DS1688/DS1691

030598 2/5

IRQ

– Interrupt Request Output

SQW

– Square Wave Output

– +3 or +5 Volt Main Supply

GND

– Ground

– Battery + Supply

– Auxiliary Battery Supply

PSEL

– +3 or +5 Volt Power Select

CEI

– RAM Chip Enable In

CEO

– RAM Chip Enable Out

DESCRIPTION

The DS1688/DS1691 is a real time clock (RTC) de-

signed as a successor to the industry standard DS1285,

DS1385, DS1485, and DS1585 PC real time clocks.

This device provides the industry standard DS1285

clock function with the new feature of either +3.0 or +5.0

volt operation and automatic backup and write protec-

tion to an external SRAM. The DS1688 also incorpo-

rates a number of enhanced features including a silicon

serial number, power on/off control circuitry, 114 bytes of

user NVSRAM, power on elapsed timer, and power

cycle counter.

Each DS1688/DS1691 is individually manufactured

with a unique 64–bit serial number as well as an addi-

tional 64–bit customer specific ROM or serial number.

The serial number is programmed and tested at Dallas

to insure that no two devices are alike. The serial num-

ber can be used to electronically identify a system for

purposes such as establishment of a network node ad-

dress or for maintenance tracking. Blocks of available

numbers from Dallas Semiconductor can be reserved

by the customer.

The serialized RTC’s also incorporate power control cir-

cuitry which allows the system to be powered on via an

external stimulus, such as a keyboard or by a time and

date (wake up) alarm. The PWR output pin can be trig-

gered by one or either of these events, and can be used

to turn on an external power supply. The PWR pin is un-

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

the system power can then be shut down.

The DS1688/DS1691 incorporates a power on elapsed

time counter, a power on cycle counter, and a battery

powered continuous counter. These three counters

provide valuable information for maintenance and war-

ranty requirements.

Automatic backup and write protection for an external

lithium energy source used to permanently power the real

time clock is also used to retain RAM data in the absence

put to RAM (CEO) is controlled during power transients to

prevent data corruption.

The DS1688 is a clock/calendar chip with the features

described above. An external crystal and battery are

the only components required to maintain time–of–day

and memory status in the absence of power.

The

DS1691 incorporates the DS1688 chip, a 32.768 KHz

crystal, and a lithium battery in a complete, self–con-

tained timekeeping module.

The entire unit is fully

tested at Dallas Semiconductor such that a minimum of

10 years of timekeeping and data retention in the

For a complete description of operating conditions,

electrical characteristics, bus timing, and pin descrip-

tions

other

than

the

SQW

output,

see

the

DS1689/DS1693 data sheet.

SIGNAL DESCRIPTION

SQW (Square Wave Output) – The SQW output signal

functions identical to the DS1689/DS1693 with an

exception occurring at power–up. A 32 KHz square

condition has been detected. This condition sets the fol-

lowing bits enabling the 32 KHz output; DV1=1,

SQWE=1, and E32K=1. The square wave will be output

on this pin if either SQWE=1 or E32K=1.