The DS12C887 from Dallas Semiconductor (now Analog Devices via Maxim) is a real-time clock and non-volatile RAM module designed as a drop-in replacement for the MC146818 clock in IBM-compatible PCs, and the low-power CMOS upgrade of the DS12887. DS12C887+ is the RoHS-compliant version. Everything a battery-backed clock needs - the RTC silicon, a 32.768 kHz quartz crystal and a lithium energy cell - is sealed inside one 24-pin encapsulated DIP module.

The timekeeping covers seconds, minutes, hours, day, date, month and year with automatic leap-year compensation, valid to 2100, in BCD or binary and 12-hour or 24-hour format. An internal lithium source keeps the clock running and the RAM retained for more than 10 years of cumulative power-off time, with about ±1 minute/month accuracy. When VCC drops below roughly 3 V the module switches to the internal battery automatically, so the host never has to manage backup power.

The device also provides 128 bytes of RAM: 15 bytes for the clock, calendar, alarm and control registers and 113 bytes of general-purpose NVRAM for BIOS settings or system data. The host reads and writes over a multiplexed AD0-AD7 bus; the MOT pin selects Motorola (AS/DS/R-W) or Intel (ALE/RD/WR) bus timing. A time-of-day alarm, three maskable interrupts on /IRQ (alarm, periodic, update-ended) and a programmable square-wave output (SQW, 13 rates from 2 Hz to 32.768 kHz) round out the function. It is pin-compatible with the MC146818B and DS1287.

Note: Values are taken from the Dallas/Maxim DS12C887 datasheet. Confirm every parameter against the latest official documentation before committing a design.

DS12C887+ is supplied in the 24-pin encapsulated DIP (EDIP) module; "+" marks the RoHS-compliant lead-free finish. The raised module body houses the RTC die, the 32.768 kHz crystal and the lithium cell, which is why pins 2, 3, 16 and 20-22 are absent by design. Place it in a standard 24-pin DIP socket matching an MC146818/DS1287 footprint; no external crystal or backup battery is needed. Package dimensions and pin-1 orientation are shown below.

📄 Full specifications are detailed in the official Dallas/Maxim DS12C887 datasheet.

The 24-pin EDIP brings out the multiplexed bus, control and clock signals (pins 2, 3, 16, 20-22 are absent by design):

• Pin 1 - MOT: Bus mode select (high = Motorola, low = Intel; internal 20 kΩ pulldown).
• Pins 4-11 - AD0-AD7: Multiplexed address/data bus.
• Pin 12 - GND: Ground.
• Pin 13 - /CS: Chip select, active low.
• Pin 14 - AS: Address strobe (ALE).
• Pin 15 - R//W: Read/write (Motorola) or /WR (Intel).
• Pin 17 - DS: Data strobe (Motorola) or /RD (Intel).
• Pin 18 - /RESET: Reset input, active low.
• Pin 19 - /IRQ: Interrupt output, active low (open drain).
• Pin 23 - SQW: Square-wave output.
• Pin 24 - VCC: +5 V supply.

Refer to the package drawing below for the physical pin numbering of the 24-pin EDIP.

The parts below are related Dallas/Maxim real-time clocks in our catalog. They are not all drop-in replacements; verify the interface, battery scheme and pinout before substituting.

Selection Tips: Stay with the DS12C887 (or DS12C887A) to drop into an MC146818/DS1287 socket with the crystal and battery built in. The DS12887 is the older NMOS version of the same module. For a serial interface and smaller footprint, the I2C DS1307 or 3-wire DS1302/DS1305 trade the parallel bus and built-in battery for a smaller package.

Dallas Semiconductor, now part of Analog Devices through Maxim Integrated, pioneered the integrated RTC module that seals the crystal and battery with the clock silicon. The DS12C887 became the de-facto PC real-time clock, a direct MC146818/DS1287 replacement that added Year-2000 compliance and CMOS low power.

The device is backed by full datasheet characterization and application notes on bus mode, register map and battery life - the details that determine whether a clock keeps accurate time and retains data across years of power cycles.