DS17485-3+ - Maxim Integrated

General Description

The DS17285, DS17485, DS17885, DS17287, DS17487,

and DS17887 real-time clocks (RTCs) are designed to

be successors to the industry-standard DS12885 and

DS12887. The DS17285, DS17485, and DS17885 (here-

after referred to as the DS17x85) provide a real-time

clock/calendar, one time-of-day alarm, three maskable

interrupts with a common interrupt output, a program-

mable square wave, and 114 bytes of battery-backed

NV SRAM. The DS17x85 also incorporates a number

of enhanced functions including a silicon serial number,

power-on/off control circuitry, and 2k, 4k, or 8kbytes of

battery-backed NV SRAM. The DS17287, DS17487, and

DS17887 (hereafter referred to as the DS17x87) integrate

a quartz crystal and lithium energy source into a 24-pin

encapsulated DIP package. The DS17x85 and DS17x87

power-control circuitry allows the system to be powered

on by an external stimulus such as a keyboard or by a

time-and-date (wake-up) alarm. The PWR output pin is

triggered by one or either of these events, and is used to

turn on an external power supply. The PWR pin is under

software control, so that when a task is complete, the

system power can then be shut down.

For all devices, the date at the end of the month is auto-

matically adjusted for months with fewer than 31 days,

including correction for leap years. It also operates in

either 24-hour or 12-hour format with an AM/PM indicator.

A precision temperature-compensated circuit monitors

the status of VCC. If a primary power failure is detected,

the device automatically switches to a backup supply. A

lithium coin cell battery can be connected to the VBAT

input pin on the DS17x85 to maintain time and date oper-

ation when primary power is absent. The DS17x85 and

DS17x87 include a VBAUX input used to power auxiliary

functions such as PWR control. The device is accessed

through a multiplexed byte-wide interface.

Applications

●Embedded Systems

●Utility Meters

●Security Systems

●Network Hubs, Bridges, and Routers

Features

●Incorporates Industry-Standard DS12887 PC

Clock Plus Enhanced Functions

●RTC Counts Seconds, Minutes, Hours, Day, Date,

Month, and Year with Leap Year Compensation

Through 2099

●Optional +3.0V or +5.0V Operation

●SMI Recovery Stack

●64-Bit Silicon Serial Number

●Power-Control Circuitry Supports System Power-

On from Date/Time Alarm or Key Closure

●Crystal Select Bit Allows Operation with 6pF or

12.5pF Crystal

●12-Hour or 24-Hour Clock with AM and PM in

12-Hour Mode

●114 Bytes of General-Purpose, Battery-Backed NV

SRAM

●Extended Battery-Backed NV SRAM

• 2048 Bytes (DS17285/DS17287)

• 4096 Bytes (DS17485/DS17487)

• 8192 Bytes (DS17885/DS17887)

●RAM Clear Function

●Interrupt Output with Six Independently Maskable

Interrupt Flags

●Time-of-Day Alarm Once per Second to Once per

Day

●End of Clock Update Cycle Flag

●Programmable Square-Wave Output

●Automatic Power-Fail Detect and Switch Circuitry

●Available in PDIP, SO, or TSOP Package

(DS17285, DS17485, DS17885)

●Optional Encapsulated DIP (EDIP) Package with

Integrated Crystal and Battery (DS17287,

DS17487, DS17887)

●Optional Industrial Temperature Range Available

●Underwriters Laboratory (UL) Recognized

19-5222; Rev 2; 5/16

Ordering Information, Pin Configurations, and Typical

Operating Circuit appear at end of data sheet.

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

Voltage Range on VCC Pin Relative to Ground ...-0.3V to +6.0V

Operating Temperature Range (Noncondensing)

Commercial.........................................................0°C to +70°C

Industrial ......................................................... -40°C to +85°C

Storage Temperature Range

EDIP ............................................................... -40°C to +85°C

PDIP, SO, TSOP ........................................... -55°C to +125°C

Lead Temperature (soldering, 10s) .................................+260°C

(Note: EDIP is hand or wave-soldered only.)

Soldering Temperature (reflow) ....................................... +260°C

(VCC = +4.5V to +5.5V, or VCC = +2.7V to +3.7V, TA = Over the operating temperature range, unless otherwise noted. Typical

values are with TA = +25°C, VCC = 5.0V or 3.0V and VBAT = 3.0V, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Voltage (Note 3) VCC

(-5) 4.5 5.0 5.5 V

(-3) 2.7 3.0 3.7

VBAT Input Voltage VBAT (Note 3) 2.5 3.0 3.7 V

VBAUX Input Voltage (Note 3) VBAUX

(-5) 2.5 3.0 5.2 V

(-3) 3.7

Input Logic 1 (Note 3) VIH

(-5) 2.2 VCC +

0.3 V

(-3) 2.0 VCC +

0.3

Input Logic 0 (Note 3) VIL

(-5) -0.3 +0.8 V

(-3) -0.3 +0.6

VCC Power-Supply Current

(Note 4) ICC1

(-5) 25 50 mA

(-3) 15 30

VCC Standby Current (Notes 4, 5) ICCS

(-5) 1.0 3.0 mA

(-3) 0.5 2.0

Input Leakage IIL -1.0 +1.0 µA

I/O Leakage IOL (Note 6) -1.0 +1.0 µA

Output Logic 1 Voltage (Note 3) VOH

(-5), -1.0mA 2.4 V

(-3), -0.4mA 2.4

Output Logic 0 Voltage

AD0–AD7, IRQ, SQW (Note 3) VOL

(-5), +2.1mA 0.4 V

(-3), +0.8mA 0.4

Output Logic 0 Voltage

PWR (Note 3) VOL

(-5), +10mA 0.4 V

(-3), +4mA 0.4

Power-Fail Voltage (Note 3) VPF

(-5) 4.25 4.37 4.5 V

(-3) 2.5 2.6 2.7

VRT Trip Point VRTTRIP (Note 3) 1.3 V

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

Absolute Maximum Ratings

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these

or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect

device reliability.

DC Electrical Characteristics

(VCC = 0V, VBAT = 3.0V, TA = Over the operating range, unless otherwise noted.) (Note 1)

(VCC = +4.5V to +5.5V, TA = Over the operating range, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

VBAT or VBAUX Current

(Oscillator On); TA = +25°C,

VBAT = 3.0V

IBAT (Note 7) 500 700 nA

VBAT or VBAUX Current

(Oscillator Off) IBATDR (Note 7) 50 400 nA

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Cycle Time tCYC 240 DC ns

Pulse Width, RD or WR Low PWRWL 120 ns

Pulse Width, RD or WR High PWRWH 80 ns

Input Rise and Fall tR, tF30 ns

Chip-Select Setup Time Before

RD or WR tCS 20 ns

Chip-Select Hold Time tCH 0 ns

Read-Data Hold Time tDHR 10 50 ns

Write-Data Hold Time tDHW 0 ns

Address Setup Time to ALE Fall tASL 20 ns

Address Hold Time to ALE Fall tAHL 10 ns

RD or WR High Setup to ALE

Rise tASD 25 ns

Pulse Width ALE High PWASH 40 ns

Delay Time ALE Low to RD Low tASED 30 ns

Output Data Delay Time from RD tDDR (Note 8) 20 120 ns

Data Setup Time tDSW 30 ns

IRQ Release from RD tIRD 2 µs

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

DC Electrical Characteristics

AC Electrical Characteristics

(VCC = +2.7V to +3.7V, TA = Over the operating range, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Cycle Time tCYC 360 DC ns

Pulse Width, RD or WR Low PWRWL 200 ns

Pulse Width, RD or WR High PWRWH 150 ns

Input Rise and Fall tR, tF30 ns

Chip-Select Setup Time Before

RD or WR tCS 20 ns

Chip-Select Hold Time tCH 0 ns

Read-Data Hold Time tDHR 10 90 ns

Write-Data Hold Time tDHW 0 ns

Address Setup Time to ALE Fall tASL 40 ns

Address Hold Time to ALE Fall tAHL 10 ns

RD or WR High Setup to ALE

Rise tASD 30 ns

Pulse Width ALE High PWASH 40 ns

Delay Time ALE Low to RD Low tASED 30 ns

Output Data Delay Time from RD tDDR (Note 8) 20 200 ns

Data Setup Time tDSW 70 ns

IRQ Release from RD tIRD 2 µs

PWASH

tASED

PWRWH PWRWL

tCS

tAHL

tASL tDSW tDHW

tCH

tASD

tCYC

AD0–AD7

WRITE

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

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AC Electrical Characteristics

Write Timing

AD0–AD7

CS, WR, RD

HIGH IMPEDANCE

DON'T CARE

VALID

RECOGNIZED RECOGNIZED

VALID

VCC

VPF(MAX)

VPF(MIN)

tREC

tASL tDDR

PWASH

ALE

AD0–AD7

tASD

PWRWL

tCS

tDHR

tAHL

tCH

tCYC

PWRWH

tASED

IRQ

tIRD

tASD

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

Power-Up/Power-Down Timing

Read Timing

Note 1: RTC modules can be successfully processed through conventional wave-soldering techniques as long as temperature

exposure to the lithium energy source contained within does not exceed +85°C. However, post-solder cleaning with water-

washing techniques is acceptable, provided that ultrasonic vibrations not used to prevent damage to the crystal.

Note 2: Limits at -40°C are guaranteed by design and not production tested.

Note 3: All voltages are referenced to ground.

Note 4: All outputs are open.

Note 5: Specified with CS = RD = WR = VCC, ALE, AD0–AD7 = 0.

Note 6: Applies to the AD0–AD7 pins, IRQ, and SQW when each is in a high-impedance state.

Note 7: Measured with a 32.768kHz crystal attached to X1 and X2.

Note 8: Measured with a 50pF capacitance load plus 1TTL gate.

Note 9: If the oscillator is disabled in software, or if the countdown chain is in reset, tREC is bypassed, and the part becomes

immediately accessible.

Note 10: Guaranteed by design. Not production tested.

(TA = -40°C to +85°C) (Note 2)

(TA = +25°C)

(TA = +25°C) (Note 10)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Recovery at Power-Up tREC (Note 9) 20 150 ms

VCC Fall Time, VPF(MAX) to

VPF(MIN)

tF300 µs

VCC Fall Time, VPF(MAX) to

VPF(MIN)

tR0 µs

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Expected Data Retention tDR (Note 9) 10 Years

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Capacitance on All Input Pins

Except X1 CIN (Note 10) 12 pF

Capacitance on IRQ, SQW, and

DQ0–DQ7 Pins CIO (Note 10) 12 pF

PARAMETER CONDITIONS

Input Pulse Levels: 0 to 3.0V

Output Load Including Scope and Jig: 50pF + 1TTL Gate

Input and Output Timing Measurement Reference Levels: Input/Output: VIL max and VIH min

Input Pulse Rise and Fall Times: 5ns

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

Power-Up/Power-Down Characteristics

Data Retention (DS17x87 Only)

Capacitance

AC Test Conditions

WARNING: Negative undershoots below -0.3V while the part is in battery-backed mode can cause loss of data.

(VCC = +3.3V, TA = +25°C, unless otherwise noted.)

PIN NAME FUNCTION

24 28

1 8 PWR

Active-Low Power-On Reset. This open-drain output pin is intended for use as an on/off control

for the system power. With VCC voltage removed from the device, PWR can be automatically

activated from a kickstart input by the KS pin or from a wake-up interrupt. Once the system is

powered on, the state of PWR can be controlled by bits in the control registers. The PWR pin

can be connected through a pullup resistor to a positive supply. For 5V operation, the voltage

of the pullup supply should be no greater than 5.7V. For 3V operation, the voltage on the pullup

supply should be no greater than 3.9V.

2, 3 9, 10 X1, X2

Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is designed

for operation with a crystal having a specied load capacitance (CL) of 6pF or 12.5pF. Pin X1 is

the input to the oscillator and can optionally be connected to an external 32.768kHz oscillator.

The output of the internal oscillator, pin X2, is left unconnected if an external oscillator is

connected to pin X1. These pins are missing (N.C.) on the EDIP package.

4–11 12–17,

19, 20 AD0–AD7

Multiplexed Bidirectional Address/Data Bus. The addresses are presented during the rst

portion of the bus cycle and latched into the device by the falling edge of ALE. Write data is

latched by the rising edge of WR. In a read cycle, the device outputs data during the latter

portion of the RD low. The read cycle is terminated and the bus returns to a high-impedance

state as RD transitions high.

12, 16 21, 22, 26 GND Ground

SUPPLY CURRENT

vs. TEMPERATURE

DS17285/87 toc02

TEMPERATURE (°C)

SUPPLY CURRENT (nA)

65503520

-10-25

300

350

400

250

-40 80

VBAT = 3.0V

OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

DS17285/87 toc03

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (Hz)

5.04.54.03.53.0

32768.1

32768.2

32768.3

32768.4

32768.5

32768.6

32768.7

32768.0

2.5 5.5

SUPPLY CURRENT

vs. INPUT VOLTAGE

DS17285/87 toc01

VBAT (V)

SUPPLY CURRENT (nA)

3.53.33.02.8

250

300

350

400

200

2.5 3.8

VCC = 0V

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

Typical Operating Characteristics

Pin Description

PIN NAME FUNCTION

24 28

13 23 CS

Active-Low Chip-Select Input. This pin must be asserted low during a bus cycle for the device

to be accessed. CS must be kept in the active state during RD and WR. Bus cycles that take

place without asserting CS latch addresses, but no access occurs.

14 24 ALE Address Latch Enable Input, Active High. This input pin is used to demultiplex the address/data

bus. The falling edge of ALE causes the address to be latched within the device.

15 25 WR Active-Low Write Input. This pin denes the period during which data is written to the

addressed register.

17 27 RD Active-Low Read Input. This pin identies the period when the device drives the bus with read

data. It is an enable signal for the output buffers of the device.

18 28 KS

Active-Low Kickstart Input. When VCC is removed from the device, the system can be powered

on in response to an active-low transition on the KS pin, as might be generated from a key

closure. VBAUX must be present and auxiliary-battery-enable bit (ABE) must be set to 1 if the

kickstart function is used, and the KS pin must be pulled up to the VBAUX supply. While VCC

is applied, the KS pin can be used as an interrupt input. If not used, KS must be grounded and

ABE set to 0.

19 1 IRQ

Active-Low Interrupt Request. This pin is an active-low output that can be used as an interrupt

input to a processor. The IRQ output remains low as long as the status bit causing the interrupt

is present and the corresponding interrupt-enable bit is set. To clear the IRQ pin, the application

software must clear all enabled ag bits contributing to the pin’s active state. When no interrupt

conditions are present, the IRQ level is in the high-impedance state. Multiple interrupting

devices can be connected to an IRQ bus, provided that they are all open drain. The IRQ pin

requires an external pullup resistor to VCC.

20 2 VBAT

Connection for Primary Battery. This supply input is used to power the normal clock functions

when VCC is absent. Diodes placed in series between VBAT and the battery can prevent proper

operation. If VBAT is not required, the pin must be grounded. UL recognized to ensure against

reverse charging current when used with a lithium battery (www.maximintegrated.com/qa/

info/ul). This pin is missing (N.C.) on the EDIP package.

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

│

Pin Description (continued)

PIN NAME FUNCTION

24 28

21 3 RCLR

Active-Low RAM Clear Input. This pin is used to clear (set to logic 1) all the 114 bytes of

general-purpose RAM but does not affect the RAM associated with the real time clock or

extended RAM. RCLR may be invoked while the part is powered from any supply. The RCLR

function is designed to be used via a human interface (shorting to ground manually or by a

switch) and not to be driven with external buffers. This pin is internally pulled up. Do not use an

external pullup resistor on this pin.

22 4 VBAUX

Auxiliary Battery Input. Required for kickstart and wake-up functions. This input also supports

clock/calendar and user RAM if VBAT is at lower voltage or is not used. A standard +3V lithium

cell or other energy source can be used. Diodes placed in series between VBAUX and the

battery may prevent proper operation. UL recognized to ensure against reverse charging

current when used with a lithium battery (www.maximintegrated.com/qa/info/ul/). For 3V

VCC operation, VBAUX must be held between +2.5V and +3.7V. For 5V VCC operation, VBAUX

must be held between +2.5V and +5.2V. If VBAUX is not used it should be grounded and the

auxiliary-battery-enable bit bank 1, register 4BH, should = 0.

23 5 SQW

Square-Wave Output. When VCC rises above VPF, bits DV1 and E32k are set to 1. This

condition enables a 32kHz square-wave output. A square wave is output if either SQWE = 1

or E32k = 1. If E32k = 1, then 32kHz is output regardless of the other control bits. If E32k =

0, then the output frequency is dependent on the control bits in Register A. The SQW pin can

output a signal from one of 13 taps provided by the 15 internal divider stages of the RTC. The

frequency of the SQW pin can be changed by programming Register A, as shown in Table 3.

The SQW signal can be turned on and off using the SQWE bit in Register B or the E32k bit

in extended register 4Bh. A 32kHz square wave is also available when VCC is less than VPF

if E32k = 1, ABE = 1, and voltage is applied to the VBAUX pin. When disabled, SQW is high

impedance when VCC is below VPF.

24 6, 7 VCC

DC Power Pin for Primary Power Supply. When VCC is applied within normal limits, the device

is fully accessible and data can be written and read. When VCC is below VPF reads and writes

are inhibited.

2, 3,

16, 20

(DS17x87

only)

11, 18 N.C. No Connection

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

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Pin Description (continued)

Figure 1. Functional Diagram

OSCILLATOR

POWER

CONTROL

DS17x87

ONLY

VBAT

GND

DIVIDE

BY 8

REGISTERS A, B, C, D

CLOCK/CALENDAR

UPDATE LOGIC

EXTENDED

USER RAM

2k/4k/8k

BYTES

SELECT

EXTENDED RAM ADDR/

DATA REGISTERS

EXTENDED CONTROL/

STATUS REGISTERS

64-BIT SERIAL NUMBER

CENTURY COUNTER

DATE ALARM

RTC ADDRESS-2

RTC ADDRESS-3

DIVIDE BY

16:1 MUX

SQUARE-

WAVE

GENERATOR

SQW

IRQ

PWR

RLCR

IRQ

GENERATOR

VCC

VBAUX

BUS

INTERFACE

ALE

AD0–AD7

CLOCK/CALENDAR AND

ALARM REGISTERS

BUFFERED CLOCK/

CALENDAR AND ALARM

REGISTERS

USER RAM

114 BYTES

RAM

CLEAR

LOGIC

DS17x85/87

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

www.maximintegrated.com Maxim Integrated

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Detailed Description

The DS17x85 is a successor to the DS1285 real-time

clock (RTC). The device provides 18 bytes of real-time

clock/calendar, alarm, and control/status registers and 114

bytes of nonvolatile battery-backed RAM. The device also

provides additional extended RAM in either 2k/4k/8kbytes

(DS17285/DS17485/DS17885). A time-of-day alarm, six

maskable interrupts with a common interrupt output, and

a programmable square-wave output are available. It also

operates in either 24-hour or 12-hour format with an AM/

PM indicator. A precision temperature-compensated cir-

cuit monitors the status of VCC. If a primary power-supply

failure is detected, the device automatically switches to

a backup supply. The backup supply input supports a

primary battery, such as a lithium coin cell. The device is

accessed by a multiplexed address/data bus.

Oscillator Circuit

The DS17x85 uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 1 specifies several crystal

parameters for the external crystal, and Figure 2 shows

a functional schematic of the oscillator circuit. The oscil-

lator is controlled by an enable bit in the control register.

Oscillator startup times are highly dependent upon crystal

characteristics, PC board leakage, and layout. High ESR

and excessive capacitive loads are the major contributors

to long startup times. A circuit using a crystal with the

recommended characteristics and proper layout usually

starts within one second.

An external 32.768kHz oscillator can also drive the

DS17x85. In this configuration, the X1 pin is connected

to the external oscillator signal and the X2 pin is left

unconnected.

Clock Accuracy

The accuracy of the clock is dependent upon the accu-

racy of the crystal and the accuracy of the match between

the capacitive load of the oscillator circuit and the capaci-

tive load for which the crystal was trimmed. Additional

error will be added by crystal frequency drift caused by

temperature shifts. External circuit noise coupled into

the oscillator circuit may result in the clock running fast.

Figure 3 shows a typical PC board layout for isolation of

the crystal and oscillator from noise. Refer to Application

Note 58: Crystal Considerations with Dallas Real-Time

Clocks for detailed information.

Clock Accuracy (DS17287,

DS17487, and DS17887)

The encapsulated DIP (EDIP) modules are trimmed at the

factory to ±1 minute per month accuracy at 25°C.

Figure 2. Oscillator Circuit Showing Internal Bias Network

Figure 3. Layout Example

Table 1. Crystal Specifications* (DS17x85

Only)

*The crystal, traces, and crystal input pins should be isolated

from RF generating signals. Refer to Application Note 58:

Crystal Considerations for Dallas Real-Time Clocks for addi-

tional specifications.

PARAMETER SYMBOL MIN TYP MAX UNITS

Nominal

Frequency fO32.768 kHz

Series

Resistance ESR 50 kΩ

Load

Capacitance CL

6 or

12.5 pF

COUNTDOWN

CHAIN

X1 X2

CRYSTAL

CL1 CL2RTC REGISTERS

DS17285/87

DS17485/87

DS17885/87

LOCAL GROUND PLANE (TOP LAYER)

CRYSTAL

GND

NOTE: AVOID ROUTING SIGNAL LINES

IN THE CROSSHATCHED AREA

(UPPER LEFT QUADRANT) OF

THE PACKAGE UNLESS THERE IS

A GROUND PLANE BETWEEN THE

SIGNAL LINE AND THE DEVICE PACKAGE.

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Real-Time Clocks

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Power-Down/Power-Up

Considerations

The RTC function continues to operate, and all the RAM,

time, calendar, and alarm memory locations remain non-

volatile regardless of the level of the VCC input. VBAT or

VBAUX must remain within the minimum and maximum

limits when VCC is not applied. When VCC falls below

VPF, the device inhibits all access, putting the part into

a low-power mode. When VCC is applied and exceeds

VPF (power-fail trip point), the device becomes acces-

sible after tREC, if the oscillator is running and the oscil-

lator countdown chain is not in reset (Register A). This

time period allows the system to stabilize after power

is applied. If the oscillator is not enabled, the oscilla-

tor enable bit is enabled on powerup, and the device

becomes immediately accessible.

Power Control

The power control function is provided by a precise,

temperature-compensated voltage reference and a com-

parator circuit that monitors the VCC level. The device is

fully accessible and data can be written and read when

VCC is greater than VPF. However, when VCC falls below

VPF, the device inhibits read and write access. If VPF is

less than VBAT, the device power is switched from VCC

to the higher of VBAT or VBAUX when VCC drops below

VPF. If VPF is greater than the higher of VBAT or VBAUX,

the device power is switched from VCC to the higher of

VBAT or VBAUX when VCC drops below the higher backup

source. The registers are maintained from the VBAT or

VBAUX source until VCC is returned to nominal levels.

After VCC returns above VPF, read and write access is

allowed after tREC.

Time, Calendar, and Alarm

Locations

The time and calendar information is obtained by read-

ing the appropriate register bytes. The time, calendar,

and alarm are set or initialized by writing the appropriate

alarm bytes can be either binary or binary-coded deci-

mal (BCD) format. Tables 3A and 3B show the BCD and

binary formats of the 12 time, date, and alarm registers,

control registers A to D, plus the two extended registers

that reside in bank 1 only (bank 0 and bank 1 switching is

explained later in this text).

The day-of-week register increments at midnight, incre-

menting from 1 through 7. The day-of-week register is

used by the daylight saving function, and so the value 1

is defined as Sunday. The date at the end of the month

is automatically adjusted for months with fewer than 31

days, including correction for leap years.

Before writing the internal time, calendar, and alarm

registers, the SET bit in Register B should be written to

logic 1 to prevent updates from occurring while access is

being attempted. In addition to writing the 12 time, calen-

dar, and alarm registers in a selected format (binary or

BCD), the data mode bit (DM) of Register B must be set

to the appropriate logic level. All 12 time, calendar, and

alarm bytes must use the same data mode. The set bit in

been written to allow the real time clock to update the time

and calendar bytes. Once initialized, the real time clock

makes all updates in the selected mode. The data mode

cannot be changed without reinitializing the 12 data bytes.

Tables 3A and 3B show the BCD and binary formats of the

12 time, calendar, and alarm locations.

The 24-12 bit cannot be changed without reinitializing the

hour locations. When the 12-hour format is selected, the

high order bit of the hours byte represents PM when it is

logic 1. The time, calendar, and alarm bytes are always

accessible because they are double-buffered. Once per

second, the eight bytes are advanced by one second and

checked for an alarm condition.

If a read of the time and calendar data occurs during an

update, a problem exists where seconds, minutes, hours,

etc., may not correlate. The probability of reading incor-

rect time and calendar data is low. Several methods of

avoiding any possible incorrect time and calendar reads

are covered later in this text.

Table 2. Power Control

SUPPLY CONDITION READ/WRITE

ACCESS POWERED BY

VCC < VPF, VCC <

(VBAT | VBAUX)No VBAT or VBAUX

VCC < VPF, VCC >

(VBAT | VBAUX)No VCC

VCC > VPF, VCC <

(VBAT | VBAUX)Yes VCC

VCC > VPF, VCC >

(VBAT | VBAUX)Yes VCC

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The alarm bytes can be used in two ways. First, when the

alarm time is written in the appropriate hours, minutes, and

seconds alarm locations, the alarm interrupt is initiated at

the specified time each day, if the alarm enable bit is high.

In this mode, the “0” bits in the alarm registers and the cor-

responding time registers must always be written to 0 (see

Table 3A and 3B). Writing the 0 bits in the alarm and/or time

registers to 1 can result in undefined operation.

The second use condition is to insert a “don’t care” state

in one or more of the alarm bytes. The don’t care code is

any hexadecimal value from C0 to FF. The two most sig-

nificant bits of each byte set the don’t care condition when

at logic 1. An alarm will be generated each hour when the

“don’t care” bits are set in the hours byte. Similarly, an

alarm is generated every minute with don’t care codes in

the hours and minute alarm bytes. An alarm is generated

every second with don’t care codes in the hours, minutes,

and seconds alarm bytes.

All 128 bytes can be directly written or read except for the

following:

1) Registers C and D are read-only.

2) Bit 7 of register A is read-only.

3) The MSB of the seconds byte is read-only.

Note: Unless otherwise specified, the state of the registers is not defined when power is first applied. Except for the seconds reg-

ister, 0 bits in the time and date registers can be written to 1, but can be modified when the clock updates. 0 bits should always be

written to 0 except for alarm mask bits.

Table 3A. Time, Calendar, and Alarm Data Modes—BCD Mode (DM = 0)

ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE

00h 0 10 Seconds Seconds Seconds 00–59

01h 0 10 Seconds Seconds Seconds Alarm 00–59

02h 0 10 Minutes Minutes Minutes 00–59

03h 0 10 Minutes Minutes Minutes Alarm 00–59

04h AM/PM 00 10 Hour Hours Hours 1–12 + AM/PM

00–23

0 10 Hour

05h AM/PM 00 10 Hour Hours Hours Alarm 1–12 + AM/PM

00–23

0 10 Hour

06h 0 0 0 0 0 Day Day 01–07

07h 0 0 10 Date Date Date 01–31

08h 0 0 0 10 Month Month Month 01–12

09h 10 Year Year Year 00–99

0Ah UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0 Control —

0Bh SET PIE AIE UIE SQWE DM 24/12 DSE Control —

0Ch IRQF PF AF UF 0 0 0 0 Control —

0Dh VRT 0 0 0 0 0 0 0 Control —

Bank 1, 48h 10 Century Century Century 00–99

Bank 1, 49h 10 Date Date Date Alarm 01–31

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Control Registers

The four control registers (A, B, C, and D) reside in both

bank 0 and bank 1. These registers are accessible at all

times, even during the update cycle.

Bit 7: Update In Progress (UIP). This bit is a status flag

that can be monitored. When the UIP bit is 1, the update

transfer will soon occur. When UIP is 0, the update trans-

fer does not occur for at least 244μs. The time, calendar,

and alarm information in RAM is fully available for access

when the UIP bit is 0. The UIP bit is read-only. Writing the

SET bit in Register B to 1 inhibits any update transfer and

clears the UIP status bit.

Bits 6, 5, and 4: DV2, DV1, and DV0. These bits are

used to turn the oscillator on or off and to reset the count-

down chain. A pattern of 01X is the only combination of

bits that turns the oscillator on and allows the RTC to keep

time. A pattern of 11X enables the oscillator but holds

the countdown chain in reset. The next update occurs at

500ms after a pattern of 01X is written to DV0, DV1, and

DV2. DV0 is used to select bank 0 or bank 1 as defined in

Table 5. When DV0 is set to 0, bank 0 is selected. When

DV0 is set to 1, bank 1 is selected.

Note: Unless otherwise specified, the state of the registers is not defined when power is first applied. Except for the seconds reg-

ister, 0 bits in the time and date registers can be written to 1, but can be modified when the clock updates. 0 bits should always be

written to 0 except for alarm mask bits.

Table 3B. Time, Calendar, and Alarm Data Modes—Binary Mode (DM = 1)

ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE

00h 0 0 Seconds Seconds 00–3B

01h 0 0 Seconds Seconds Alarm 00–3B

02h 0 0 Minutes Minutes 00–3B

03h 0 0 Minutes Minutes Alarm 00–3B

04h AM/PM 0 0 0 Hours Hours 1–0C + AM/PM

00–17

0 Hours

05h AM/PM 0 0 0 Hours Hours Alarm 1–0C + AM/PM

00–17

0 Hours

06h 0 0 0 0 0 Day Day 01–07

07h 0 0 0 Date Date 01–1F

08h 0 0 0 0 Month Month 01–0C

09h 0 Year Year 00–63

0Ah UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0 Control —

0Bh SET PIE AIE UIE SQWE DM 24/12 DSE Control —

0Ch IRQF PF AF UF 0 0 0 0 Control —

0Dh VRT 0 0 0 0 0 0 0 Control —

Bank 1, 48h 10 Century Century Century 00–63

Bank 1, 49h 10 Date Date Date Alarm 01–1F

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0

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Bits 3 to 0: Rate Selector Bits (RS3 to RS0). These

four rate-selection bits select one of the 13 taps on the

15-stage divider or disable the divider output. The tap

selected can be used to generate an output square wave

(SQW pin) and/or a periodic interrupt. The user can do

one of the following:

1) Enable the interrupt with the PIE bit;

2) Enable the SQW output pin with the SQWE or E32k

bits;

3) Enable both at the same time and the same rate; or

4) Enable neither.

Table 4 lists the periodic interrupt rates and the square-

wave frequencies that can be chosen with the RS bits.

*RS3 to RS0 determine periodic interrupt rates as listed for E32K = 0.

Table 4. Periodic Interrupt Rate and Square-Wave Output Frequency

EXT REG B SELECT BITS REGISTER A tPI PERIODIC INTERRUPT

RATE SQW OUTPUT FREQUENCY

E32K RS3 RS2 RS1 RS0

0 0 0 0 0 None None

0 0 0 0 1 3.90625ms 256Hz

0 0 0 1 0 7.8125ms 128Hz

0 0 0 1 1 122.070Fs 8.192kHz

0 0 1 0 0 244.141Fs 4.096kHz

0 0 1 0 1 488.281Fs 2.048kHz

0 0 1 1 0 976.5625Fs 1.024kHz

0 0 1 1 1 1.953125ms 512Hz

0 1 0 0 0 3.90625ms 256Hz

0 1 0 0 1 7.8125ms 128Hz

0 1 0 1 0 15.625ms 64Hz

0 1 0 1 1 31.25ms 32Hz

0 1 1 0 0 62.5ms 16Hz

0 1 1 0 1 125ms 8Hz

0 1 1 1 0 250ms 4Hz

0 1 1 1 1 500ms 2Hz

1 X X X X * 32.768kHz

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Bit 7: SET. When the SET bit is 0, the update transfer

functions normally by advancing the counts once per sec-

ond. When the SET bit is written to 1, any update transfer

is inhibited, and the program can initialize the time and

calendar bytes without an update occurring in the midst

of initializing. Read cycles can be executed in a similar

manner. SET is a read/write bit and is not affected by any

internal functions of the DS17x85.

Bit 6: Periodic Interrupt Enable (PIE). This bit is a read/

write bit that allows the periodic interrupt flag (PF) bit in

periodic interrupts are generated by driving the IRQ pin low

at a rate specified by the RS3–RS0 bits of Register A. A 0

in the PIE bit blocks the IRQ output from being driven by

a periodic interrupt, but the PF bit is still set at the periodic

rate. PIE is not modified by any internal DS17x85 functions.

Bit 5: Alarm Interrupt Enable (AIE). This bit is a read/

write bit that, when set to 1, permits the alarm flag (AF)

bit in Register C to assert IRQ. An alarm interrupt occurs

for each second that the three time bytes equal the three

alarm bytes, including a don’t care alarm code of binary

11XXXXXX. When the AIE bit is set to 0, the AF bit does

not initiate the IRQ signal. The internal functions of the

DS17x285/87 do not affect the AIE bit.

Bit 4: Update-Ended Interrupt Enable (UIE). This bit is

a read/write bit that enables the update-end flag (UF) bit

in Register C to assert IRQ. The SET bit going high clears

the UIE bit.

Bit 3: Square-Wave Enable (SQWE). When this bit is set

to 1 and E32k = 0, a square-wave signal at the frequency

set by RS3–RS0 is driven out on the SQW pin. When the

SQWE bit is set to 0 and E32k = 0, the SQW pin is held

low. SQWE is a read/write bit. SQWE is set to 1 when

VCC is powered up.

Bit 2: Data Mode (DM). This bit indicates whether time

and calendar information is in binary or BCD format. The

program sets the DM bit to the appropriate format and can

be read as required. This bit is not modified by internal

functions. A 1 in DM signifies binary data, while a 0 in DM

specifies binary-coded decimal (BCD) data.

Bit 1: 24/12 Control (24/12). This bit establishes the for-

mat of the hours byte. A 1 indicates the 24-hour mode and

a 0 indicates the 12-hour mode. This bit is read/write and

is not affected by internal functions.

Bit 0: Daylight Saving Enable (DSE). This bit is a read/

write bit that enables two daylight saving adjustments

when DSE is set to 1. On the first Sunday in April, the

time increments from 1:59:59AM to 3:00:00AM. On

the last Sunday in October when the time first reaches

1:59:59AM, it changes to 1:00:00AM. When DSE is

enabled, the internal logic tests for the first/last Sunday

condition at midnight. If the DSE bit is not set when the

test occurs, the daylight saving function does not operate

correctly. These adjustments do not occur when the DSE

bit is zero. This bit is not affected by internal functions.

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

SET PIE AIE UIE SQWE DM 24/12 DSE

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Bit 7: Interrupt Request Flag (IRQF). This bit is set to 1

when any of the following are true:

PF = PIE = 1 WF = WIE = 1

AF = AIE = 1 KF = KSE = 1

UF = UIE = 1 RF = RIE = 1

Any time the IRQF bit is 1, the IRQ pin is driven low. Flag

bits PF, AF, and UF are cleared after reading Register C.

Bit 6: Periodic Interrupt Flag (PF). This is a read-only bit

that is set to 1 when an edge is detected on the selected

tap of the divider chain. The RS3–RS0 bits establish the

periodic rate. PF is set to 1 independent of the state of the

PIE bit. When both PF and PIE are 1s, the IRQ signal is

active and sets the IRQF bit. Reading Register C clears

this bit.

Bit 5: Alarm Interrupt Flag (AF). A 1 in this bit indicates

that the current time has matched the alarm time. If the

AIE bit is also 1, the IRQ pin goes low and a 1 appears in

the IRQF bit. Reading Register C clears this bit.

Bit 4: Update-Ended Interrupt Flag (UF). This bit is set

after each update cycle. When the UIE bit is set to 1, the

1 in UF causes the IRQF bit to be 1, which asserts IRQ.

Reading Register C clears this bit.

Bits 3 to 0: Unused. These unused bits always read 0

and cannot be written.

Bit 7: Valid RAM and Time (VRT). This bit indicates

the condition of the battery connected to the VBAT and

VBAUX pin. If either supply is above the internal voltage

threshold, VRTTRIP, the bit will be high. This bit is not

writeable and should always be a 1 when read. If a 0 is

ever present, an exhausted internal lithium energy source

is indicated and both the contents of the RTC data and

RAM data are questionable.

Bits 6 to 0: Unused. These bits cannot be written and,

when read, always read 0.

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

IRQF PF AF UF 0 0 0 0

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

VRT 0 0 0 0 0 0 0

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Nonvolatile RAM

The user RAM bytes are not dedicated to any special

function within the DS17x85. They can be used by the

processor program as battery-backed memory and are

fully available during the update cycle.

The user RAM is divided into two separate memory

banks. When the bank 0 is selected, the 14 real-time

clock registers and 114 bytes of user RAM are acces-

sible. When bank 1 is selected, an additional 2kbytes,

4kbytes, or 8kbytes of user RAM are accessible through

the extended RAM address and data registers.

Interrupts

The RTC includes six separate, fully automatic sources of

interrupt for a processor:

1) Alarm Interrupt

2) Periodic Interrupt

3) Update-Ended Interrupt

4) Wake-Up Interrupt

5) Kickstart Interrupt

6) RAM Clear Interrupt

The conditions that generate each of these independent

interrupt conditions are described in detail in other sec-

tions of this data sheet. This section describes the overall

control of the interrupts.

The application software can select which interrupts, if

any, are to be used. There are 6 bits, including 3 bits

in Register B and 3 bits in Extended Register 4B, that

enable the interrupts. The extended register locations

are described later. Writing logic 1 to an interruptenable

bit permits that interrupt to be initiated when the event

occurs. A logic 0 in the interrupt-enable bit prohibits the

IRQ pin from being asserted from that interrupt condi-

tion. If an interrupt flag is already set when an interrupt

is enabled, IRQ is immediately set at an active level,

although the event initiating the interrupt condition might

have occurred much earlier. Therefore, there are cases

where the software should clear these earlier generated

interrupts before first enabling new interrupts.

When an interrupt event occurs, the relating flag bit is

set to logic 1 in Register C or in Extended Register 4A.

These flag bits are set regardless of the setting of the

corresponding enable bit located either in Register B or in

Extended Register 4B. The flag bits can be used in a poll-

ing mode without enabling the corresponding enable bits.

However, care should be taken when using the flag bits of

ately after they are read. Double latching is implemented

on these bits so that set bits remain stable throughout the

read cycle. All bits that were set are cleared when read

and new interrupts that are pending during the read cycle

are held until after the cycle is completed. One, two, or

three bits can be set when reading Register C. Each used

flag bit should be examined when read to ensure that no

interrupts are lost.

The flag bits in Extended Register 4A are not automati-

cally cleared following a read. Instead, each flag bit can

be cleared to 0 only by writing 0 to that bit.

When using the flag bits with fully enabled interrupts,

the IRQ line is driven low when an interrupt flag bit is set

and its corresponding enable bit is also set. IRQ is held

low as long as at least one of the six possible interrupt

sources has its flag and enable bits both set. The IRQF

bit in Register C is 1 whenever the IRQ pin is being driven

low as a result of one of the six possible active sources.

Therefore, determination that the DS17x85/DS17x87 initi-

ated an interrupt is accomplished by reading Register C

and finding IRQF = 1. IRQF remains set until all enabled

interrupt flag bits are cleared to 0.

Oscillator Control Bits

A pattern of 01X in bits 4 to 6 of Register A turns the oscil-

lator on and enables the countdown chain. A pattern of

11X (DV2 = 1, DV1 = 1, DV0 = X) turns the oscillator on,

but holds the countdown chain of the oscillator in reset.

All other combinations of bits 4 to 6 keep the oscillator off.

When the DS17x87 is shipped from the factory, the

internal oscillator is turned off. This feature prevents the

lithium energy cell from being used until it is installed in

a system.

Square-Wave Output Selection

Thirteen of the 15 divider taps are made available to a

1-of-16 multiplexer, as shown in Figure 1. The square

wave and periodic interrupt generators share the output of

the multiplexer. The RS0–RS3 bits in Register A establish

the output frequency of the multiplexer. These frequen-

cies are listed in Table 4. Once the frequency is selected,

the output of the SQW pin can be turned on and off under

program control with the square-wave enable bit (SQWE).

If E32K = 0, the square-wave output is determined by the

RS3 to RS0 bits. If E32K = 1, a 32kHz square wave is

output on the SQW pin, regardless of the RS3 to RS0 bits’

state. If E32K = ABE = 1 and a valid voltage is applied to

VBAUX, a 32kHz square wave is output on SQW when

VCC is below VTP.

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Periodic Interrupt Selection

The periodic interrupt causes the IRQ pin to go to an

active state from once every 500ms to once every 122μs.

This function is separate from the alarm interrupt, which

can be output from once per second to once per day. The

periodic interrupt rate is selected using the same Register

A bits that select the squarewave frequency (see Table 4).

Changing the Register A bits affects both the square-wave

frequency and the periodic interrupt output. However,

each function has a separate enable bit in Register B.

The SQWE and E32k bits control the square-wave output.

Similarly, the periodic interrupt is enabled by the PIE bit in

ware counters to measure inputs, create output intervals,

or await the next needed software function.

Update Cycle

The DS17x85 executes an update cycle once per second

regardless of the SET bit in Register B. When the SET

bit in Register B is set to 1, the user copy of the double-

buffered time, calendar, and alarm bytes is frozen and

does not update as the time increments. However, the

time countdown chain continues to update the internal

copy of the buffer. This feature allows time to maintain

accuracy independent of reading or writing the time, cal-

endar, and alarm buffers, and also guarantees that time

and calendar information is consistent. The update cycle

also compares each alarm byte with the corresponding

time byte and issues an alarm if a match or if a don’t care

code is present in all alarm locations.

There are three methods that can handle access of the

RTC that avoid any possibility of accessing inconsistent

time and calendar data. The first method uses the update-

ended interrupt. If enabled, an interrupt occurs after every

update cycle that indicates that over 999ms are available

to read valid time and date information. If this interrupt is

used, the IRQF bit in Register C should be cleared before

leaving the interrupt routine.

A second method uses the update-in-progress (UIP) bit in

The UIP bit pulses once per second. After the UIP bit goes

high, the update transfer occurs 244μs later. If a low is

read on the UIP bit, the user has at least 244μs before the

time/calendar data is changed. Therefore, the user should

avoid interrupt service routines that would cause the time

needed to read valid time/calendar data to exceed 244μs.

The third method uses a periodic interrupt to determine if

an update cycle is in progress. The UIP bit in Register A

is set high between the setting of the PF bit in Register C

(see Figure 4). Periodic interrupts that occur at a rate of

greater than tBUC allow valid time and date information to

be reached at each occurrence of the periodic interrupt.

The reads should be complete within 1 (tPI/2 + tBUC) to

ensure that data is not read during the update cycle.

Figure 4. UIP and Periodic Interrupt Timing

UIP

tBUC = DELAY TIME BEFORE UPDATE CYCLE = 244µs.

1 SECOND

tPI

tPI/2 tPI/2

tBUC

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Extended Functions

The extended functions provided by the DS17x85/

DS17x87 that are new to the RAMified RTC family

are accessed by a software-controlled bank-switching

scheme, as illustrated in Table 5. In bank 0, the clock/

calendar registers and 50 bytes of user RAM are in the

same locations as for the DS1287. As a result, existing

routines implemented within BIOS, DOS, or application

software packages can gain access to the DS17x85/

DS17x87 clock registers with no changes. Also in bank

0, an extra 64 bytes of RAM are provided at addresses

just above the original locations for a total of 114 directly

addressable bytes of user RAM.

When bank 1 is selected, the clock/calendar registers and

the original 50 bytes of user RAM still appear as bank 0.

However, the extended registers that provide control and

status for the extended functions are accessed in place of

the additional 64 bytes of user RAM. The major extended

functions controlled by the extended registers are listed

below:

• 64-Bit Silicon Serial Number

• Century Counter

• RTC Write Counter

• Date Alarm

• Auxiliary Battery Control/Status

• Wake-Up

• Kickstart

• RAM Clear Control/Status

• Extended RAM Access

The bank selection is controlled by the state of the DV0 bit in

a 0. To access bank 1, DV0 should be written to 1. Register

locations designated as reserved in the bank 1 map are

reserved for future use by Dallas Semiconductor. Bits in

these locations cannot be written and return a 0 if read.

Silicon Serial Number

A unique 64-bit lasered serial number is located in bank 1,

registers 40h–47h. This serial number is divided into three

parts. The first byte in register 40h contains a model num-

ber to identify the device type of the DS17x85/DS17x87.

Registers 41h–46h contain a unique binary number.

data in registers 40h–46h. The CRC polynomial is X8 +

X5 + X4 + 1. See Figure 5. All 8 bytes of the serial number

are read-only registers. The DS17x85/DS17x87 is manu-

factured such that no two devices contain an identical

number in locations 41h–47h.

Figure 5. CRC Polynomial

DEVICE MODEL NUMBER

DS17285/87 72h

DS17485/87 74h

DS17885/87 78h

1ST

STAGE

2ND

STAGE

3RD

STAGE

4TH

STAGE

5TH

STAGE

6TH

STAGE

7TH

STAGE

8TH

STAGE

INPUT DATA

POLYNOMIAL = X8 + X5 + X4 + 1

X0 X1 X2 X3 X4 X5 X6 X7 X8

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Note: Reserved bits can be written to any value, but always read back as zeros.

Table 5. Extended Bank Register Bank Definition

Bank 0 Bank 1

DV0 = 0 DV0 = 1

00h

0Dh

Timekeeping and Control

00h

0Dh

Timekeeping and Control

0Eh

3Fh

50 Bytes – User RAM

0Eh

3Fh

50 Bytes – User RAM

40h 40h Model Number Byte

41h 1st Byte Serial Number

42h 2nd Byte Serial Number

43h 3rd Byte Serial Number

44h 4th Byte Serial Number

45h 5th Byte Serial Number

46h 6th Byte Serial Number

47h CRC Byte

48h Century Byte

49h Date Alarm

4Ah Extended Control Register 4A

4Bh Extended Control Register 4B

4Ch Reserved

4Dh Reserved

4Eh RTC Address – 2

4Fh RTC Address – 3

64 Bytes – User RAM 50h Extended RAM Address LSB

51h Extended RAM Address MSB

52h Reserved

53h Extended RAM Data Port

54h Reserved

55h Reserved

56h Reserved

57h Reserved

58h Reserved

59h Reserved

5Ah Reserved

5Bh Reserved

5Ch Reserved

5Dh Reserved

5Eh RTC Write Counter

7Fh

5Fh

7Fh

Reserved

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Century Counter

A register has been added in bank 1, location 48H, to

keep track of centuries. The value is read in either binary

or BCD according to the setting of the DM bit.

RTC Write Counter

An 8-bit counter located in extended register bank 1, 5Eh,

counts the number of times the RTC is written to. This

counter is incremented on the rising edge of the WR sig-

nal every time that the CS signal qualifies it. This counter

is a read-only register and rolls over after 256 RTC write

pulses. This counter can be used to determine if and how

many RTC writes have occurred since the last time this

Auxiliary Battery

The VBAUX input is provided to supply power from an

auxiliary battery for the DS17x85/DS17x87 kickstart,

wake-up, and SQW output in the absence of VCC func-

tions. This power source must be available to use these

auxiliary functions when no VCC is applied to the device.

The auxiliary battery enable (ABE; bank 1, register 04BH)

bit in Extended Control Register 4B is used to turn the

auxiliary battery on and off for the above functions in the

absence of VCC. When set to 1, VBAUX battery power

is enabled; when cleared to 0, VBAUX battery power is

disabled to these functions.

In the DS17x85/DS17x87, this auxiliary battery can be

used as the primary backup power source for maintain-

ing the clock/calendar, user RAM, and extended external

RAM functions. This occurs if the VBAT pin is at a lower

voltage than VBAUX. If the DS17x85 is to be backed

up using a single battery with any auxiliary functions

enabled, then VBAUX should be used and VBAT should

be grounded. If VBAUX is not to be used, it should be

grounded and ABE should be cleared to 0.

Wake-Up/Kickstart

The DS17x85/DS17x87 incorporates a wake-up feature

that powers on the system at a predetermined date and

time through activation of the PWR output pin. In addition,

the kickstart feature allows the system to be powered up

in response to a low-going transition on the KS pin, with-

out operating voltage applied to the VCC pin.

As a result, system power can be applied upon such

events as a key closure or modem ring-detect signal.

To use either the wake-up or the kickstart functions, the

DS17x85/DS17x87 must have an auxiliary battery con-

nected to the VBAUX pin, the oscillator must be running, and

the countdown chain must not be in reset (Register A DV2,

DV1, DV0 = 01X). If DV2 and DV1 are not in this required

state, the PWR pin is not driven low in response to a kick-

start or wake-up condition while in battery-backed mode.

The wake-up feature is controlled through the wake-up

interrupt-enable bit in Extended Control Register 4B (WIE,

bank 1, 04BH). Setting WIE to 1 enables the wake-up

feature, clearing WIE to 0 disables it. Similarly, the kick-

start interrupt-enable bit in Extended Control Register 4B

(KSE, bank 1, 04BH) controls the kickstart feature.

A wake-up sequence occurs as follows: When wake-up

is enabled through WIE = 1 while the system is powered

down (no VCC voltage), the clock/calendar monitors the

current date for a match condition with the date alarm reg-

ister (bank 1, register 049H). With the date alarm register,

the hours, minutes, and seconds alarm bytes in the clock/

calendar register map (bank 0, registers 05H, 03H, and

01H) are also monitored. As a result, a wake-up occurs at

the date and time specified by the date, hours, minutes,

and seconds alarm register values. This additional alarm

occurs regardless of the programming of the AIE bit (bank

0, register B, 0BH). When the match condition occurs,

the PWR pin is automatically driven low. This output can

be used to turn on the main system power supply that

provides VCC voltage to the DS17x85/DS17x87 as well

as the other major components in the system. Also at this

time, the wake-up flag (WF, bank 1, register 04AH) is set,

indicating that a wake-up condition has occurred.

A kickstart sequence occurs when kickstarting is enabled

through KSE = 1. While the system is powered down,

the KS input pin is monitored for a low-going transition of

minimum pulse width tKSPW. When such a transition is

detected, the PWR line is pulled low, as it is for a wake-up

condition. Also at this time, the kickstart flag (KF, bank 1,

has occurred.

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(TA = +25°C)

The timing associated with both the wake-up and kick-

starting sequences is illustrated in the Wake-Up/Kickstart

Timing Diagram (Figure 6). The timing associated with

these functions is divided into five intervals, labeled 1 to

5 on the diagram.

The occurrence of either a kickstart or wake-up condition

causes the PWR pin to be driven low, as described above.

During interval 1, if the supply voltage on the DS17x85/

DS17x87 VCC pin rises above the greater of VBAT or VPF

before the power-on timeout period (tPOTO) expires, then

PWR remains at the active-low level. If VCC does not

rise above the greater of VBAT or VPF in this time, then

the PWR output pin is turned off and returns to its high-

impedance level. In this event, the IRQ pin also remains

tri-stated. The interrupt flag bit (either WF or KF) associ-

ated with the attempted power-on sequence remains set

until cleared by software during a subsequent system

power-on.

If VCC is applied within the timeout period, then the sys-

tem power-on sequence continue as shown in intervals 2

to 5 in the timing diagram. During interval 2, PWR remains

active and IRQ is driven to its active-low level, indicating

that either WF or KF was set in initiating the power-on.

In the diagram KS is assumed to be pulled up to the

VBAUX supply. Also at this time, the PAB bit is automati-

cally cleared to 0 in response to a successful power-on.

The PWR line remains active as long as the PAB remains

cleared to 0.

Figure 6. Wake-Up/Kickstart Timing Diagram

Note: Wake-up/kickstart timeout is generated only when the oscillator is enabled and the countdown chain is not reset.

Table 6. Wake-Up/Kickstart Timing

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Kickstart-Input Pulse Width tKSPW 2 µs

Wake-Up/Kickstart Power-On

Timeout tPOTO 2 s

VBAT

VIH tKSPW

tPOTP

VIH

1234 5

VIL

HIGH-IMPEDANCE

VIL

VPF

*CONDITION

VPF < VBAT

*THIS CONDITION CAN OCCUR WITH THE 3V DEVICE.

NOTE: THE TIME INTERVALS SHOWN ABOVE ARE REFERENCED IN THE WAKE-UP/KICKSTART SECTION.

*CONDITION

VBAT > VPF

WF/KF

(INTERNAL)

PWR

IRQ

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At the beginning of interval 3, the system processor has

begun code execution and clears the interrupt condition

of WF and/or KF by writing zeros to both of these control

bits. As long as no other interrupt within the DS17x85/

DS17x87 is pending, the IRQ line is taken inactive once

these bits are reset. Execution of the application software

can proceed. During this time, the wake-up and kickstart

functions can be used to generate status and interrupts.

WF is set in response to a date, hours, minutes, and

seconds match condition. KF is set in response to a low-

going transition on KS. If the associated interrupt-enable

bit is set (WIE and/or KSE), the IRQ line is driven active

low in response to enabled event. In addition, the other

possible interrupt sources within the DS17885/DS17887

can cause IRQ to be driven low. While system power is

applied, the on-chip logic always attempts to drive the

PWR pin active in response to the enabled kickstart or

wake-up condition. This is true even if PWR was previ-

ously inactive as the result of power being applied by

some means other than wake-up or kickstart.

The system can be powered down under software control

by setting the PAB bit to logic 1. This causes the open-

drain PWR pin to be placed in a high-impedance state, as

shown at the beginning of interval 4 in the timing diagram.

As VCC voltage decays, the IRQ output pin is placed in

a high-impedance state when VCC goes below VPF. If

the system is to be again powered on in response to a

wake-up or kickstart, then the WF and KF flags should be

cleared, and WIE and/or KSE should be enabled prior to

setting the PAB bit.

During interval 5, the system is fully powered down.

Battery backup of the clock calendar and NV RAM is in

effect and IRQ is tri-stated, and monitoring of wake-up

and kickstart takes place. If PRS = 1, PWR stays active;

otherwise, if PRS = 0, PWR is high impedance.

RAM Clear

The DS17x85/DS17x87 provide a RAM clear function for

the 114 bytes of user RAM. When enabled, this function can

be performed regardless of the condition of the V

pin.

The RAM clear function is enabled or disabled through

the RAM clear-enable bit (RCE; bank 1, register 04BH).

When this bit is set to logic 1, the 114 bytes of user RAM

is cleared (all bits set to 1) when an active-low transition

is sensed on the RCLR pin. This action has no effect on

either the clock/calendar settings or the contents of the

extended RAM. The RAM clear flag (RF, bank 1, register

04AH) is set when the RAM clear operation has been

completed. If VCC is present at the time of the RAM clear

and RIE = 1, the IRQ line is also driven low upon comple-

tion. Writing a zero to the RF bit clears the interrupt con-

dition. The IRQ line then returns to its inactive high level,

provided there are no other pending interrupts. Once the

RCLR pin is activated, all read/write accesses are locked

out for a minimum recover time, specified as tREC in

Electrical Characteristics.

When RCE is cleared to 0, the RAM clear function is

disabled. The state of the RCLR pin has no effect on the

contents of the user RAM, and transitions on the RCLR

pin have no effect on RF.

Extended RAM

The DS17x85/DS17x87 provide 2k, 4k, or 8k x 8 of onchip

SRAM that is controlled as nonvolatile storage sustained

from a lithium battery. On power-up, the RAM is taken

out of write-protect status by the internal power-OK sig-

nal (POK) generated from the write-protect circuitry. The

on-chip SRAM is accessed through the eight multiplexed

address/data lines AD7 to AD0. Three on-chip latch regis-

ters control access to the SRAM. Two registers are used

to hold the SRAM address, and the other register is used

to hold read/write data.

Access to the extended RAM is controlled by three of

the registers shown in Table 5. The extended registers in

bank 1 must first be selected by setting the DV0 bit in reg-

ister A to logic 1. The address of the RAM location to be

accessed must be loaded into the extended RAM address

registers located at 50h and 51h. The least significant

address byte should be written to location 50h, and the

most significant bits (right-justified) should be loaded in

location 51h. Data in the addressed location can be read

by performing a read operation from location 53h, or writ-

ten to by performing a write operation to location 53h.

Data in any addressed location can be read or written

repeatedly without changing the address in location 50h

and 51h.

To read or write consecutive extended RAM locations,

a burst mode feature can be enabled to increment the

extended RAM address. To enable the burst mode fea-

ture, set the BME bit in the Extended Control Register

4Ah to logic 1. With burst mode enabled, write the

extended RAM starting address location to registers 50h

and 51h. Then read or write the extended RAM data from/

to register 53h. The extended RAM address locations are

automatically incremented on the rising edge of RD or

WR only when register 53h is being accessed. See the

Burst Mode Timing Waveform.

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Extended Control Registers

Two extended control registers are provided to supply

control and status information for the extended functions

offered by the DS17x85/DS17x87. These are designated

as Extended Control Registers 4A and 4B, and are locat-

ed in register bank 1, locations 04AH and 04BH, respec-

tively. The functions of the bits within these registers are

described as follows.

Bit 7: Valid RAM and Time 2 (VRT2). This status bit

gives the condition of the auxiliary battery. It is set to logic

1 condition when the external lithium battery is connected

to the VBAUX. If this bit is read as logic 0, the external

battery should be replaced.

Bit 6: Increment in Progress Status (INCR). This bit is

set to 1 when an increment to the time/date registers is

in progress and the alarm checks are being made. INCR

is set to 1 at 122μs before the update cycle starts and is

cleared to 0 at the end of each update cycle.

Bit 5: Burst Mode Enable (BME). The burst mode

enable bit allows the extended user RAM address regis-

ters to automatically increment for consecutive reads and

writes. When BME is set to logic 1, the automatic incre-

menting is enabled and when BME is set to a logic 0, the

automatic incrementing is disabled.

Bit 3: Power Active-Bar Control (PAB). When this bit is

0, the PWR pin is in the active low state. When this bit is

1, the PWR pin is in the high-impedance state. The user

can write this bit to logic 1 or 0. If either WF and WIE = 1

or KF and KSE = 1, the PAB bit is cleared to 0.

Bit 2: RAM Clear Flag (RF). This bit is set to logic 1 when

a high-to-low transition occurs on the RCLR input if RCE

= 1. Writing this bit to logic 0 clears it. This bit can also be

written to logic 1 to force an interrupt condition.

Bit 1: Wake-Up Alarm Flag (WF). This bit is set to 1

when a wake-up alarm condition occurs or when the user

writes it to 1. WF is cleared by writing it to 0.

Bit 0: Kickstart Flag (KF). This bit is set to 1 when a

kickstart condition occurs or when the user writes it to 1.

This bit is cleared by writing it to logic 0.

Figure 7. Burst Mode Timing Waveform

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

VRT2 INCR BME * PAB RF WF KF

ADDRESS + 1

53H DATA

PWRWL PWRWH

DATA

AD0-7

DS OR R/W

ADDRESS + 2

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Extended Control Register (4Ah)

Bit 7: Auxiliary Battery Enable (ABE). When written

to logic 1, this bit enables the VBAUX pin for extended

functions.

Bit 6: Enable 32.768kHz Output (E32k). When writ-

ten to logic 1, this bit enables the 32.768kHz oscillator

frequency to be output on the SQW pin. E32k is set to 1

when VCC is powered up.

Bit 5: Crystal Select (CS). When CS is set to 0, the oscil-

lator is configured for operation with a crystal that has a

6pF specified load capacitance. When CS = 1, the oscilla-

tor is configured for a 12.5pF crystal. CS is disabled in the

DS17x87 module and should be set to CS = 0.

Bit 4: RAM Clear Enable (RCE). When set to 1, this bit

enables a low level on RCLR to clear all 114 bytes of user

RAM. When RCE = 0, RCLR and the RAM clear function

are disabled.

Bit 3: PAB Reset Select (PRS). When set to 0, the PWR

pin is set high impedance when the DS17x85 goes into

power fail. When set to 1, the PWR pin remains active

upon entering power fail.

Bit 2: RAM Clear Interrupt Enable (RIE). When RIE is

set to 1, the IRQ pin is driven low when a RAM clear func-

tion is completed.

Bit 1: Wake-Up Alarm Interrupt Enable (WIE). When

VCC voltage is absent and WIE is set to 1, the PWR pin is

driven active low when a wake-up condition occurs, caus-

ing the WF bit to be set to 1. When VCC is then applied,

the IRQ pin is also driven low. If WIE is set while system

power is applied, both IRQ and PWR are driven low in

response to WF being set to 1. When WIE is cleared to 0,

the WF bit has no effect on the PWR or IRQ pins.

Bit 0: Kickstart Interrupt Enable (KSE). When VCC volt-

age is absent and KSE is set to 1, the PWR pin is driven

active low when a kickstart condition occurs (KS pulsed

low), causing the KF bit to be set to 1. When VCC is then

applied, the IRQ pin is also driven low. If KSE is set to 1

while system power is applied, both IRQ and PWR are

driven low in response to KF being set to 1. When KSE

is cleared to 0, the KF bit has no effect on the PWR or

IRQ pins.

MSB LSB

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

ABE E32k CS RCE PRS RIE WIE KSE

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Extended Control Register (4Bh)

System Maintenance Interrupt

(SMI) Recovery Stack

An SMI recovery register stack is located in the extended

shown below, can be used by the BIOS to recover from an

SMI occurring during an RTC read or write.

The RTC address is latched on the falling edge of the ALE

signal. Each time an RTC address is latched, the register

address stack is pushed. The stack is only four registers

deep, holding the three previous RTC addresses in addi-

tion to the current RTC address being accessed. Figure 8

illustrates how the BIOS could recover the RTC address

when an SMI occurs.

1) The RTC address is latched.

2) An SMI is generated before an RTC read or write

occurs.

3) RTC address 0Ah is latched and the address from

1 is pushed to the “RTC Address–1” stack location.

This step is necessary to change the bank select bit,

DV0 = 1.

4) RTC address 4Eh is latched and the address from 1

is pushed to location 4Eh, “RTC Address–2” while 0Ah

is pushed to the “RTC Address–1” location. The data

in this register, 4Eh, is the RTC address lost due to

the SMI.

Figure 8. ALE Waveform

RTC ADDRESS

RTC ADDRESS-1

4Eh RTC ADDRESS-2

4Fh RTC ADDRESS-3

SMI Recovery Stack

7 6 5 4 3 2 1 0

DV0 AD6 AD5 AD4 AD3 AD2 AD1 AD0

1234

ALE

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VCC

SQW

VBAUX

RCLRAD0

PWR

TOP VIEW

VBAT

IRQ

RDAD4

AD3

AD2

AD1

GND

ALE

CSGND

AD7

AD6

AD5

SO, PDIP

DS17285

DS17485

DS17885

VCC

SQW

VBAUX

RCLR

AD0

N.C.

PWR

N.C.

IRQ

RDAD4

AD3

AD2

AD1

N.C.

ALE

CSGND

AD7

AD6

AD5

EDIP

DS17287

DS17487

DS17887

VBAUX

GND

ALE

GND

21 GND

20 AD7

19 AD6

18 N.C.

17 AD5

16 AD4

15 AD3

RCLR

VBAT

IRQ

VCC

PWR

N.C.

AD0

AD1

AD2

SQW

TSOP

DS17285

DS17485

DS17885

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Pin Congurations

+Denotes a lead(Pb)-free/RoHS-compliant package.

*A “+” anywhere on the top mark denotes a lead(Pb)-free package. An “N” or “IND” denotes an industrial temperature range package.

Note: A “-5” suffix denotes a VCC = 5V±10% device, and a “-3” suffix denotes a VCC = 3V±10% device.

PART TEMP RANGE PIN-PACKAGE TOP MARK*

DS17285-3+ 0°C to +70°C 24 PDIP DS17285-3

DS17285-5+ 0°C to +70°C 24 PDIP DS17285-5

DS17285E-3+ 0°C to +70°C 28 TSOP DS17285E3

DS17285E-5+ 0°C to +70°C 28 TSOP DS17285E5

DS17285EN-3+ -40°C to +85°C 28 TSOP DS17285E3

DS17285S-3+ 0°C to +70°C 24 SO (300 mils) DS17285S-3

DS17285S-5+ 0°C to +70°C 24 SO (300 mils) DS17285S-5

DS17285SN-3+ -40°C to +85°C 24 SO (300 mils) DS17285SN3

DS17285SN-5+ -40°C to +85°C 24 SO (300 mils) DS17285SN5

DS17287-3+ 0°C to +70°C 24 EDIP DS17287-3

DS17287-5+ 0°C to +70°C 24 EDIP DS17287-5

DS17485-3+ 0°C to +70°C 24 PDIP DS17485-3

DS17485-5+ 0°C to +70°C 24 PDIP DS17485-5

DS17485E-3+ 0°C to +70°C 28 TSOP DS17485E3

DS17485E-5+ 0°C to +70°C 28 TSOP DS17485E5

DS17485S-3+ 0°C to +70°C 24 SO (300 mils) DS17485S-3

DS17485S-5+ 0°C to +70NC 24 SO (300 mils) DS17485S-5

DS17485SN-5+ -40°C to +85°C 24 SO (300 mils) DS17485SN5

DS17487-3+ 0°C to +70°C 24 EDIP DS17487-3

DS17487-3IND+ -40°C to +85°C 24 EDIP DS17487-3 REAL TIME IND

DS17487-5+ 0°C to +70°C 24 EDIP DS17487-5

DS17487-5IND+ -40°C to +85°C 24 EDIP DS17487-5 REAL TIME IND

DS17885-3+ 0°C to +70°C 24 PDIP DS17885-3

DS17885-5+ 0°C to +70°C 24 PDIP DS17885-5

DS17885E-3+ 0°C to +70°C 28 TSOP DS17885E3

DS17885E-5+ 0°C to +70°C 28 TSOP DS17885E5

DS17885EN-3+ -40°C to +85°C 28 TSOP DS17885E3

DS17885S-3+ 0°C to +70°C 24 SO (300 mils) DS17885S-3

DS17885S-5+ 0°C to +70°C 24 SO (300 mils) DS17885S-5

DS17885SN-3+ -40°C to +85°C 24 SO (300 mils) DS17885SN3

DS17887-3+ 0°C to +70°C 24 EDIP DS17887-3

DS17887-3IND+ -40°C to +85°C 24 EDIP DS17887-3 REAL TIME IND

DS17887-5+ 0°C to +70°C 24 EDIP DS17887-5

DS17887-5IND+ -40°C to +85°C 24 EDIP DS17887-5 REAL TIME IND

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Ordering Information

PACKAGE THETA-JA (°C/W) THETA-JC (°C/W)

DIP 75 30

SO 105 22

PACKAGE TYPE DOCUMENT NO.

24 PDIP (600 mils) 21-0044

24 SO (300 mils) 21-0042

24 EDIP (740 mils) 21-0241

28 TSOP (465 mils) 21-0273

IRQ

X1 X2 VCC

VCC

ALE

DS83C520

DS17285

DS17485

DS17885

AD0–AD7

GND

PWR

VSB

VCC

SUPPLY

CONTROL

CIRCUIT

SQW

CRYSTAL

VCC

RCLR

VBAUX

VBAT

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Package Information

For the latest package outline information and land patterns

(footprints), go to www.maximintegrated.com/packages. Note

that a “+”, “#”, or “-” in the package code indicates RoHS status

only. Package drawings may show a different suffix character, but

the drawing pertains to the package regardless of RoHS status.

Chip Information

SUBSTRATE CONNECTED TO GROUND

PROCESS: CMOS

Typical Operating Circuit Thermal Information

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 4/06 Initial release of revised data sheet template —

1 4/10

Updated the storage temperature ranges, added the lead temperature, and updated

the soldering temperature for all packages in the Absolute Maximum Ratings; removed

the leaded parts from the Ordering Information table; updated the Document No. for the

Package Information table.

2, 29, 30

2 5/16 Updated Package Information table 30

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses

are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)

shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.

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DS17485/DS17487/

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Real-Time Clocks

│

Revision History

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.