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ABC of electronics terms


Real-time clock

The Real Time Clock is the part in a computer that keeps on running even when the power is removed from the computer to keep track of the real time (as opposed to the power-on time of the system).
To be able to keep running an alternative form of energy like a battery is needed. This can be integrated into the chip like package (like the often in PC's used RTC's from Dallas).
The original DATRAM in the PC was the Motorola MC146818.
Since RTC's need a battery anyway they often also have an integrated SRAM* which will also keep it's contents during power down.
An alternative for this SRAM* is EEPROM, but that can't contain the RTC of course.
Perhaps currently the RTC is integrated in the chipset of the PC, but that would require a separate battery again.
Some batteries will load themselves up again when power is applied and others not. When a PC isn't used for a while such a self-uploading battery may have gone empty. The normal batteries will work much longer even when AC power is down, I guess. I think that they have a typical lifespan of about 10 years.
When you're using whatever Dallas battery powered SRAM*, make sure that you have a long (130 ms) waiting loop before you do anything with the SRAM*, because they come up very slow! Counting to 65536 on a 6811 will do the trick, but that MCU is quite slow, so calculate it carefully on other MCU's!

The book: 'Personal Computer Troubleshooting and Repair' tells a lot about the early PC's and perhaps it has even be updated in the mean time...

BIOS and motherboard sites may also have more data:

From:    Jaap van Ganswijk <>
To:      Allen Poland
Subject: Re: Odin

On 19991121 Allen Poland wrote:

I am trying to find a replacement CMOS* battery for my motherboard. It is marked ODIN OEC12C887A.
If you can tell me the voltage of this battery, how it is soldered to the board (can't see anything on bottom of motherboard except two (2) solder pads, no trace), a suggested replacement, etc., I would appreciate it.

Am I right in assuming that the component is a combined real-time clock and battery? If so, Dallas may have been the original manufacturer [Yes, this has been confirmed it's the Dallas DS12C887A, Jaap]. You can check their site.

If it's only a battery (as your remark about the 2 solder pads suggests) then desoldering the two pads should help. You can then measure the voltage that is still left in the component and up that to a multiple of 1.5 volt. Each cell in a battery delivers usually 1.5 volt. Since a battery's voltage goes down over time, there may be a voltage regulator on the board near the battery. You can also look at which component is fed using the voltage and look up what power it needs. The reason that you see no trace is because motherboards are usually multilayered with some of the layers inside the board. Holding it before a very strong light (like a home movie lamp, or a dia projector, or a dentists lamp) might help. You can also use a multimeter and measure with which components the leads are connected.

As regards to a replacement. In electronics stores like Radio Shack you can usually buy penlight battery holders for 1,2,3,4 or 6 batteries. I'd use that and solder leads to the motherboard. You can also ask for other solutions in the electronics store, but penlights should work fine. Make sure that when the penlights should start to leak that they don't leak on any crucial parts.

If you can't find out the voltage in any other way, you can try increasing the voltage by adding battery after battery until the BIOS starts remembering it's settings and the clocks keeps running between power-on's.

You can also look in the mainboard booklet if you have that or can get that. Some motherboards are documented on the WWW site of their manufacturer.

On 20010622, Archie Hopkins wrote:

Did you suggest the penlight solution the OEC12C887A installation?

I suggested it in general as a cheap way to replace a PC battery (which may be hard to find).

But normally the regular batteries should last 5 to 10 years nowadays, so longer than the economical life-span of a mother board. I have seen worn out batteries on 486 boards though.

The problem with using two different batteries however is that the more powerful one can start uploading the other resulting in chemical problems.

This is also the case when using two different batteries (different brand or age) in a household product. (You probably already know this.)

Or only for a direct battery replacement?

It will only replace the battery part of course. Some of those Dallas and Dallas-like chips have an inbuilt battery. It might be possible to provide a temporary solution by adding an external battery. Those motherboards are usually at the end of their life-cycle anyway, so experimenting a bit shouldn't hurt. However consider that the internal battery may get chemical problems. As long as a capable technician is around it shouldn't be a problem, however I wouldn't like to give a warrantee on such a solution

See also RTC resource page by Bob Paddock


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