BQ4285S-SB2TR

Overview

The BQ4285S-SB2TR is a low-power CMOS real-time clock (RTC) with nonvolatile RAM (NVRAM) control, manufactured by Texas Instruments. It provides time-of-day and calendar functions for IBM AT-compatible systems. The device features 114 bytes of general NVRAM and 14 bytes of clock/calendar registers. It operates on a +5V supply with less than 0.5 μA battery load. Key electrical parameters include a 160 ns cycle time, 32.768 kHz crystal input, and backup voltage range of 2.5 V to 4.0 V. It supports Intel bus protocols and includes programmable interrupts.

Feature Summary

• Direct replacement for DS1285/IBM AT RTCs with MC146818A pin compatibility
• 114 bytes of general nonvolatile storage with automatic write-protect during power failure
• Integrated battery-backup controller switching from VCC to BC input below 3V typical
• Three maskable interrupt sources: periodic, alarm, and update-ended

Applications

• IBM AT-compatible computers
• Systems requiring precise timekeeping with nonvolatile data retention
• Industrial controllers needing battery-backed real-time clock functionality

Procurement Notes

Verify the package type matches the 'S' suffix for 24-pin SOIC. Confirm the temperature grade corresponds to commercial operation (0°C to +70°C). Ensure the ordering suffix aligns with current TI availability as PLCC packages are obsolete. Check for specific tape-and-reel requirements if indicated by the TR suffix in the part number.

Selection Notes

Select this component when an Intel-bus compatible RTC with integrated SRAM management is required. It is suitable for designs needing automatic power-fail detection and battery switchover without external logic. Consider the 24-pin SOIC package constraints and the need for an external 32.768 kHz crystal with 6 pF load capacitance.

Part Context

The BQ4285S-SB2TR belongs to the bq4285 family of real-time clocks. The 'S' indicates a 24-pin Small Outline Integrated Circuit (SOIC) package. The 'TR' suffix typically denotes tape-and-reel packaging. This device replaces older discrete RTC solutions by integrating memory control and backup switching into a single CMOS chip.

Replacement Considerations

Functionally compatible with the Dallas Semiconductor DS1285 and Motorola MC146818A. Verify pinout compatibility if substituting directly. Note that the PLCC package option is obsolete; ensure design-in allows for SOIC or DIP alternatives.