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Motherboard is : ASUS A7N8X-E Deluxe, Rev1.01,

On start up get: "cmos checksum error - defaults loaded" error BIOS is reset everytime and can't boot to Windows off HDD.

POST report is ok once I tell it there is no Floppy drive attached and not to seek it. The boot sequence starts then just as get to load windows safe mode options whole PC shuts down. BIOS is reset to defaults again.

I've checked the battery and jumpers. Tried a known working battery. Changed Graphics card, changed memory, changed HD to one without an OS. Disconnected DVD drive. PSU is known to be ok.

IN BIOS CPU is recognised correctly and memory is also recognised ok.

So is it likely to be a bad motherboard or corrupted BIOS or something else?

Thanks :)

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4 Answers 4

From your description, it sounds like this happens every time you boot.

  1. Do you only boot your system by switching on the AC power to it?
  2. Does this also occur after you power down from the OS or by pressing your power button for > 4 seconds causing the system to go into "Soft Off" or ACPI S5 mode?
  3. Is the time reset each time you have invalid CMOS checksum?

The reason I'm asking is because the VBAT power connection is typically powered by an on board regulator when the system is plugged in and by the battery when the system is disconnected from the AC outlet. You can measure the VBAT voltage and if it is ~3V or less then it is running off the battery, but if it is closer to 3.3V, then it is powered by the 3.3V DC-DC regulator on the motherboard.

If #1 above is the case, then it is likely the battery connection. #2 is more cause for concern that the motherboard isn't behaving as designed.

There is a third potential problem. Some motherboards have a Clear CMOS input which does not GND the path from the battery to the VBAT power input. Instead, when asserted, it clears the CMOS, but not the time. Look for a header with this connection to make sure it isn't shorted.

Also, bad electrolytic capacitors being the cause is unlikely because the VBAT connection is a low current connection and any capacitors on it are not likely to be electrolytic. The 3.3V voltage rail may have capacitors, but even if the voltage dipped, the VBAT power usually can go as low as 2.6V without CMOS corruption.

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1  
THis is a "new" second hand board I acquired to make use of spare components lying around. This was first time tried using so: 1. Only been able to try booting by switching on 2. Never reached OS, think it reaches start of OS load then switches self off 3. Yes How do I check VBAT stuff? –  ianfuture Sep 18 '09 at 9:34
    
So this motherboard doesn't retain CMOS setting and can't get into the OS before powering itself off? Sounds like it is more trouble than it is worth to get it working...unless you're just want to work on it some more for a learning experience. –  hanleyp Sep 18 '09 at 14:21
    
You need a voltage meter to check the VBAT. You can probe around the pins on the parts by the battery. Make sure not to short anything (pins) together. –  hanleyp Sep 18 '09 at 14:22
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Thanks for suggestions. Interestingly I've an identical known working board that was removed from a system and left alone for a few weeks, when I came back to boot it after putting in a machine, I got the CMOS checksum error message too. After resetting BIOS and restarting that all worked fine, which led me to think it may well have been a battery issue. However that working board now can't boot to OS too, which may be a Hard dsik problem as it was working fine a few weeks ago.Feel like I'm going around in circles but need to rescue both as can't afford to do full system upgrade to latest kit –  ianfuture Oct 4 '09 at 14:19
    
Yeah, you could be running into a few issues at the same time. That always makes it harder to debug. A good strategy is to isolate the issues if you can. –  hanleyp Oct 12 '09 at 12:51

Obviously impossible to say 100% without seeing it, however, I had one of these last week that was very similar (also on a Asus motherboard...).

I tried reflashing BIOS but go nowhere, I attributed it to a bad motherboard and got a replacement under RMA.

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up vote 0 down vote accepted

After all that and lots of fiddling it appears the CPU was overclocked in BIOS and had become unstable. Only by chance I noticed this. Thanks for help :)

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Dude, that is an old board. I remembered I had it like 4 or 5 years back. Check for capacitor leaks or any forms of burnish on the board - they might be causing those errors.

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It could even be that the wiring paths from the batteries have corroded somehow. It's 5 years, time to give it a rest.

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