DS1688_05 Datasheet, PDF (2/6 Pages) Dallas Semiconductor – 3V/5V Serialized Real-Time Clocks with NV RAM Control

DS1688/DS1691
DETAILED DESCRIPTION
Each DS1688/DS1691 is individually manufactured with a unique 64-bit serial number as well as an
additional 64-bit customer-specific ROM or serial number. The serial number is programmed and tested
at Dallas to ensure that no two devices are alike. The serial number can be used to electronically identify
a system for purposes such as establishment of a network node address or for maintenance tracking.
Customers can reserve blocks of available numbers from Dallas Semiconductor.
The serialized RTCs also incorporate power-control circuitry, 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 triggered by one or either of these events, and can be 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.
The DS1688/DS1691 incorporate 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 warranty 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 RTC is also used to retain RAM data in
the absence of V_CC power through the V_CCO pin. The chip enable output 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 32.768kHz crystal, and a lithium battery in a complete, self-
contained 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
descriptions other than the SQW output, refer to the DS1689/DS1693 data sheet, available online at
www.maxim-ic.com/DS1689.
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