BQ3285S-SB2TR

Overview

The Texas Instruments BQ3285S-SB2TR is a CMOS real-time clock (RTC) module providing time-of-day, a 100-year calendar with automatic leap-year adjustment, and alarm features. It includes 114 bytes of general nonvolatile storage and three individually maskable interrupt sources. The device operates on a +5V supply (4.5V to 5.5V) with battery backup via the BC pin (2.5V to 4.0V). Key electrical parameters include a 160ns cycle time, less than 0.5μA load under battery operation, and support for both Intel and Motorola bus timing architectures.

Feature Summary

• Direct replacement for IBM AT-compatible computers with functional compatibility to DS1285 and MC146818A pin configurations.
• Provides 14 bytes of clock/calendar registers and 114 bytes of general nonvolatile storage with write-protection during power failure.
• Supports selectable Intel or Motorola bus timing, BCD or binary data formats, and 12- or 24-hour time modes.
• Features programmable square wave output (1Hz to 8.192kHz), periodic interrupts (122μs to 500ms), and time-of-day alarms.

Applications

• IBM AT-compatible computer systems
• Microprocessor peripheral timekeeping modules
• Applications requiring low-power battery-backed real-time clocking

Procurement Notes

Verify the specific suffix 'SB2TR' against current manufacturer documentation, as standard listings often indicate PLCC packages are discontinued. Confirm package type availability, as the datasheet specifies 24-pin DIP (P) or SOIC (S) options. Ensure the commercial temperature range (0°C to +70°C) meets system requirements. Check for end-of-life status or last-time-buy notices associated with older RTC families.

Selection Notes

Select this component when a fully compatible drop-in replacement for legacy IBM AT architecture is required. Ensure the design accommodates the 24-pin package option (SOIC or DIP) and provides external connections for a 32.768kHz crystal and a 3V backup battery. Verify that the host processor can interface with the multiplexed address/data bus and supports the required Intel or Motorola timing signals.

Part Context

The bq3285 is part of Texas Instruments' legacy real-time clock family designed for microprocessor peripherals. It serves as a direct functional equivalent to the National Semiconductor DP8573A and Dallas Semiconductor DS1285. The device integrates oscillator control, frequency division, and nonvolatile RAM management into a single CMOS chip, simplifying RTC implementation in personal computers and industrial controllers.

Replacement Considerations

Functionally compatible substitutes include the DS1285 and MC146818A due to pin configuration matching. The bq3285E is an electrically similar variant within the same product line. Note that the 28-pin PLCC version is no longer available; verify if the SOIC or DIP package meets board layout constraints.

FAQ

How does the BQ3285 handle power failures?

When VCC falls below the power-fail-detect voltage (VPFD, typically 4.17V), the device automatically write-protects the clock and storage registers. If VCC drops below the backup cell voltage (VBC, typically 3V), power switches to the BC pin to maintain data integrity and clock operation.

What external components are required for operation?

Only two external components are needed: a 32.768kHz quartz crystal (e.g., Daiwa DT-26) connected to X1-X2 pins, and a 3V backup battery connected to the BC pin.

Can the BQ3285 operate with both Intel and Motorola buses?

Yes, the MOT pin selects the bus timing. Tying MOT to VCC enables Motorola timing, while tying it to VSS enables Intel timing. This setting should not be changed during system operation.