DS1688 Datasheet, PDF (2/5 Pages) Dallas Semiconductor – 3 Volt/5 Volt Serialized Real Time Clock with NV RAM Control

DS1688/DS1691
IRQ
SQW
V_CCI
GND
V_BAT
V_BAUX
PSEL
CEI
CEO
– Interrupt Request Output
– Square Wave Output
– +3 or +5 Volt Main Supply
– Ground
– Battery + Supply
– Auxiliary Battery Supply
– +3 or +5 Volt Power Select
– RAM Chip Enable In
– 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
SRAM is provided through the V_CCO and CEO pins. The
lithium energy source used to permanently power the real
time clock is also used to retain RAM data in the absence
of V_CC power through the V_CCO pin. The chip enable out-
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
absence of V_CC is guaranteed.
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
wave will be output on this pin, t_REC, after a power–up
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.
030598 2/5