BQ3285P-SB2

Overview

The Texas Instruments BQ3285P-SB2 is a CMOS real-time clock (RTC) module designed for IBM AT-compatible systems. It provides time-of-day, a 100-year calendar with automatic leap-year adjustment, and an alarm function. The device includes 114 bytes of general nonvolatile storage and three maskable interrupt sources: periodic, alarm, and update-ended. It operates on a +5V supply with a backup battery input for data retention during power failures. Key features include selectable Intel or Motorola bus timing, programmable square-wave output, and support for both BCD and binary data formats.

Feature Summary

• Integrated 100-year calendar and time-of-day clock with automatic leap-year correction
• 114 bytes of general-purpose nonvolatile RAM for user storage
• Three individually maskable interrupts: periodic, alarm, and update-ended
• Selectable bus timing for Intel or Motorola architectures via the MOT pin

Applications

• Direct replacement for DS1285 in IBM AT-compatible computers
• Real-time clock modules requiring low-power battery backup
• Systems needing nonvolatile storage alongside timekeeping functions

Procurement Notes

Verify package configuration as the 'P' suffix indicates a 24-pin plastic DIP. Confirm that the SB2 designation aligns with specific manufacturing lot or revision requirements if critical for legacy system compatibility. Ensure external components match datasheet specifications, specifically a 32.768kHz crystal with 6pF load capacitance and a 3V backup cell connected to the BC pin.

Selection Notes

Select this component when a direct pin-compatible replacement for the DS1285 is required. Ensure the design supports a +5V main supply and a 3V backup source. Verify that the host processor can interface with the multiplexed address/data bus and respects the specified cycle time of 160ns. Consider the operating temperature range of 0°C to +70°C for commercial applications.

Part Context

The BQ3285 is part of Texas Instruments' legacy RTC family, functionally compatible with the National Semiconductor DS1285. It serves as a microprocessor peripheral providing precise timekeeping and nonvolatile memory. The device simplifies system design by integrating clock, calendar, and storage functions into a single 24-pin package, reducing component count in legacy PC architectures.

Replacement Considerations

The BQ3285 is functionally compatible with the DS1285 and closely matches the MC146818A pin configuration. When replacing older RTCs, verify that the existing circuitry supports the specific bus timing signals (AS/DS vs ALE/RD-WR) selected by the MOT pin.