BQ4285P-SB2

Overview

The bq4285 is a low-power CMOS microprocessor peripheral providing a time-of-day clock, 100-year calendar with automatic leap-year adjustment, and alarm features. It includes 114 bytes of general nonvolatile storage and integrates a battery-backup controller for external SRAM. The device operates on a +5V supply (4.5V to 5.5V) with less than 0.5 μA load under battery operation. It supports Intel bus protocols with a 160 ns cycle time and offers three maskable interrupt sources: periodic, alarm, and update-ended. Data formats are selectable as BCD or binary.

Feature Summary

• Functionally compatible with DS1285 and MC146818A pin configurations
• 114 bytes of general nonvolatile storage plus 14 bytes for clock/calendar
• Automatic backup and write-protect control for external CMOS SRAM
• Three individually maskable interrupts: periodic (122 μs to 500 ms), alarm, and update-ended

Applications

• IBM AT-compatible computers
• Microprocessor systems requiring real-time clock functionality
• Applications necessitating battery-backed nonvolatile memory

Procurement Notes

Verify package availability as the 28-pin PLCC option is obsolete. Confirm operating temperature range requirements against the commercial specification of 0°C to +70°C. Ensure design accommodates the required 32.768 kHz crystal with 6 pF load capacitance. Validate that the host system supports the multiplexed address/data bus interface timing specifications.

Selection Notes

Select the P suffix for 24-pin plastic DIP or S suffix for 24-pin SOIC packages. The device requires a 3V backup cell input on the BC pin to maintain data integrity during power failures. Design must include provisions for the VOUT pin to supply external RAM during battery operation. Consider the 200 ms chip enable recovery time after power-up when initializing external memory access.

Part Context

The bq4285 serves as a direct replacement for legacy IBM AT-compatible real-time clock chips. It combines RTC functionality with NVRAM management in a single CMOS component. The device eliminates the need for separate battery management circuits by integrating power-fail detection and switching logic internally.

Replacement Considerations

The bq4285 is functionally compatible with the DS1285 and closely matches the MC146818A pin configuration. Verify register mapping compatibility if substituting into existing designs using these reference components.