BQ3285EP

Overview

The Texas Instruments BQ3285EP is a CMOS real-time clock (RTC) and calendar peripheral designed for IBM AT-compatible systems. It operates within a supply voltage range of 4.5 V to 5.5 V and provides a 100-year calendar with automatic leap-year adjustment, time-of-day tracking in 12- or 24-hour formats, and 242 bytes of general nonvolatile SRAM. The device features a multiplexed address/data bus interface compatible with Intel or Motorola architectures. Key electrical characteristics include a typical standby current of 300 µA and a battery backup current of less than 0.5 µA. It supports three individually maskable interrupt sources: periodic, alarm, and update-ended.

Feature Summary

• Multiplexed bus interface supporting selectable Intel or Motorola timing protocols via the MOT pin.
• Integrated 100-year calendar and time-of-day clock with BCD or binary data format options.
• Three maskable interrupt sources: periodic (122µs to 500ms), time-of-day alarm, and update-ended.
• Programmable square-wave output ranging from 32.768 kHz to 2 Hz, plus a dedicated 32.768 kHz power management output.

Applications

• Direct replacement for DS1285 RTCs in IBM AT-compatible computers.
• Systems requiring low-power battery-backed timekeeping and nonvolatile storage.
• Applications utilizing alarm interrupts for system wake-up capabilities during battery backup.

Procurement Notes

Verify package availability as the BQ3285EP denotes a plastic DIP variant, while other suffixes indicate SOIC or SSOP packages. Confirm that the specific ordering suffix matches the required physical footprint and lead configuration for the target PCB design. Ensure the 4.5V–5.5V operating voltage aligns with the system's main power rail specifications.

Selection Notes

Select the BQ3285E series when the system operates at 5V; use the BQ3285L series for 3V applications. The choice between Intel and Motorola bus timing is determined by the external MOT pin connection rather than the part number itself. Ensure an external 32.768 kHz crystal with a 6 pF load capacitance is included in the design. Verify that the host processor can accommodate the multiplexed address/data bus cycle requirements.

Part Context

The BQ3285EP belongs to the bq3285E/L family of real-time clocks manufactured by Texas Instruments. This family includes variants such as the bq3285ESSTR (SSOP) and bq3285ELP (DIP). The 'EP' suffix specifically identifies the 24-pin plastic dual in-line package (DIP) version. All family members share identical functional specifications, register maps, and electrical characteristics, differing only in the physical package type.

Replacement Considerations

The datasheet confirms functional compatibility with the Dallas Semiconductor DS1285 and close pin configuration matching to the MC146818A. When replacing these components, verify that the host system's bus timing signals (AS, DS, R/W) are correctly mapped to the Intel or Motorola modes selected by the MOT pin.