UPD4990

uPD4990 RTC in a (dirty) 1FZS MVS slot.

The uPD4990 is a RTC (Real Time Clock) chip made by NEC, used on MVS boards to keep the calendar's time and date.

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

Interface

The READ_CALENDAR BIOS call can be used to transparently load the uPD4990 time and date to BIOS RAM locations.

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

Failure

A damaged chip, oscillator or their connections provokes the Calendar error or a freeze on MVS boards.

Datasheet

Official uPD4990A datasheet: [[1]]