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
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