UPD4990: Difference between revisions
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[[File:mvs_upd4990.jpg|right|thumb|µPD4990A RTC on a (dirty) {{PCB|MV1FZS}} board.]] | [[File:mvs_upd4990.jpg|right|thumb|µPD4990A RTC on a (dirty) {{PCB|MV1FZS}} board.]] | ||
The µPD4990 is a RTC (Real Time Clock) chip made by NEC, used on [[ | 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 | 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). | 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 | 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|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. | ||
=Failure= | |||
A damaged chip, oscillator or their connections can bring a [[calendar error]], but most of the time just locks up | A damaged chip, oscillator or their connections can bring a [[calendar error]], but most of the time the system ROM just locks up due to a [[System_self-test|self-test]] bug. | ||
=Datasheet= | |||
Official µPD4990A datasheet: [[http://www.datasheetcatalog.org/datasheet/nec/UPD4990AC.pdf]] | Official µPD4990A datasheet: [[http://www.datasheetcatalog.org/datasheet/nec/UPD4990AC.pdf]] | ||
[[Category:Chips]] | [[Category:Chips]] | ||
[[Category:Repairs]] | |||
Revision as of 06:38, 3 October 2016
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.
Failure
A damaged chip, oscillator or their connections can bring a calendar error, but most of the time the system ROM just locks up due to a self-test bug.
Datasheet
Official µPD4990A datasheet: [[1]]