BQ3285LFSS-A1G4

Overview

The BQ3285LFSS-A1G4 is a CMOS real-time clock (RTC) and calendar integrated circuit manufactured by Texas Instruments. It provides time-of-day tracking with a 100-year calendar, alarm features, and 242 bytes of general nonvolatile storage. The device operates on a 2.7V to 3.6V supply voltage and draws less than 0.5µA in battery mode. Key electrical parameters include a 32.768kHz oscillator input, selectable Intel or Motorola bus timing, and an operating temperature range of 0°C to 70°C. It is housed in a 24-pin SSOP package.

Feature Summary

• Direct replacement for IBM AT-compatible computers with functional compatibility to DS1285 and MC146818A pin configurations.
• Includes 242 bytes of nonvolatile SRAM and 14 bytes of clock/calendar registers supporting BCD or binary formats.
• Features three individually maskable interrupt sources: periodic, time-of-day alarm, and update-ended.
• Provides system wake-up capability via alarm interrupt active in battery-backup mode.

Applications

• IBM AT-compatible computer systems
• Microprocessor peripheral timekeeping
• Battery-backed data logging applications

Procurement Notes

Verify the component status as this part number may be discontinued. Confirm that the G4 suffix indicates RoHS compliance without altering the functional specifications. Ensure the specific SSOP package variant matches the design requirements. Check for valid backup energy source indicators during operation.

Selection Notes

Select the bq3285L variant for 3V system operations. Ensure the external crystal meets the 32.768kHz frequency with a 6pF load capacitance. Configure the MOT pin for the appropriate bus architecture (Intel or Motorola) before system operation. Verify that the backup battery voltage remains within the 2.4V to 4.0V range for reliable switching.

Part Context

This component belongs to the bq3285E/L family of real-time clocks. The 'LF' designation typically refers to the lead-free and halogen-free compliant version of the standard bq3285L. The 'SSOP' indicates the 24-pin shrink small-outline package. The 'A1' suffix often denotes a specific revision or manufacturing lot code within the TI product line.

Replacement Considerations

Functionally compatible parts include the DS1285 and MC146818A due to pin configuration matching. Verify voltage levels when substituting, as the bq3285E operates at 4.5V–5.5V while the bq3285L operates at 2.7V–3.6V.

FAQ

How is the square-wave output frequency selected?

The frequency is selected by setting the RS0–RS3 bits in Register A. A 32.768kHz output requires setting OSC2–OSC0 to 011, SQWE to 1, and 32KE to 1.

What happens to the registers during a power failure?

When VCC falls below the power-fail-detect voltage (VPFD), the bq3285L write-protects the clock and storage registers and switches power to the BC backup cell.

Can the alarm interrupt function during battery backup?

Yes, the INT output is open-drain and can be connected to a separate power supply, allowing the alarm interrupt to remain active and provide wake-up capability in battery-backup mode.