UPD4990: Difference between revisions

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[[File:mvs_upd4990.jpg|right|thumb|uPD4990 RTC in a (dirty) 1FZS MVS slot.]]
[[File:mvs_upd4990.jpg|right|thumb|µPD4990A RTC on a (dirty) {{PCB|MV1FZS}} board.]]


The uPD4990 is a RTC (Real Time Clock) chip made by NEC, used on [[MVS]] boards to keep the [[calendar]]'s time and date.
The µPD4990 is a RTC (Real Time Clock) chip made by NEC, used on [[MVS_hardware]] 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.
It shares the [[battery-backed RAM]]'s battery and has a separate 32.768kHz crystal clock 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==
==Interface==


The [[READ_CALENDAR]] BIOS call can be used to transparently load the uPD4990 time and date to [[BIOS RAM locations]].
The [[READ_CALENDAR]] system ROM call can be used to conveniently load the µPD4990 time and date to [[BIOS RAM locations]].


Raw access is done through {{Reg|REG_STATUS_A}} for input and {{Reg|REG_RTCCTRL}} for output.
Raw access is done through {{Reg|REG_STATUS_A}} for input and {{Reg|REG_RTCCTRL}} for output.
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==Failure==
==Failure==


A damaged chip, oscillator or their connections provokes the [[Calendar error]] or a freeze on MVS boards.
A damaged chip, oscillator or their connections can bring a [[calendar error]], but most of the time just locks up the system ROM due to a [[System_self-test|self-test]] bug.


==Datasheet==
==Datasheet==


Official uPD4990A datasheet: [[http://www.datasheetcatalog.org/datasheet/nec/UPD4990AC.pdf]]
Official µPD4990A datasheet: [[http://www.datasheetcatalog.org/datasheet/nec/UPD4990AC.pdf]]


[[Category:Chips]]
[[Category:Chips]]

Revision as of 03:22, 30 September 2016

µPD4990A RTC on a (dirty) MV1FZS board.

The µPD4990 is a RTC (Real Time Clock) chip made by NEC, used on MVS_hardware 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.

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 µPD4990 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 can bring a calendar error, but most of the time just locks up the system ROM due to a self-test bug.

Datasheet

Official µPD4990A datasheet: [[1]]