DESCRIPTION
The DS1688/DS1691 is a real time clock (RTC) designed 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 protection to an external SRAM. The DS1688 also incorporates 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.

FEATURES
Incorporates industry standard DS1287 PC clock plus enhanced features:
• +3 or +5 volt operation
• 64–bit Silicon serial number
• 64–bit customer specific ROM or additional serial number available
• Power control circuitry supports system power on from date/time alarm or key closure
• Automatic battery backup and write protection to external SRAM
• Crystal select bit allows RTC to operate with 6 pF or 12.5 pF crystal
• 114 bytes user NVRAM
• Auxiliary battery input
• RAM clear input
• Century register
• 32 KHz output for power management
• 32–bit VCC powered elapsed time counter
• 32–bit VBAT powered elapsed time counter
• 16–bit power cycle counter
• Compatible with existing BIOS for original DS1287 functions
• Available as chip (DS1688) or stand–alone module with embedded battery and crystal (DS1691)
• Timekeeping algorithm includes leap year compensation valid up to 2100