BQ4285S-SB2

Overview

The Texas Instruments bq4285 is a CMOS real-time clock (RTC) and nonvolatile RAM (NVRAM) controller designed for IBM AT-compatible systems. It provides a 100-year calendar with automatic leap-year adjustment, time-of-day tracking in 12- or 24-hour formats, and three maskable interrupt sources. The device features 114 bytes of general nonvolatile storage and integrates battery-backup control for external SRAM. Operating on a single +5V supply with less than 0.5 μA load under battery operation, it supports BCD or binary data formats and includes daylight saving time adjustments. Key electrical parameters include a 160 ns cycle time, 32.768 kHz crystal input, and an operating temperature range of 0°C to +70°C.

Feature Summary

• Direct replacement for DS1285/MC146818A with compatible pin configuration
• Integrated battery backup controller for external SRAM write-protection and power switching
• Three individually maskable interrupts: periodic, alarm, and update-ended
• Programmable square-wave output with 13 selectable frequencies from 2 Hz to 256 Hz

Applications

• IBM AT-compatible computer systems
• Real-time clock modules requiring NVRAM control
• Battery-backed memory systems with automatic failover protection

Procurement Notes

Verify package availability as the PLCC option is obsolete. Confirm the specific suffix matches the required package type: P for 24-pin plastic DIP or S for 24-pin SOIC. Ensure the order includes the correct temperature grade, which is commercial (0°C to +70°C) for standard variants. Check for RoHS compliance status if environmental regulations apply to the target assembly process.

Selection Notes

Select the bq4285 when a direct functional equivalent to legacy DS1285 or MC146818A devices is required for system compatibility. Choose this component for applications needing integrated external SRAM management during power failures. Ensure the design accommodates a 32.768 kHz crystal with 6 pF load capacitance. Verify that the host processor interface supports the multiplexed address/data bus timing requirements specified in the datasheet.

Part Context

The bq4285 belongs to Texas Instruments' legacy real-time clock family, originally designed to support early PC architectures. It serves as a bridge between microprocessor buses and time-critical functions while managing auxiliary nonvolatile memory. The device simplifies system design by combining RTC logic with battery-switching circuitry, reducing the component count compared to discrete implementations.

Replacement Considerations

The bq4285 is functionally compatible with the National Semiconductor DS1285 and Motorola MC146818A. When replacing these parts, verify that the host system's bus timing aligns with the bq4285's 160 ns cycle time. Ensure the backup battery voltage meets the minimum requirement of 2.5 V for reliable operation.