UPD4990

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

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

It shared the battery-backed RAM's battery and had a separate 32.768kHz crystal clock to run when power was off.

Datasheet

Official uPD4990A datasheet: [[1]]

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