BQ4285S-SB2G4

Overview

The Texas Instruments bq4285S-SB2G4 is a CMOS real-time clock (RTC) with nonvolatile RAM control, designed for IBM AT-compatible systems. It provides a 100-year calendar and time-of-day clock with alarm features. The device includes 114 bytes of general nonvolatile storage and integrates battery-backup control for external SRAM. Key electrical parameters include a 160 ns cycle time, less than 0.5 μA load under battery operation, and a +5V supply voltage. It supports BCD or binary data formats and offers three maskable interrupt sources.

Feature Summary

• Direct replacement for DS1285 with MC146818A pin compatibility
• 114 bytes of general nonvolatile storage plus 14 bytes for clock/calendar
• Automatic backup and write-protect control to external SRAM during power failure
• Three individually maskable interrupt events: periodic, alarm, and update-ended

Applications

• IBM AT-compatible computers
• Microprocessor peripheral timekeeping
• Systems requiring battery-backed NVRAM control

Procurement Notes

Verify the G4 suffix indicates RoHS compliance against current TI documentation. Confirm package availability as PLCC options are obsolete; only 24-pin DIP or SOIC remain in active production status per datasheet revision history.

Selection Notes

Select this component when an Intel-bus compatible RTC with integrated external SRAM management is required. Ensure the design accommodates a 32.768 kHz crystal and a 3V backup cell on the BC pin. Verify that the host processor can interface with the multiplexed address/data bus using AS, DS, and R/W signals.

Part Context

The bq4285 series serves as a legacy real-time clock solution for industrial and computing applications requiring precise timekeeping and nonvolatile memory expansion. The S variant denotes the 24-pin SOIC package, while the G4 suffix typically signifies environmental compliance. This part bridges standard RTC functionality with NVRAM controller duties.

Replacement Considerations

Functionally compatible with DS1285 and closely matches MC146818A pin configuration. Direct replacements should maintain bus timing requirements and external SRAM interface compatibility.

FAQ

How does the bq4285 handle external SRAM during power failure?

During power failure, the bq4285 automatically write-protects the external SRAM by driving CEOUT inactive and switches VOUT to the backup battery source to maintain data integrity.

What is the function of the BC pin?

The BC pin accepts a 3V primary backup cell (typically lithium) to sustain the oscillator and external SRAM during main power loss.

Can the bq4285 operate in both 12-hour and 24-hour formats?

Yes, the hour format bit (HF) in Register B allows selection between 12-hour and 24-hour formats for time-of-day and alarm settings.