The CMOS bq4285 is a low-power microprocessor peripheral providing a time-of-day clock and 100-year calendar with alarm features and battery operation. Other features include three maskable interrupt sources, square wave output, and 114 bytes of general nonvolatile storage.

The bq4285 write-protects the clock, calendar, and storage registers during power failure. A backup battery then maintains data and operates the clock and calendar.

The bq4285 is a fully compatible real-time clock for IBM AT-compatible computers and other applications. The only external components are a 32.768kHz crystal and a backup battery.

The bq4285 integrates a battery-backup controller to make a standard CMOS SRAM nonvolatile during power-fail conditions. During power-fail, the bq4285 automatically write-protects the external SRAM and provides a VCC output sourced from the clock backup battery.

• Direct clock/calendar replacement for IBM® AT-compatible computers and other applications
• Functionally compatible with the DS1285
  • Closely matches MC146818A pin configuration
• 114 bytes of general nonvolatile storage
• Automatic backup and write-protect control to external SRAM
• 160 ns cycle time allows fast bus operation
• Less than 0.5µA load under battery operation
• 14 bytes for clock/calendar and control
• Calendar in day of the week, day of the month, months, and years, with automatic leap-year adjustment
• Time of day in seconds, minutes, and hours
  • 12- or 24-hour format
  • Optional daylight saving adjustment
• BCD or binary format for clock and calendar data
• Programmable square wave out-put
• Three individually maskable interrupt event flags:
  • Periodic rates from 122 us to 500 ms
  • Time-of-day alarm once per second to once per day
  • End-of-clock update cycle
• 24-pin plastic DIP or SOIC