Latest revision as of 19:13, 15 June 2020

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.

There are two versions used, the DIP 14 pin uPD4990AC and the SOP 16 pin uPD4990AG.

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.

Pinouts

SOP16

File:UPD4990.odg

DIP14

Failure

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.

Datasheet

Official µPD4990A datasheet: [[1]]

@@ Line 1: / Line 1: @@
-[[File:mvs_upd4990.jpg|right|thumb|uPD4990 RTC in a (dirty) 1FZS MVS slot.]]
+[[File:mvs_upd4990.jpg|right|thumb|µPD4990AG RTC on a (dirty) {{PCB|MV1FZS}} board.]]
-The uPD4990 is a Real Time Clock (RTC) chip made by NEC, to keep the [[calendar]]'s time and date informations on MVS boards.
+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]].
+There are two versions used, the DIP 14 pin uPD4990AC and the SOP 16 pin uPD4990AG.
+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|REG_STATUS_A}} for input and {{Reg|REG_RTCCTRL}} for output.
+=Pinouts=
+==SOP16==
+[[File:uPD4990.png|300px]]
+[[File:uPD4990.odg]]
+==DIP14==
+[[File:UPD4990AC_pinout.png|256px]]
+=Failure=
+A damaged chip, oscillator or their connections can bring a [[CALENDAR ERROR]], but most of the time the system ROM just [[calendar freeze|freezes]] due to a [[System_self-test|self-test]] bug.
+=Datasheet=
+Official µPD4990A datasheet: [[http://www.datasheetcatalog.org/datasheet/nec/UPD4990AC.pdf]]
-It shared the [[SRAM]]'s battery and had a separate 32.768kHz crystal clock to run when power was off.
 [[Category:Chips]]
+[[Category:Repairs]]

UPD4990: Difference between revisions