DS1688

Real Time Clock Overview

The DS1688 is a serialized real-time clock (RTC) designed as a successor to the industry-standard DS1285 PC RTC. It operates at +3.0V or +5.0V and provides automatic battery backup and write protection for external SRAM via VCCO and CEO pins. The device includes 114 bytes of user NV RAM, a 64-bit silicon serial number, and power control circuitry that supports system power-on from date/time alarms or key closures. It features a multiplexed bus interface, a 32kHz output, and timekeeping algorithms with leap year compensation valid up to 2100.

Real Time Clock Feature Summary

• Incorporates a unique 64-bit silicon serial number and optional 64-bit customer-specific ROM.
• Provides 114 bytes of non-volatile RAM with automatic backup and write protection.
• Supports system power-on via date/time alarm or external key closure using PWR output.
• Includes 32-bit VCC-powered elapsed time counter, 32-bit VBAT-powered elapsed time counter, and 16-bit power-cycle counter.

Real Time Clock Applications

• PC real-time clock systems requiring legacy compatibility.
• Network node address establishment using unique serial numbers.
• Maintenance tracking and warranty monitoring applications.
• Systems requiring wake-up capabilities based on time/date alarms.

Real Time Clock Procurement Notes

Verify the specific package variant required, such as the 28-pin SOIC (DS1688S) or PDIP. Confirm voltage operation requirements (+3V or +5V) and ensure the design accommodates the multiplexed address/data bus interface. Check for lead-free/RoHS compliance if indicated by the '+' suffix in ordering information.

Real Time Clock Selection Notes

Select the DS1688 when an industry-standard PC RTC function is needed alongside enhanced serialization and power management features. Consider the DS1691 if a self-contained module with an embedded battery and crystal is preferred over discrete components. Ensure the application can support the multiplexed bus timing and external SRAM connection requirements.

Real Time Clock Part Context

The DS1688 serves as a direct replacement for older Dallas Semiconductor PC RTCs like the DS1285, DS1385, DS1485, and DS1585. It maintains pin and functional compatibility with the DS1287 BIOS standards while adding modern features like serialization and advanced power control. The device is part of the DS1688/DS1691 family, where the DS1691 offers a complete module solution.

Real Time Clock Replacement Considerations

Public confirmed substitute part numbers are not provided in the source material. The DS1688 is described as a successor to the DS1285 series; however, specific cross-reference data for alternative manufacturers is unavailable.

Real Time Clock FAQ

How does the DS1688 handle external SRAM protection?

The DS1688 provides automatic battery backup and write protection to external SRAM through the VCCO and CEO pins. The VCCO pin supplies power to the RAM during battery backup, while the CEO pin controls chip enable during power transients to prevent data corruption.

What is the function of the PWR pin?

The PWR pin is an active-low power-on interrupt output. It can be triggered by a date/time alarm or a key closure (via the KS input) to turn on an external power supply, allowing the system to wake up from a powered-down state.

Does the DS1688 account for leap years?

Yes, the timekeeping algorithm includes leap year compensation that is valid up to the year 2100.