The µPD4990 is a RTC (Real Time Clock) chip made by NEC, used on MVS hardware to keep track of the time and date for bookkeeping.

It shares the battery-backed RAM's battery and has its own 32.768kHz crystal to run even when power is off.

The parallel command inputs aren't used, they are all set high to enable serial commands (see MVS schematics page 1, and datasheet).

Interface

The READ_CALENDAR system ROM call can be used to conveniently load the time and date to BIOS RAM locations.

Raw access is done through REG_STATUS_A for input and REG_RTCCTRL for output.

Pinout

A damaged chip, oscillator or their connections can bring a CALENDAR ERROR, but most of the time the system ROM just freezes due to a self-test bug.

Official µPD4990A datasheet: [[1]]