DS12887A-DALLAS-RTC-DEAD

DS12887A-DALLAS-RTC-DEAD

The Problem

On OLD 1990's Motherboards (Mine are SHUTTLE HOT-555) in the Socket 7 era, the CMOS is shown at startup as "CMOS BATTERY BAD" CMOS Corrupted. DEFAULTS LOADED.

And my motherboards have the DS12887A module SOLDERED IN!

Sure enough, using BIOs setup, anything changed is not retained. Loop around again. NOPE! So you can't boot from a hard drive. Or probably from a floppy drive.

A Possible Solution

The 555 motherboard I have has 2 pins labelled JP21 and are to "Clear CMOS". They have a plastic protector on them that is easily removed.

Checking, I find just a few millivolts across those pins. Also find that the one farthest from the Dallas RTC chip is GROUND. The one nearest the RTC chip?? Hmmmm...

First Try:

I made a 3 volt battery with two AA batteries and soldered connections in a 4 AA case. Added wires that end in female connectors that mate with usual PC board pins.

Connected that (+3V to pin nearest Dalla chip, - to GND)) , Tried. NO CHANGE. Bummer....

Well, may as well leave it connected while I do other stuff... That Dallas chip is very high impedance. The CMOS must be fed from the battery through a large resistance, right?? Maybe??

A day later, same: NO CHANGE. So we have a 3V battery across the "CLEAR CMOS" pins.

Another Try

OK, let's try PATIENCE. Leave that battery connected. DO other stuff.

FOUR Days later, try again and !! No Bad Battery Message. But CMOS corrupted. So use BIOS setup to load "default CMOS". SAVE and EXIT.

Reboot and the CMOS messages are GONE! Try making some minor changes with BIOS Setup. They are RETAINED.. WOW!!

NO $%^&* SOLDERING ! !

Conclusion: " WORKS FOR ME!! "

WORKS FOR ME!! Still OK after a week. Now to get a hard drive working...

UPDATE: OK after 6 days+. Talking to others who have done the Dremel Surgery approach.