Dead fan causing CMOS data loss?


M

Man-wai Chang

Could a dead fan cause loss of BIOS settings in the CMOS?

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P

Paul

Man-wai Chang said:
Could a dead fan cause loss of BIOS settings in the CMOS?

I wouldn't think so. Not in a very direct way.

The CMOS is powered by the CMOS battery, wired-OR with a
regulator powered by +5VSB. To cause a "normal" failure, you'd
need to short that rail to ground. That would be an
electrically induced failure.

It is possible to erase the CMOS, under program control. So
a bad software design can do it. The question would be,
whether any software goes nuts, when the fan reads zero RPM.

On an Asus motherboard, the "overclocking failure" detection,
will reset the BIOS (but not change the "clock time"). So
if you're only losing a few settings, and it's an Asus
motherboard, it could be a computer crash that is causing it.
Can a dead fan cause a computer crash ? Yes, if the CPU
overheats (because it abruptly powers off). Many other fans
could die, without side effects. The CPU fan is a bit more
important.

Paul
 
M

Man-wai Chang

I wouldn't think so. Not in a very direct way.

Same here, unless there is a design flaw... Um...
... need to short that rail to ground. That would be an
electrically induced failure.

If the fan failed to turn, would it send a back-emf to that rail and
clear the CMOS? The PC (HP Pavilion m9088hk) was reporting "System Fan
Failure" lately.
On an Asus motherboard, the "overclocking failure" detection,
will reset the BIOS (but not change the "clock time"). So
if you're only losing a few settings, and it's an Asus
motherboard, it could be a computer crash that is causing it.
Can a dead fan cause a computer crash ? Yes, if the CPU
overheats (because it abruptly powers off). Many other fans
could die, without side effects. The CPU fan is a bit more
important.

HP m9088hk is a generic Asus motherboard, but it doesn't have any
overclocking feature. The PC didn't hang, so the heat factor could be
ticked out.

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D

david

If the fan failed to turn, would it send a back-emf to that rail and
clear the CMOS? The PC (HP Pavilion m9088hk) was reporting "System Fan
Failure" lately.

No. Fix the fan, but it didn't cause your CMOS memory data to be lost.
 
M

Man-wai Chang

No. Fix the fan, but it didn't cause your CMOS memory data to be lost.

I began to think the fan was causing the CMOS data loss... :)

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M

Man-wai Chang

I began to think the fan was causing the CMOS data loss... :)

If it's not electronics issue, then someone was playing security game
with that PC.

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M

Man-wai Chang

If conditions are just right;
1. This dead shorted fan.
2. No recharge from battery circuit.
3. Some poor design or blown component.
4. Weak CMOS battery.
Now this leaves the CMOS battery with a short across its leads,
draining it.

Too deep for me as I am not a EE person. Let me investigate further.

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D

david

Actually, it could.

The user would have to power off the computer from a power strip for it
to happen.
As said; the fan is powered off the +5 rail as is the CMOS battery
charger/battery.
Bad/shorted fan is not drawing enough current to 'catch fire' but when
computer is powered off with a strip which effectively is like pulling
the plug, the battery circuit is now a dead short.

If conditions are just right;
1. This dead shorted fan.
2. No recharge from battery circuit.
3. Some poor design or blown component. 4. Weak CMOS battery.

Now this leaves the CMOS battery with a short across its leads, draining
it.

Have seen much stranger things happen. Onion

There is no "CMOS Batter Charger". The battery is a primary cell. The
CMOS memory is powered from 5V standby. The fans are powered from the
12V rail, though a fan speed controller. One has nothing to do with the
other.
 
D

david

I was being simplistic. The 'charger' is a circuit powered from the
main power supply.
All DC voltage creation originates at the main transformer hence they
are all connected.

Drastically so in the case of a short.

I can explain futher, but I hope you see the point.

I can see that you know nothing of power supply design or construction.
As I am an electrical engineer, I would love to hear your "further
explanation" of how a fan on the 12V rail could affect a battery-backed
circuit with a reverse-biased diode to prevent battery drain when 5VSB is
on.

Oh and while you're at it, explain how a transformer creates DC voltage.
 
L

Loren Pechtel

Suspect that the protect diodes in the CMOS power circuit would
eliminate any back circuit drain of the battery.
Has been a long time since I've dug into the circuitry but recall that
there were 3 of 4 diodes blocking the supplies from affecting each
other.

I would think there would have to be to avoid CMOS power being drawn
off into the rest of the board. A powered-down board would look
awfully similar to a short as far as the CMOS battery was concerned.
 
P

Paul

Onion said:
I was being simplistic. The 'charger' is a circuit powered from the
main power supply.
All DC voltage creation originates at the main transformer hence they
are all connected.

Drastically so in the case of a short.

I can explain futher, but I hope you see the point.

There's a picture of a power supply here, so you can understand how
it works. Transformers are used for isolation. The turns ratio
helps define the output voltages. Rectifiers convert the high frequency
AC from the output transformer, back into DC. It's a switch mode supply,
where the switching occurs at high frequencies, to help keep the
size of the transformer smaller, and make it easier to filter
on the output. A separate transformer, separates the controller
portion, from the bipolar transistor base drive, as the switching
transistors have high voltage on them. (This is one of my favorite
free schematics on the web.)

http://www.pavouk.org/hw/en_atxps.html

As for the DC blocking of the CMOS (powering) thing, you can see
that on PDF page 82 here. The upper left corner shows the diodes
selecting the battery or +5VSB derived source of power for the CMOS storage.
PDF page 85, shows the regulator that takes 5VSB and makes V_3P3_Standby
from it. The diodes only allow current to flow in one direction.

http://www.intel.com/design/chipsets/schematics/252812.htm

The 12V fan, is well removed from CMOS battery, 5VSB, 3P3_Standby
and so on. It has nothing to do with them. I don't see any
obvious backfeed paths to consider, either.

The CMOS battery is *not* to be charged. It says so right on the
datasheet for the CR2032. The spec to meet, is 1 microamp, which
means no more than 1 microamp may flow backwards into the
CMOS battery (which would charge it). Therefore, it cannot and
must not be charged. The diodes on page 82, help meet that spec.
The battery runs, until it runs out of juice. When the computer
is line powered, and +5VSB is available, the +5VSB supplies the
current required, which is why the battery lasts so long.
Even when the computer is completely unplugged, the load
on the CMOS battery is around 5-10 microamps or so. 2 microamps
is enough to run a digital watch circuit, and the SB well
uses a bit more than that.

To get back to the original symptoms, there is one additional
piece of information we can use. The CMOS well, powers both
256 bytes of CMOS RAM, as well as the RTC clock. If the CMOS
BIOS settings were lost, as well as the RTC clock being
set to the year 1980, that would be evidence for a failure
of the CMOS voltage (the output of the two diodes). My
suspicion is, Man-wai Chang saw the BIOS settings reset
to defaults, but the clock maintained the correct time.
And that is evidence of the Asus "overclocking failure,
reset to defaults" feature. It doesn't reset the clock
to the year 1980.

I don't see how the dead fan would have triggered that
feature, unless the computer crashed during the period in
question. Even switching off the power, without safely
shutting down the PC, could trigger that Asus feature.

Paul
 
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M

Man-wai Chang

of the CMOS voltage (the output of the two diodes). My
suspicion is, Man-wai Chang saw the BIOS settings reset
to defaults, but the clock maintained the correct time.
And that is evidence of the Asus "overclocking failure,
reset to defaults" feature. It doesn't reset the clock
to the year 1980.

But why was this feature triggered when the motherboard doesn't support
overclocking at all?

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P

Paul

Man-wai Chang said:
But why was this feature triggered when the motherboard doesn't support
overclocking at all?

The Asus feature, checks for "safe shutdown". It isn't an
overclocking feature, at all. All the feature does, is check
to see whether the BIOS shut down the computer safely. The
BIOS sets a flag at shutdown - the BIOS checks for the flag
on the next startup. If the flag isn't there, it implies
the machine was shut down improperly. And that triggers
a reset of the BIOS settings, on the assumption the
BIOS settings are what triggered the crash. But
something as simple as switching off the power in the
middle of a Windows session, could trigger it.

I had a few of those on my P4C800-E Deluxe, so I'm used
to it.

Paul
 
M

Man-wai Chang

The Asus feature, checks for "safe shutdown". It isn't an
overclocking feature, at all. All the feature does, is check
to see whether the BIOS shut down the computer safely. The
BIOS sets a flag at shutdown - the BIOS checks for the flag
on the next startup. If the flag isn't there, it implies
the machine was shut down improperly. And that triggers
a reset of the BIOS settings, on the assumption the
BIOS settings are what triggered the crash. But
something as simple as switching off the power in the
middle of a Windows session, could trigger it.

SO when there was "System Fan Failure" warning, the BIOS would think it
needed a safe shutdown, and triggered the feature?

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P

Paul

Man-wai Chang said:
SO when there was "System Fan Failure" warning, the BIOS would think it
needed a safe shutdown, and triggered the feature?

I'm having trouble connecting the event of a fan failure,
to the Asus clearing of parameters. What I'm seeing is:

1) It's an Asus motherboard
2) BIOS parameters cleared to default, RTC clock remains correct

And the Asus feature, is the nearest thing to explaining that.

But how the fan failure fits into that, I don't exactly know.

*******

There are a couple of options on a fan failure. If the
CPU fan stops spinning, and there is SMM BIOS code running,
it's possible that code could shut off the computer. If the code
was badly designed, it could shut down the computer without
properly setting the flag that indicates there was a clean
shutdown.

The THERMTRIP on the computer, may trigger if the CPU gets
hot enough. If the CPU fan fails, and you wait a minute or
two, the system may get hot enough to shut off. And then,
that might trigger the clearing of CMOS parameters.

It it's a fan not connected to the CPU fan header, the
BIOS doesn't take that as seriously. And if the case fan
failed, it's also less likely that a THERMTRIP will happen
due to CPU overheat.

Were you present, when the fan failed ? Or did you figure out,
after that fact, that some fan had failed ?

Paul
 
M

Man-wai Chang

Were you present, when the fan failed ? Or did you figure out,
after that fact, that some fan had failed ?

The PC is not in my home. I needed to go uptown to check.
I am keeping your notes.

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G

GMAN

SO when there was "System Fan Failure" warning, the BIOS would think it
needed a safe shutdown, and triggered the feature?
yes

My brother to his P4C800 , the P4P800 Deluxe did the same thing.
 
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M

Man-wai Chang

SO when there was "System Fan Failure" warning, the BIOS would think it
yes
My brother to his P4C800 , the P4P800 Deluxe did the same thing.

Going to replace the fan and see what would happen...

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