DESCRIPTION
The DS12C887 Real Time Clock plus RAM is designed as a direct upgrade replacement for the DS12887 in existing IBM compatible personal computers to add hardware year 2000 compliance. A century byte was added to memory location 50, 32h, as called out by the PC AT specification. A lithium energy source, quartz crystal, and write-protection circuitry are contained within a 24–pin dual in-line package.
   
FEATURES
■ Drop-in replacement for IBM AT computer
    clock/calendar
■ Pin compatible with the MC146818B and
    DS1287
■ Totally nonvolatile with over 10 years of
    operation in the absence of power
■ Self-contained subsystem includes lithium,
    quartz, and support circuitry.
■ Counts seconds, minutes, hours, days, day of
    the week, date, month, and year with leap
    year compensation valid up to 2100
■ Binary or BCD representation of time,
    calendar, and alarm
■ 12– or 24–hour clock with AM and PM in
    12–hour mode
■ Daylight Savings Time option
■ Selectable between Motorola and Intel bus
    timing
■ Multiplex bus for pin efficiency
■ Interfaced with software as 128 RAM
    locations
    – 15 bytes of clock and control registers
    – 113 bytes of general purpose RAM
■ Programmable square wave output signal
■ Bus–compatible interrupt signals (IRQ)
■ Three interrupts are separately software
    maskable and testable
    – Time–of–day alarm once/second to
        once/day
    – Periodic rates from 122 ms to 500 ms
    – End of clock update cycle
■ Century register