI 
instructions will only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The 
CBI and SBI instructions work with registers 0x00 to 0x1F only.
4. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O 
Registers as data space using LD and ST instructions, 0x20 must be added to these addresses. The ATmega48/88/168 is a 
complex microcontroller with more peripheral units than can be supported within the 64 location reserved in Opcode for the 
IN and OUT instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD 
instructions can be used.
5. Only valid for ATmega88/168
0x1B (0x3B) PCIFR – – – – – PCIF2 PCIF1 PCIF0
0x1A (0x3A) Reserved – – – – – – – –
0x19 (0x39) Reserved – – – – – – – –
0x18 (0x38) Reserved – – – – – – – –
0x17 (0x37) TIFR2 – – – – – OCF2B OCF2A TOV2 151
0x16 (0x36) TIFR1 ––ICF1 –– OCF1B OCF1A TOV1 133
0x15 (0x35) TIFR0 – – – – – OCF0B OCF0A TOV0
0x14 (0x34) Reserved – – – – – – – –
0x13 (0x33) Reserved – – – – – – – –
0x12 (0x32) Reserved – – – – – – – –
0x11 (0x31) Reserved – – – – – – – –
0x10 (0x30) Reserved – – – – – – – –
0x0F (0x2F) Reserved – – – – – – – –
0x0E (0x2E) Reserved – – – – – – – –
0x0D (0x2D) Reserved – – – – – – – –
0x0C (0x2C) Reserved – – – – – – – –
0x0B (0x2B) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 80
0x0A (0x2A) DDRD DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 80
0x09 (0x29) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 81
0x08 (0x28) PORTC – PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 80
0x07 (0x27) DDRC – DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 80
0x06 (0x26) PINC – PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 80
0x05 (0x25) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 80
0x04 (0x24) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 80
0x03 (0x23) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 80
0x02 (0x22) Reserved – – – – – – – –
0x01 (0x21) Reserved – – – – – – – –
0x0 (0x20) Reserved – – – – – – – –
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

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30. Instruction Set Summary

Mnemonics Operands Description Operation Flags #Clocks

ARITHMETIC AND LOGIC INSTRUCTIONS

ADD Rd, Rr Add two Registers Rd ← Rd + Rr Z,C,N,V,H 1

ADC Rd, Rr Add with Carry two Registers Rd ← Rd + Rr + C Z,C,N,V,H 1

ADIW Rdl,K Add Immediate to Word Rdh:Rdl ← Rdh:Rdl + K Z,C,N,V,S 2

SUB Rd, Rr Subtract two Registers Rd ← Rd - Rr Z,C,N,V,H 1

SUBI Rd, K Subtract Constant from Register Rd ← Rd - K Z,C,N,V,H 1

SBC Rd, Rr Subtract with Carry two Registers Rd ← Rd - Rr - C Z,C,N,V,H 1

SBCI Rd, K Subtract with Carry Constant from Reg. Rd ← Rd - K - C Z,C,N,V,H 1

SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl ← Rdh:Rdl - K Z,C,N,V,S 2

AND Rd, Rr Logical AND Registers Rd ← Rd • Rr Z,N,V 1

ANDI Rd, K Logical AND Register and Constant Rd ← Rd • K Z,N,V 1

OR Rd, Rr Logical OR Registers Rd ← Rd v Rr Z,N,V 1

ORI Rd, K Logical OR Register and Constant Rd ← Rd v K Z,N,V 1

EOR Rd, Rr Exclusive OR Registers Rd ← Rd ⊕ Rr Z,N,V 1

COM Rd One’s Complement Rd ← 0xFF − Rd Z,C,N,V 1

NEG Rd Two’s Complement Rd ← 0x00 − Rd Z,C,N,V,H 1

SBR Rd,K Set Bit(s) in Register Rd ← Rd v K Z,N,V 1

CBR Rd,K Clear Bit(s) in Register Rd ← Rd • (0xFF - K) Z,N,V 1

INC Rd Increment Rd ← Rd + 1 Z,N,V 1

DEC Rd Decrement Rd ← Rd − 1 Z,N,V 1

TST Rd Test for Zero or Minus Rd ← Rd • Rd Z,N,V 1

CLR Rd Clear Register Rd ← Rd ⊕ Rd Z,N,V 1

SER Rd Set Register Rd ← 0xFF None 1

MUL Rd, Rr Multiply Unsigned R1:R0 ← Rd x Rr Z,C 2

MULS Rd, Rr Multiply Signed R1:R0 ← Rd x Rr Z,C 2

MULSU Rd, Rr Multiply Signed with Unsigned R1:R0 ← Rd x Rr Z,C 2

FMUL Rd, Rr Fractional Multiply Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2

FMULS Rd, Rr Fractional Multiply Signed R1:R0 ← (Rd x Rr) << 1 Z,C 2

FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2

BRANCH INSTRUCTIONS

RJMP k Relative Jump PC ← PC + k + 1 None 2

IJMP Indirect Jump to (Z) PC ← Z None 2

JMP(1) k Direct Jump PC ← kNone3

RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3

ICALL Indirect Call to (Z) PC ← ZNone3

CALL(1) k Direct Subroutine Call PC ← kNone4

RET Subroutine Return PC ← STACK None 4

RETI Interrupt Return PC ← STACK I 4

CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC ← PC + 2 or 3 None 1/2/3

CP Rd,Rr Compare Rd − Rr Z, N,V,C,H 1

CPC Rd,Rr Compare with Carry Rd − Rr − C Z, N,V,C,H 1

CPI Rd,K Compare Register with Immediate Rd − K Z, N,V,C,H 1

SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC ← PC + 2 or 3 None 1/2/3

SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC ← PC + 2 or 3 None 1/2/3

SBIC P, b Skip if Bit in I/O Register Cleared if (P(b)=0) PC ← PC + 2 or 3 None 1/2/3

SBIS P, b Skip if Bit in I/O Register is Set if (P(b)=1) PC ← PC + 2 or 3 None 1/2/3

BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PC←PC+k + 1 None 1/2

BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PC←PC+k + 1 None 1/2

BREQ k Branch if Equal if (Z = 1) then PC ← PC + k + 1 None 1/2

BRNE k Branch if Not Equal if (Z = 0) then PC ← PC + k + 1 None 1/2

BRCS k Branch if Carry Set if (C = 1) then PC ← PC + k + 1 None 1/2

BRCC k Branch if Carry Cleared if (C = 0) then PC ← PC + k + 1 None 1/2

BRSH k Branch if Same or Higher if (C = 0) then PC ← PC + k + 1 None 1/2

BRLO k Branch if Lower if (C = 1) then PC ← PC + k + 1 None 1/2

BRMI k Branch if Minus if (N = 1) then PC ← PC + k + 1 None 1/2

BRPL k Branch if Plus if (N = 0) then PC ← PC + k + 1 None 1/2

BRGE k Branch if Greater or Equal, Signed if (N ⊕ V= 0) then PC ← PC + k + 1 None 1/2

BRLT k Branch if Less Than Zero, Signed if (N ⊕ V= 1) then PC ← PC + k + 1 None 1/2

BRHS k Branch if Half Carry Flag Set if (H = 1) then PC ← PC + k + 1 None 1/2

BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC ← PC + k + 1 None 1/2

BRTS k Branch if T Flag Set if (T = 1) then PC ← PC + k + 1 None 1/2

BRTC k Branch if T Flag Cleared if (T = 0) then PC ← PC + k + 1 None 1/2

BRVS k Branch if Overflow Flag is Set if (V = 1) then PC ← PC + k + 1 None 1/2

BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC ← PC + k + 1 None 1/2

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BRIE k Branch if Interrupt Enabled if ( I = 1) then PC ← PC + k + 1 None 1/2

BRID k Branch if Interrupt Disabled if ( I = 0) then PC ← PC + k + 1 None 1/2

BIT AND BIT-TEST INSTRUCTIONS

SBI P,b Set Bit in I/O Register I/O(P,b) ← 1None2

CBI P,b Clear Bit in I/O Register I/O(P,b) ← 0None2

LSL Rd Logical Shift Left Rd(n+1) ← Rd(n), Rd(0) ← 0 Z,C,N,V 1

LSR Rd Logical Shift Right Rd(n) ← Rd(n+1), Rd(7) ← 0 Z,C,N,V 1

ROL Rd Rotate Left Through Carry Rd(0)←C,Rd(n+1)← Rd(n),C←Rd(7) Z,C,N,V 1

ROR Rd Rotate Right Through Carry Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0) Z,C,N,V 1

ASR Rd Arithmetic Shift Right Rd(n) ← Rd(n+1), n=0..6 Z,C,N,V 1

SWAP Rd Swap Nibbles Rd(3..0)←Rd(7..4),Rd(7..4)←Rd(3..0) None 1

BSET s Flag Set SREG(s) ← 1 SREG(s) 1

BCLR s Flag Clear SREG(s) ← 0 SREG(s) 1

BST Rr, b Bit Store from Register to T T ← Rr(b) T 1

BLD Rd, b Bit load from T to Register Rd(b) ← TNone1

SEC Set Carry C ← 1C1

CLC Clear Carry C ← 0 C 1

SEN Set Negative Flag N ← 1N1

CLN Clear Negative Flag N ← 0 N 1

SEZ Set Zero Flag Z ← 1Z1

CLZ Clear Zero Flag Z ← 0 Z 1

SEI Global Interrupt Enable I ← 1I1

CLI Global Interrupt Disable I ← 0 I 1

SES Set Signed Test Flag S ← 1S1

CLS Clear Signed Test Flag S ← 0 S 1

SEV Set Twos Complement Overflow. V ← 1V1

CLV Clear Twos Complement Overflow V ← 0 V 1

SET Set T in SREG T ← 1T1

CLT Clear T in SREG T ← 0 T 1

SEH Set Half Carry Flag in SREG H ← 1H1

CLH Clear Half Carry Flag in SREG H ← 0 H 1

DATA TRANSFER INSTRUCTIONS

MOV Rd, Rr Move Between Registers Rd ← Rr None 1

MOVW Rd, Rr Copy Register Word Rd+1:Rd ← Rr+1:Rr None 1

LDI Rd, K Load Immediate Rd ← KNone1

LD Rd, X Load Indirect Rd ← (X) None 2

LD Rd, X+ Load Indirect and Post-Inc. Rd ← (X), X ← X + 1 None 2

LD Rd, - X Load Indirect and Pre-Dec. X ← X - 1, Rd ← (X) None 2

LD Rd, Y Load Indirect Rd ← (Y) None 2

LD Rd, Y+ Load Indirect and Post-Inc. Rd ← (Y), Y ← Y + 1 None 2

LD Rd, - Y Load Indirect and Pre-Dec. Y ← Y - 1, Rd ← (Y) None 2

LDD Rd,Y+q Load Indirect with Displacement Rd ← (Y + q) None 2

LD Rd, Z Load Indirect Rd ← (Z) None 2

LD Rd, Z+ Load Indirect and Post-Inc. Rd ← (Z), Z ← Z+1 None 2

LD Rd, -Z Load Indirect and Pre-Dec. Z ← Z - 1, Rd ← (Z) None 2

LDD Rd, Z+q Load Indirect with Displacement Rd ← (Z + q) None 2

LDS Rd, k Load Direct from SRAM Rd ← (k) None 2

ST X, Rr Store Indirect (X) ← Rr None 2

ST X+, Rr Store Indirect and Post-Inc. (X) ← Rr, X ← X + 1 None 2

ST - X, Rr Store Indirect and Pre-Dec. X ← X - 1, (X) ← Rr None 2

ST Y, Rr Store Indirect (Y) ← Rr None 2

ST Y+, Rr Store Indirect and Post-Inc. (Y) ← Rr, Y ← Y + 1 None 2

ST - Y, Rr Store Indirect and Pre-Dec. Y ← Y - 1, (Y) ← Rr None 2

STD Y+q,Rr Store Indirect with Displacement (Y + q) ← Rr None 2

ST Z, Rr Store Indirect (Z) ← Rr None 2

ST Z+, Rr Store Indirect and Post-Inc. (Z) ← Rr, Z ← Z + 1 None 2

ST -Z, Rr Store Indirect and Pre-Dec. Z ← Z - 1, (Z) ← Rr None 2

STD Z+q,Rr Store Indirect with Displacement (Z + q) ← Rr None 2

STS k, Rr Store Direct to SRAM (k) ← Rr None 2

LPM Load Program Memory R0 ← (Z) None 3

LPM Rd, Z Load Program Memory Rd ← (Z) None 3

LPM Rd, Z+ Load Program Memory and Post-Inc Rd ← (Z), Z ← Z+1 None 3

SPM Store Program Memory (Z) ← R1:R0 None -

IN Rd, P In Port Rd ← PNone1

OUT P, Rr Out Port P ← Rr None 1

PUSH Rr Push Register on Stack STACK ← Rr None 2

Mnemonics Operands Description Operation Flags #Clocks

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Note: 1. These instructions are only available in ATmega168.

POP Rd Pop Register from Stack Rd ← STACK None 2

MCU CONTROL INSTRUCTIONS

NOP No Operation None 1

SLEEP Sleep (see specific descr. for Sleep function) None 1

WDR Watchdog Reset (see specific descr. for WDR/timer) None 1

BREAK Break For On-chip Debug Only None N/A

Mnemonics Operands Description Operation Flags #Clocks

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31. Ordering Information
31.1 ATmega48
Note: 1. Green and ROHS packaging
2. See Figure 26-2 on page 301.
3. Tape and Reel with Dry-pack delivery
31.2 ATmega88
Note: 1. Green and ROHS packaging.
2. See Figure 26-2 on page 301.
3. Tape and Reel with Dry-pack deliver
31.3 ATmega168
Note: 1. Green and ROHS packaging
2. See Figure 26-2 on page 301
3. Tape and Reel with Dry-pack delivery
Speed (MHz) Power Supply Ordering Code Package(1) Operation Range
16(2) 2.7 - 5.5
ATmega48-15AT
ATmega48-15AT1
ATmega48-15AZ
MA
-40°C to +85°C
-40°C to +105°C
-40°C to +125°C
16(2) 2.7 - 5.5
ATmega48-15MT
ATmega48-15MT1
ATmega48-15MZ
PN -40°C to 85°C
-40°C to +105°C
-40°C to 125°C
Speed (MHz) Power Supply Ordering Code Package(1) Operation Range
16(2) 2.7 - 5.5
ATmega88-15AT
ATmega88-15AT1
ATmega88-15AZ
MA
-40°C to +85°C
-40°C to +105°C
-40°C to +125°C
16(2) 2.7 - 5.5
ATmega88-15MT
ATmega88-15MT1
ATmega88-15MZ
PN -40°C to 85°C
-40°C to +105°C
-40°C to 125°C
Speed (MHz) Power Supply Ordering Code Package(1) Operation Range
16(2) 2.7 - 5.5
ATmega168-15AT
ATmega168-15AT1
ATmega168-15AZ
MA -40°C to 85°C
-40°C to +105°C
-40°C to 125°C
16(2) 2.7 - 5.5
ATmega168-15MT
ATmega168-15MT1
ATmega168-15MZ
PN -40°C to 85°C
-40°C to +105°C
-40°C to 125°C
Package Information
MA
32 - Lead, 7 × 7 mm Body Size, 1.0 mm Body Thickness, 0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package 
(TQFP)
PN 32-pad, 5 × 5 ×1.0 mm Body, Lead Pitch 0.50 mm, Quad Flat No-Lead (QFN)

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32. Packaging Information

32.1 MA

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32.2 PN

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33. Errata ATmega48

The revision letter in this section refers to the revision of the ATmega48 device.

33.1 Rev. E •Interrupts may be lost when writing the timer registers in the asynchronous timer

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

33.2 Rev. D •Interrupts may be lost when writing the timer registers in the asynchronous timer

•POR sensitivity with Vcc ramp up from a very low supply voltage

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

2. POR sensitivity with Vcc ramp up from a very low supply voltage

If Vcc ramp up from a stable 150mV to 300mV plateau, the Power On Reset (POR) may not

reset the device properly.

Problem Fix/Workaround

None.

33.3 Rev. A •Interrupts may be lost when writing the timer registers in the asynchronous timer

•POR sensitivity with Vcc ramp up from a very low supply voltage

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

2. POR sensitivity with Vcc ramp up from a very low supply voltage

If Vcc ramp up from a stable 150mV to 300mV plateau, the Power On Reset (POR) may not

reset the device properly.

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Problem Fix/Workaround

None.

Note: Please note from datasheet 7530F-AVR-09/07 we introduce a new errata numbering scheme

(Errata Rev E of datasheet 7530E-AVR-03/07 is equivalent to Errata Rev D of datasheet

7530F-AVR-09/07)

34. Errata ATmega88

The revision letter in this section refers to the revision of the ATmega88 device.

34.1 Rev. G

•Interrupts may be lost when writing the timer registers in the asynchronous timer

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

34.2 Rev. E

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•POR sensitivity with Vcc ramp up from a very low supply voltage

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

2. POR sensitivity with Vcc ramp up from a very low supply voltage

If Vcc ramp up from a stable 150mV to 300mV plateau, the Power On Reset (POR) may not

reset the device properly.

Problem Fix/Workaround

None.

Note: Please note from datasheet 7530F-AVR-09/07 we introduce a new errata numbering scheme

(Errata Rev F of datasheet 7530E-AVR-03/07 is equivalent to Errata Rev E of datasheet

7530F-AVR-09/07)

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35. Errata ATmega168

The revision letter in this section refers to the revision of the ATmega168 device.

35.1 Rev. F •Interrupts may be lost when writing the timer registers in the asynchronous timer

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

35.2 Rev. E •Interrupts may be lost when writing the timer registers in the asynchronous timer

•POR sensitivity with Vcc ramp up from a very low supply voltage

1. Interrupts may be lost when writing the timer registers in the asynchronous timer

If one of the timer registers which is synchronized to the asynchronous timer2 clock is written in

the cycle before an overflow interrupt occurs, the interrupt may be lost.

Problem Fix/Workaround

Always check that the Timer2 Timer/Counter register, TCNT2, does not have the value 0xFF

before writing the Timer2 Control Register, TCCR2, or Output Compare Register, OCR2.

2. POR sensitivity with Vcc ramp up from a very low supply voltage

If Vcc ramp up from a stable 150mV to 300mV plateau, the Power On Reset (POR) may not

reset the device properly.

Problem Fix/Workaround

None.

Note: Please note from datasheet 7530F-AVR-09/07 we introduce a new errata numbering scheme

(Errata Rev F of datasheet 7530E-AVR-03/07 is equivalent to Errata Rev E of datasheet

7530F-AVR-09/07)

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36. Document Revision History

Please note that the referring page numbers in this section are referred to this document. The

referring revision in this section are referring to the document revision.

36.1 Rev. 7530A-AVR-08/05

36.2 Changes from

Rev. 7530A-AVR-08/05 to Rev. 7530B-AVR-03/06

36.3 Changes from

Rev. 7530B-AVR-03/06 to Rev. 7530C-AVR-09/06

36.4 Changes from

Rev. 7530C-AVR-09/06 to Rev. 7530D-AVR-02/07

1. Creation of the Automotive version starting from Industrial version. Temperature and

voltage ranges reflecting Automotive requirements.

2. EEPROM and FLASH endurance extended to reflect qualification results.

Added Green/ROHS compliance.

3. Parametric and analog values modified according to characterization results:

Reference input Resistance specification at 22.4, 32, 41.6 KOhm

Tacit typical value indicated at 140nS for 4.5V. Reference to 2.7V removed.

VIL1 Xtal1 limit similar to RESET pin.

VOH for Vcc=3V limit at 0.5V max

VOL for Vcc=3V limit at 2.3V max

Tape & reel + dry-pack delivery notice added.

4. Internal RC Oscillator typical graphs added.

Internal RC oscillator tolerance at ±10%

Reference input Resistance specification at 32 ±20%

Added Oscillator (Watchdog and RC) typical characteristics curves.

1. Re-synchronization algorithm detailed.

1. Correction of package codification and drawings.

1. Clarification of Power On Reset Specifications table, Table on page 43.

2. Errata list updated.

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36.5 Changes from

Rev. 7530D-AVR-02/07 to Rev. 7530E-AVR-03/07

36.6 Changes from

Rev. 7530E-AVR-03/07 to Rev. 7530F-AVR-09/07

36.7 Changes from

Rev. 7530F-AVR-09/07 to Rev. 7530G-AVR-10/07

36.8 Changes from

Rev. 7530G-AVR-10/07 to Rev. 7530H-AVR-02/09

36.9 Changes from

Rev. 7530H-AVR-02/09 to Rev. 7530I-AVR-02/10

36.10 Changes from

Rev. 7530I-AVR-02/10 to Rev. 7530J-AVR-03/12

1. Modification to POR. Section 8.0.3 on page 42.

2. D2 / E2 tolerance on Mechanical outline. Section 32.2 on page 327.

1. Modification to POR. Section on page 43. VPOT value modified.

2. Errata list numbering scheme modified. Section 33.Section 34.Section 35.

1. Update to errata section.

1. Update Section 7.6.1 “Power Reduction Register - PRR” on page 39.

1. Package updated.

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37. Table of Contents

Features ..................................................................................................... 1

1 Pin Configurations ................................................................................... 2

1.1 Disclaimer ..........................................................................................................2

2 Overview ................................................................................................... 2

2.1 Block Diagram ...................................................................................................3

2.2 Automotive Quality Grade .................................................................................4

2.3 Comparison Between ATmega48, ATmega88, and ATmega168 .....................5

2.4 Pin Descriptions .................................................................................................5

3 About Code Examples ............................................................................. 7

4 AVR CPU Core .......................................................................................... 7

4.1 Introduction ........................................................................................................7

4.2 Architectural Overview .......................................................................................7

4.3 ALU – Arithmetic Logic Unit ...............................................................................9

4.4 Status Register ..................................................................................................9

4.5 General Purpose Register File ........................................................................10

4.6 Stack Pointer ...................................................................................................11

4.7 Instruction Execution Timing ...........................................................................12

4.8 Reset and Interrupt Handling ...........................................................................12

5 AVR ATmega48/88/168 Memories ........................................................ 14

5.1 In-System Reprogrammable Flash Program Memory .....................................14

5.2 SRAM Data Memory ........................................................................................16

5.3 EEPROM Data Memory ..................................................................................17

5.4 I/O Memory ......................................................................................................23

6 System Clock and Clock Options ......................................................... 24

6.1 Clock Systems and their Distribution ...............................................................24

6.2 Clock Sources .................................................................................................25

6.3 Low Power Crystal Oscillator ...........................................................................26

6.4 Full Swing Crystal Oscillator ............................................................................28

6.5 Low Frequency Crystal Oscillator ....................................................................30

6.6 Calibrated Internal RC Oscillator .....................................................................30

6.7 128 kHz Internal Oscillator ..............................................................................32

6.8 External Clock .................................................................................................32

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6.9 Clock Output Buffer .........................................................................................33

6.10 Timer/Counter Oscillator ..................................................................................33

6.11 System Clock Prescaler ..................................................................................34

7 Power Management and Sleep Modes ................................................. 36

7.1 Idle Mode .........................................................................................................37

7.2 ADC Noise Reduction Mode ............................................................................37

7.3 Power-down Mode ...........................................................................................37

7.4 Power-save Mode ............................................................................................38

7.5 Standby Mode .................................................................................................38

7.6 Power Reduction Register ...............................................................................39

7.7 Minimizing Power Consumption ......................................................................40

8 System Control and Reset .................................................................... 41

8.1 Internal Voltage Reference ..............................................................................46

8.2 Watchdog Timer ..............................................................................................47

9 Interrupts ................................................................................................ 52

9.1 Interrupt Vectors in ATmega48 ........................................................................53

9.2 Interrupt Vectors in ATmega88 ........................................................................54

9.3 Interrupt Vectors in ATmega168 ......................................................................58

10 I/O-Ports .................................................................................................. 62

10.1 Introduction ......................................................................................................62

10.2 Ports as General Digital I/O .............................................................................63

10.3 Alternate Port Functions ..................................................................................68

10.4 Register Description for I/O Ports ....................................................................80

11 External Interrupts ................................................................................. 81

12 8-bit Timer/Counter0 with PWM ............................................................ 86

12.1 Overview ..........................................................................................................86

12.2 Timer/Counter Clock Sources .........................................................................87

12.3 Counter Unit ....................................................................................................87

12.4 Output Compare Unit .......................................................................................88

12.5 Compare Match Output Unit ............................................................................90

12.6 Modes of Operation .........................................................................................91

12.7 Timer/Counter Timing Diagrams .....................................................................95

12.8 8-bit Timer/Counter Register Description ........................................................97

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13 Timer/Counter0 and Timer/Counter1 Prescalers .............................. 103

14 16-bit Timer/Counter1 with PWM ........................................................ 105

14.1 Overview ........................................................................................................105

14.2 Accessing 16-bit Registers ............................................................................107

14.3 Timer/Counter Clock Sources .......................................................................110

14.4 Counter Unit ..................................................................................................111

14.5 Input Capture Unit .........................................................................................112

14.6 Output Compare Units ...................................................................................114

14.7 Compare Match Output Unit ..........................................................................116

14.8 Modes of Operation .......................................................................................117

14.9 Timer/Counter Timing Diagrams ...................................................................125

14.10 16-bit Timer/Counter Register Description ....................................................127

15 8-bit Timer/Counter2 with PWM and Asynchronous Operation ...... 134

15.1 Overview ........................................................................................................134

15.2 Timer/Counter Clock Sources .......................................................................136

15.3 Counter Unit ..................................................................................................136

15.4 Output Compare Unit .....................................................................................137

15.5 Compare Match Output Unit ..........................................................................138

15.6 Modes of Operation .......................................................................................140

15.7 Timer/Counter Timing Diagrams ...................................................................144

15.8 8-bit Timer/Counter Register Description ......................................................146

15.9 Asynchronous operation of the Timer/Counter ..............................................152

15.10 Timer/Counter Prescaler ...............................................................................155

16 Serial Peripheral Interface – SPI ......................................................... 156

16.1 SS Pin Functionality ......................................................................................161

16.2 Data Modes ...................................................................................................163

17 USART0 ................................................................................................. 165

17.1 Overview ........................................................................................................165

17.2 Clock Generation ...........................................................................................167

17.3 Frame Formats.............................................................................................. 170

17.4 USART Initialization .......................................................................................171

17.5 Data Transmission – The USART Transmitter ..............................................172

17.6 Data Reception – The USART Receiver .......................................................175

17.7 Asynchronous Data Reception ......................................................................179

17.8 Multi-processor Communication Mode ..........................................................183

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17.9 USART Register Description .........................................................................184

17.10 Examples of Baud Rate Setting .....................................................................188

18 USART in SPI Mode ............................................................................. 192

18.1 Overview ........................................................................................................192

18.2 Clock Generation ...........................................................................................192

18.3 SPI Data Modes and Timing ..........................................................................193

18.4 Frame Formats ..............................................................................................193

18.5 Data Transfer .................................................................................................196

18.6 USART MSPIM Register Description ............................................................198

18.7 AVR USART MSPIM vs. AVR SPI ................................................................200

19 2-wire Serial Interface .......................................................................... 201

19.1 Features ........................................................................................................201

19.2 2-wire Serial Interface Bus Definition ............................................................201

19.3 Data Transfer and Frame Format ..................................................................202

19.4 Multi-master Bus Systems, Arbitration and Synchronization .........................206

19.5 Overview of the TWI Module .........................................................................208

19.6 TWI Register Description ...............................................................................210

19.7 Using the TWI ................................................................................................214

19.8 Transmission Modes .....................................................................................218

19.9 Multi-master Systems and Arbitration ............................................................232

20 Analog Comparator ............................................................................. 233

20.1 Analog Comparator Multiplexed Input ...........................................................236

21 Analog-to-Digital Converter ................................................................ 237

21.1 Features ........................................................................................................237

21.2 Starting a Conversion ....................................................................................239

21.3 Prescaling and Conversion Timing ................................................................240

21.4 Changing Channel or Reference Selection ...................................................243

21.5 ADC Noise Canceler .....................................................................................244

21.6 ADC Conversion Result................................................................................. 249

22 debugWIRE On-chip Debug System .................................................. 254

22.1 Features ........................................................................................................254

22.2 Overview ........................................................................................................254

22.3 Physical Interface ..........................................................................................254

22.4 Software Break Points ...................................................................................255

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22.5 Limitations of debugWIRE .............................................................................255

22.6 debugWIRE Related Register in I/O Memory ................................................255

23 Self-Programming the Flash, ATmega48 ........................................... 256

23.1 Addressing the Flash During Self-Programming ...........................................257

24 Boot Loader Support – Read-While-Write Self-Programming,

ATmega88 and ATmega168 ................................................................ 263

24.1 Boot Loader Features ....................................................................................263

24.2 Application and Boot Loader Flash Sections .................................................263

24.3 Read-While-Write and No Read-While-Write Flash Sections ........................264

24.4 Boot Loader Lock Bits ...................................................................................266

24.5 Entering the Boot Loader Program ................................................................267

24.6 Addressing the Flash During Self-Programming ...........................................269

24.7 Self-Programming the Flash ..........................................................................270

25 Memory Programming ......................................................................... 278

25.1 Program And Data Memory Lock Bits ...........................................................278

25.2 Fuse Bits ........................................................................................................280

25.3 Signature Bytes .............................................................................................282

25.4 Calibration Byte .............................................................................................282

25.5 Parallel Programming Parameters, Pin Mapping, and Commands ...............282

25.6 Serial Programming Pin Mapping ..................................................................285

25.7 Parallel Programming ....................................................................................285

25.8 Serial Downloading ........................................................................................293

26 Electrical Characteristics .................................................................... 298

26.1 Absolute Maximum Ratings* .........................................................................298

26.2 DC Characteristics .........................................................................................298

26.3 External Clock Drive Waveforms ...................................................................300

26.4 External Clock Drive ......................................................................................300

26.5 Maximum Speed vs. VCC ...............................................................................301

26.6 LIN Re-synchronization algorithm .................................................................301

26.7 Synchronization Algorithm .............................................................................301

26.8 Precaution Against OSCCAL Discontinuity ...................................................302

27 2-wire Serial Interface Characteristics ............................................... 303

27.1 SPI Timing Characteristics ............................................................................305

27.2 ADC Characteristics .....................................................................................306

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28 ATmega48/88/168 Typical Characteristics ........................................ 307

28.1 Active Supply Current ....................................................................................307

29 Register Summary ............................................................................... 318

30 Instruction Set Summary .................................................................... 322

31 Ordering Information ........................................................................... 325

31.1 ATmega48 .....................................................................................................325

31.2 ATmega88 .....................................................................................................325

31.3 ATmega168 ...................................................................................................325

32 Packaging Information ........................................................................ 326

32.1 MA .................................................................................................................326

32.2 PN ..................................................................................................................327

33 Errata ATmega48 ................................................................................. 328

33.1 Rev. E ............................................................................................................328

33.2 Rev. D ............................................................................................................328

33.3 Rev. A ............................................................................................................328

34 Errata ATmega88 ................................................................................. 329

34.1 Rev. G ...........................................................................................................329

34.2 Rev. E ............................................................................................................329

35 Errata ATmega168 ............................................................................... 330

35.1 Rev. F ............................................................................................................330

35.2 Rev. E ............................................................................................................330

36 Document Revision History ................................................................ 331

36.1 Rev. 7530A-AVR-08/05 .................................................................................331

36.2 Changes from

Rev. 7530A-AVR-08/05 to Rev. 7530B-AVR-03/06 ......................................331

36.3 Changes from

Rev. 7530B-AVR-03/06 to Rev. 7530C-AVR-09/06 ......................................331

36.4 Changes from

Rev. 7530C-AVR-09/06 to Rev. 7530D-AVR-02/07.......................................331

36.5 Changes from

Rev. 7530D-AVR-02/07 to Rev. 7530E-AVR-03/07 ......................................332

36.6 Changes from

Rev. 7530E-AVR-03/07 to Rev. 7530F-AVR-09/07 .......................................332

36.7 Changes from

Rev. 7530F-AVR-09/07 to Rev. 7530G-AVR-10/07 ......................................332

339

7530J–AVR–03/12

Atmel ATmega48/88/168 Automotive

36.8 Changes from

Rev. 7530G-AVR-10/07 to Rev. 7530H-AVR-02/09 ......................................332

36.9 Changes from

Rev. 7530H-AVR-02/09 to Rev. 7530I-AVR-02/10 ........................................332

36.10 Changes from

Rev. 7530I-AVR-02/10 to Rev. 7530J-AVR-03/12 ........................................332

37 Table of Contents ................................................................................. 333

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