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Features
High Performance, Low Power Atmel®AVR® 8-Bit Microcontroller Family
Advanced RISC Architecture
131 Powerful Instructions – Most Single Clock Cycle Execution
32 x 8 General Purpose Working Registers
Fully Static Operation
Up to 20 MIPS Throughput at 20MHz
On-chip 2-cycle Multiplier
High Endurance Non-volatile Memory Segments
4/8/16/32KBytes of In-System Self-Programmable Flash program memory
256/512/512/1KBytes EEPROM
512/1K/1K/2KBytes Internal SRAM
Write/Erase Cycles: 10,000 Flash/100,000 EEPROM
Data retention: 20 years at 85C/100 years at 25C(1)
Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
Programming Lock for Software Security
Atmel® QTouch® library support
Capacitive touch buttons, sliders and wheels
QTouch and QMatrix® acquisition
Up to 64 sense channels
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 in TQFP and QFN/MLF package
Temperature Measurement
6-channel 10-bit ADC in PDIP Package
Temperature Measurement
Programmable Serial USART
Master/Slave SPI Serial Interface
Byte-oriented 2-wire Serial Interface (Philips I2C compatible)
Programmable Watchdog Timer with Separate On-chip Oscillator
On-chip Analog Comparator
Interrupt and Wake-up on Pin Change
ATmega48A/PA/88A/PA/168A/PA/328/P
ATMEL 8-BIT MICROCONTROLLER WITH 4/8/16/32KBYTES
IN-SYSTEM PROGRAMMABLE FLASH
DATASHEET
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Special Microcontroller Features
Power-on Reset and Programmable Brown-out Detection
Internal Calibrated Oscillator
External and Internal Interrupt Sources
Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and Extended Standby
I/O and Packages
23 Programmable I/O Lines
28-pin PDIP, 32-lead TQFP, 28-pad QFN/MLF and 32-pad QFN/MLF
Operating Voltage:
1.8 - 5.5V
Temperature Range:
-40C to 85C
Speed Grade:
0 - 4MHz@1.8 - 5.5V, 0 - 10MHz@2.7 - 5.5.V, 0 - 20MHz @ 4.5 - 5.5V
Power Consumption at 1MHz, 1.8V, 25C
Active Mode: 0.2mA
Power-down Mode: 0.1µA
Power-save Mode: 0.75µA (Including 32kHz RTC)
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1. Pin Configurations
Figure 1-1. Pinout ATmega48A/PA/88A/PA/168A/PA/328/P
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
(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)
32
31
30
29
28
27
26
25
9
10
11
12
13
14
15
16
(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)
32 TQFP T op View
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
(PCINT14/RESET) PC6
(PCINT16/RXD) PD0
(PCINT17/TXD) PD1
(PCINT18/INT0) PD2
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
PB4 (MISO/PCINT4)
PB3 (MOSI/OC2A/PCINT3)
PB2 (SS/OC1B/PCINT2)
PB1 (OC1A/PCINT1)
28 PDIP
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
32
31
30
29
28
27
26
25
9
10
11
12
13
14
15
16
32 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/PCINT
9)
PC0 (ADC0/PCINT
8)
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.
1
2
3
4
5
6
7
21
20
19
18
17
16
15
28
27
26
25
24
23
22
8
9
10
11
12
13
14
28 MLF Top View
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(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)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
NOTE: Bottom pad should be soldered to ground.
Table 1-1. 32UFBGA - Pinout ATmega48A/48PA/88A/88PA/168A/168PA
1 2 3 4 5 6
APD2 PD1 PC6 PC4 PC2 PC1
BPD3 PD4 PD0 PC5 PC3 PC0
CGND GND ADC7 GND
DVDD VDD AREF ADC6
EPB6 PD6 PB0 PB2 AVDD PB5
FPB7 PD5 PD7 PB1 PB3 PB4
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1.1 Pin Descriptions
1.1.1 VCC
Digital supply voltage.
1.1.2 GND
Ground.
1.1.3 Port B (PB7:0) XTAL1/XTAL2/TOSC1/TO SC2
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 Oscillator amplifier
and input to the internal clock operating circuit.
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 82 and ”System
Clock and Clock Options” on page 27.
1.1.4 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.
1.1.5 PC6/RESET
If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics 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 29-11 on page 305. 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 85.|
1.1.6 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 88.
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1.1.7 AVCC
AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally 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.
1.1.8 AREF
AREF is the analog reference pin for the A/D Converter.
1.1.9 ADC7: 6 (T QF P an d QFN /M L F Pac k age Onl y)
In the TQFP and QFN/MLF 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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2. Overview
The ATmega48A/PA/88A/PA/168A/PA/328/P 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
ATmega48A/PA/88A/PA/168A/PA/328/P achieves throughputs approaching 1 MIPS per MHz allowing the
system designer to optimize power consumption versus processing speed.
2.1 Block Diagram
Figure 2-1. Block Diagram
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 conventional CISC microcontrollers.
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
2
GND
AREF
AVCC
D ATA BU S
ADC[6..7]PC[0..6]PB[0..7]PD[0..7]
6
RESET
XTAL[1..2]
CPU
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The ATmega48A/PA/88A/PA/168A/PA/328/P provides the following features: 4K/8Kbytes of In-System
Programmable Flash with Read-While-Write capabilities, 256/512/512/1Kbytes EEPROM, 512/1K/1K/2Kbytes
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/MLF packages), a
programmable Watchdog Timer with internal Oscillator, 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 system 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 during 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.
Atmel® offers the QTouch® library for embedding capacitive touch buttons, sliders and wheels functionality into
AVR® microcontrollers. The patented charge-transfer signal acquisition offers robust sensing and includes fully
debounced reporting of touch keys and includes Adjacent Key Suppression® (AKS™) technology for
unambiguous detection of key events. The easy-to-use QTouch Suite toolchain allows you to explore, develop
and debug your own touch applications.
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 program 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 ATmega48A/PA/88A/PA/168A/PA/328/P is a powerful microcontroller that provides a
highly flexible and cost effective solution to many embedded control applications.
The ATmega48A/PA/88A/PA/168A/PA/328/P AVR is supported with a full suite of program and system
development tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators, In-Circuit
Emulators, and Evaluation kits.
2.2 Comparison Between Processors
The ATmega48A/PA/88A/PA/168A/PA/328/P differ only in memory sizes, boot loader support, and interrupt
vector sizes. Table 2-1 summarizes the different memory and interrupt vector sizes for the devices.
Table 2-1. Memory Size Summary
Device Flash EEPROM RAM Interrupt Vector Size
ATmega48A 4KBytes 256Bytes 512Bytes 1 instruction word/vector
ATmega48PA 4KBytes 256Bytes 512Bytes 1 instruction word/vector
ATmega88A 8KBytes 512Bytes 1KBytes 1 instruction word/vector
ATmega88PA 8KBytes 512Bytes 1KBytes 1 instruction word/vector
ATmega168A 16KBytes 512Bytes 1KBytes 2 instruction words/vector
ATmega168PA 16KBytes 512Bytes 1KBytes 2 instruction words/vector
ATmega328 32KBytes 1KBytes 2KBytes 2 instruction words/vector
ATmega328P 32KBytes 1KBytes 2KBytes 2 instruction words/vector
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ATmega48A/PA/88A/PA/168A/PA/328/P 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 ATmega
48A/48PA there is no Read-While-Write support and no separate Boot Loader Section. The SPM instruction can
execute from the entire Flash
3. Resources
A comprehensive set of development tools, application notes and datasheets are available for download on
http://www.atmel.com/avr.
Note: 1.
4. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over
20 years at 85°C or 100 years at 25°C.
5. 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 documentation for more details.
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”.
6. Capacitive Touch Sensing
The Atmel® QTouch® Library provides a simple to use solution to realize touch sensitive interfaces on most
Atmel AVR® microcontrollers. The QTouch Library includes support for the Atmel QTouch and Atmel QMatrix®
acquisition methods.
Touch sensing can be added to any application by linking the appropriate Atmel QTouch Library for the AVR
Microcontroller. This is done by using a simple set of APIs to define the touch channels and sensors, and then
calling the touch sensing APIs to retrieve the channel information and determine the touch sensor states.
The QTouch Library is FREE and downloadable from the Atmel website at the following location:
www.atmel.com/qtouchlibrary. For implementation details and other information, refer to the Atmel QTouch
Library User Guide - also available for download from Atmel website.
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7. AVR CPU Core
7.1 Overview
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.
Figure 7-1. Block Diagram of the AVR Architecture
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 instruction 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 typical ALU operation, two
Flash
Program
Memory
Instruction
Register
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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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 operation, 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 format. 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 position. The lower the Interrupt Vector address, the
higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, 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 ATmega48A/PA/88A/PA/168A/PA/328/P has Extended I/O space
from 0x60 - 0xFF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
7.2 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.
7.3 S tatus Register
The Status Register contains information about the result of the most recently executed arithmetic 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.
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7.3.1 SREG – AVR Status Register
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 interrupt enable
control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of
the interrupts are enabled independent of the individual interrupt 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 destination 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 arithmetic. See the “Instruction Set
Description” for detailed information.
• 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.
7.4 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 Register File:
One 8-bit output operand and one 8-bit result input
Bit 76543210
0x3F (0x5F) I T H S V N Z C SREG
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
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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 7-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 7-2. 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 7-2, 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 implemented 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.
7.4.1 The X-register, Y-register, and Z-register
The registers R26...R31 have some added functions to their general purpose usage. These registers 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 7-3.
Figure 7-3. 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).
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
15 XH XL 0
X-register 7 0 7 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 7 0 7 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7 0 7 0
R31 (0x1F) R30 (0x1E)
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7.5 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. Note that the Stack is implemented as growing from higher to lower
memory locations. The Stack Pointer Register always points to the top of the Stack. The Stack Pointer points to
the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. A Stack PUSH command will
decrease the Stack Pointer.
The Stack in the data SRAM must be defined by the program before any subroutine calls are executed or
interrupts are enabled. Initial Stack Pointer value equals the last address of the internal SRAM and the Stack
Pointer must be set to point above start of the SRAM, see Table 8-3 on page 19.
See Table 7-1 for Stack Pointer details.
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 implementations of the AVR architecture is so
small that only SPL is needed. In this case, the SPH Register will not be present.
7.5.1 SPH and SPL – Stack Pointer High and Stack Pointer Low Register
Table 7-1. Stack Pointer instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
RCALL
Decremented by 2
Return address is pushed onto the stack with a subroutine call or
interrupt
POP Incremented by 1 Data is popped from the stack
RET
RETI
Incremented by 2 Return address is popped from the stack with return from
subroutine or return from interrupt
Bit 151413121110 9 8
0x3E (0x5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
0x3D (0x5D) 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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7.6 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 7-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard 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 7-4. The Parallel Instruction Fetches and Instruction Executions
Figure 7-5 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 destination register.
Figure 7-5. Single Cycle ALU Operation
7.7 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 Programming” on page 280 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 57. The list also determines the priority levels of the
different interrupts. The lower the address the higher is the 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 57
for more information. The Reset Vector can also be moved to the start of the Boot Flash section by
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
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clkCPU
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programming the BOOTRST Fuse, see ”Boot Loader Support – Read-While-Write Self-Programming” on page
263.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. 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 Vector 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.
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any
pending interrupts, as shown in this example.
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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7.7.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. 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.
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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8. AVR Memories
8.1 Overview
This section describes the different memories in the ATmega48A/PA/88A/PA/168A/PA/328/P. The AVR
architecture has two main memory spaces, the Data Memory and the Program Memory space. In addition, the
ATmega48A/PA/88A/PA/168A/PA/328/P features an EEPROM Memory for data storage. All three memory
spaces are linear and regular.
8.2 In-System Reprogrammable Flash Program Memory
The ATmega48A/PA/88A/PA/168A/PA/328/P contains 4/8/16/32Kbytes On-chip In-System Reprogrammable
Flash memory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as
2/4/8/16K x 16. For software security, the Flash Program memory space is divided into two sections, Boot
Loader Section and Application Program Section in ATmega88PA and ATmega168PA. See SPMEN description
in section ”SPMCSR – Store Program Memory Control and Status Register” on page 278 for more details.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The
ATmega48A/PA/88A/PA/168A/PA/328/P Program Counter (PC) is 11/12/13/14 bits wide, thus addressing the
2/4/8/16K program 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, ATmega 48A/48PA” on page 255
and ”Boot Loader Support – Read-While-Write Self-Programming” on page 263. ”Memory Programming” on
page 280 contains a detailed description on Flash Programming in SPI- or Parallel Programming mode.
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 Timing” on page 14.
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Figure 8-1. Program Memory Map ATmega 48A/48PA
Figure 8-2. Program Memory Map ATmega88A, ATmega88PA, ATmega168A, ATmega168PA, ATmega328 and
ATmega328P
0x0000
0x7FF
Program Memory
Application Flash Section
0x0000
0x0FFF/0x1FFF/0x3FFF
Program Memory
Application Flash Section
Boot Flash Section
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8.3 SRAM Data Me mory
Figure 8-3 shows how the ATmega48A/PA/88A/PA/168A/PA/328/P SRAM Memory is organized.
The ATmega48A/PA/88A/PA/168A/PA/328/P is a complex microcontroller with more peripheral units than can
be supported 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 instructions can be used.
The lower 768/1280/1280/2303 data memory locations address both the Register File, the I/O memory,
Extended I/O memory, and the internal data SRAM. The first 32 locations address the Register File, the next 64
location the standard I/O memory, then 160 locations of Extended I/O memory, and the next
512/1024/1024/2048 locations address the internal data SRAM.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, 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-increment, 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
512/1024/1024/2048 bytes of internal data SRAM in the ATmega48A/PA/88A/PA/168A/PA/328/P are all
accessible through all these addressing modes. The Register File is described in ”General Purpose Register
File” on page 11.
Figure 8-3. Data Memory Map
32 Registers
64 I/O Registers
Internal SRAM
(512/1024/1024/2048 x 8)
0x0000 - 0x001F
0x0020 - 0x005F
0x02FF/0x04FF/0x4FF/0x08FF
0x0060 - 0x00FF
Data Memory
160 Ext I/O Reg.
0x0100
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8.3.1 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 8-4.
Figure 8-4. On-chip Data SRAM Access Cycles
8.4 EEPROM Data Memory
The ATmega48A/PA/88A/PA/168A/PA/328/P contains 256/512/512/1Kbytes 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 280 contains a detailed description on EEPROM Programming in SPI or
Parallel Programming mode.
8.4.1 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 8-2. A self-timing function, however, lets the user
software detect when the next byte can be written. If the user code contains instructions 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 21 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.
clk
WR
RD
Data
Data
Address Address valid
T1 T2 T3
Compute Address
Read Write
CPU
Memory Access Instruction Next Instruction
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8.4.2 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. Secondly, 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 completed provided that the power supply voltage is
sufficient.
8.5 I/O Memory
The I/O space definition of the ATmega48A/PA/88A/PA/168A/PA/328/P is shown in ”Register Summary” on
page 612.
All ATmega48A/PA/88A/PA/168A/PA/328/P 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
ATmega48A/PA/88A/PA/168A/PA/328/P 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 registers 0x00 to 0x1F only.
The I/O and peripherals control registers are explained in later sections.
8.5.1 General Purpose I/O Registers
The ATmega48A/PA/88A/PA/168A/PA/328/P 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.
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8.6 Register Description
8.6.1 EEARH and EEARL – The EEPROM Address Register
• Bits [15:10] – Reserved
These bits are reserved bits in the ATmega48A/PA/88A/PA/168A/PA/328/P and will always read as zero.
• Bits 9:0 – EEAR[9:0]: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the
256/512/512/1Kbytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and
255/511/511/1023. The initial value of EEAR is undefined. A proper value must be written before the EEPROM
may be accessed.
Note: 1. EEAR9 and EEAR8 are unused bits in ATmega 48A/48PA and must always be written to zero.
8.6.2 EEDR – The EEPROM Data Register
• Bits 7:0 – EEDR[7:0]: 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.
8.6.3 EECR – The EEPROM Control Register
• Bits 7:6 – Reserved
These bits are reserved bits in the ATmega48A/PA/88A/PA/168A/PA/328/P 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 triggered 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 8-1. 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 151413121110 9 8
0x22 (0x42) ––––––EEAR9
(1) EEAR8(1) EEARH
0x21 (0x41) 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
0x20 (0x40) MSB LSB EEDR
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
0x1F (0x3F) – – 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
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• 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 interrupt when EEPE is cleared. The
interrupt will not be generated during EEPROM write or SPM.
• 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, otherwise 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 SPMEN in SPMCSR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
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” 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 software 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.
Table 8-1. EEPROM Mode Bits
EEPM1 EEPM0 Programming
Time Operation
0 0 3.4ms Erase and Write in one operation (Atomic Operation)
0 1 1.8ms Erase Only
1 0 1.8ms Write Only
11 –Reserved for future use
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• 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 8-2 lists the typical programming 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 globally) 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 8-2. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time
EEPROM write
(from CPU) 26,368 3.3ms
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The next code examples show assembly and C functions for reading the EEPROM. The examples 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 ch ar 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);
}
Assembly Code Example
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8.6.4 GPIOR2 – General Purpose I/O Register 2
8.6.5 GPIOR1 – General Purpose I/O Register 1
8.6.6 GPIOR0 – General Purpose I/O Register 0
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;
}
Bit 76543210
0x2B (0x4B) MSB LSB GPIOR2
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
0x2A (0x4A) MSB LSB GPIOR1
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
0x1E (0x3E) MSB LSB GPIOR0
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
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9. System Clock and Clock Options
9.1 Clock Systems and their Distribution
Figure 9-1 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 Management and Sleep Modes” on page 39. The
clock systems are detailed below.
Figure 9-1. Clock Distribution
9.1.1 CPU Cloc k – 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.
9.1.2 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 start condition detection in the USI module is carried
out asynchronously when clkI/O is halted, TWI address recognition in all sleep modes.
Note: 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 interrupt 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 27.
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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9.1.3 Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with
the CPU clock.
9.1.4 Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly from an external
clock or an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/Counter as a real-
time counter even when the device is in sleep mode.
9.1.5 ADC Cloc k – cl kADC
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.
9.2 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.
9.2.1 Defa ul t Clo c k S ou rc e
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed, 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.
9.2.2 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 47 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. The selectable delays are shown in Table 9-2. The frequency of the
Watchdog Oscillator is voltage dependent as shown in ”Typical Characteristics – (TA = -40°C to 85°C)” on page
317.
Table 9-1. 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 128kHz 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 considered 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.
9.3 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 9-2 on page 29. 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 output. 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 30.
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 9-3 on
page 30. For ceramic resonators, the capacitor values given by the manufacturer should be used.
Figure 9-2. Crystal Oscillator Connections
The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range.
The operating mode is selected by the fuses CKSEL3...1 as shown in Table 9-3 on page 30.
Table 9-2. Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles
0ms 0ms 0
4.1ms 4.3ms 512
65ms 69ms 8K (8,192)
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C2
C1
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Notes: 1. This is the recommended CKSEL settings for the difference frequency ranges.
2. This option should not be used with crystals, only with ceramic resonators.
3. If the crystal 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 9-4.
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 frequency of the device, and if
frequency stability at start-up is not important for the application.
9.4 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 9-2 on page 29. Either a quartz crystal or a ceramic resonator
may be used.
Table 9-3. Low Power Crystal Oscillator Operating Modes(3)
Frequency Range
(MHz) Recommended Ran ge for
Capacitors C1 and C2 (pF) CKSEL3...1(1)
0.4 - 0.9 –100(2)
0.9 - 3.0 12 - 22 101
3.0 - 8.0 12 - 22 110
8.0 - 16.0 12 - 22 111
Table 9-4. Start-up Times for the Low Power Crys tal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Star t-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 + 4.1ms(1) 000
Ceramic resonator, slowly
rising power 258 CK 14CK + 65ms(1) 001
Ceramic resonator, BOD
enabled 1K CK 14CK(2) 010
Ceramic resonator, fast
rising power 1K CK 14CK + 4.1ms(2) 011
Ceramic resonator, slowly
rising power 1K CK 14CK + 65ms(2) 100
Crystal Oscillator, BOD
enabled 16K CK 14CK 101
Crystal Oscillator, fast
rising power 16K CK 14CK + 4.1ms 110
Crystal Oscillator, slowly
rising power 16K CK 14CK + 65ms 111