rcoc wrote:
> On boot, I get this warning, and am prompted to hit F1 or F2.
> But...the fan has not failed, nor is it blocked or running slowly.
> The graphics card fan is after market and runs great also.
> I don't see any control for this in BIOS
> Dell XPS 400, XP Media Center SP2, original card fan.
>
> Suggestions would be very appreciated!
A monitored fan has three wires. The signals are +12V, GND, and RPM.
The RPM signal pulses twice per revolution of the fan.
The RPM signal is monitored by the hardware monitor interface on
the SuperI/O chip on the motherboard. The SuperI/O chip measures
the time period, between two pulses on the RPM signal. The time
is measured with a clock signal. For example, the measurement
register might be an 8 bit register, and if the clock ticked
once per microsecond, the chip could measure a period as long
as 255 microseconds.
If the period between pulses goes beyond the register's maximum
value (255), then the register is "overflowed". The software
doesn't know the difference between 256 microseconds and an
infinite number of microseconds, once the overflow happens.
If you take the inverse of the time period, you get the
rotation rate. A register with a limited ability to
hold the time period, equates to a "minimum RPM" value
that can be measured.
On my computer, the one I'm typing on, that number
equates to 1800RPM. My power supply delivers an RPM signal
to my motherboard. When the computer starts, the power
supply fan runs at about 1700 or so. The measurement
chip pretends that is "0", and I get one of those
"Press F1 to enter the BIOS" errors. Yet, by the time
I get into the BIOS, and go to the hardware monitor page,
to look for something out of sorts, by that time the
fan has started to register, and the RPMs are just above
1800. As the power supply warms up, that fan runs a little
bit faster.
All of this nonsense could be fixed, if the time base
used by the hardware monitor was changed. There is a
separate register, that sets the scale of the time
base. In fact, the hardware monitor may not be cranked
down as far as it can go. I think on my chip for example, the
BIOS limit is 1800RPM, but if properly programmed, the
chip can still detect 500-600 or so for an RPM value.
You should check that your fans are clean and not
binding on anything. With the power off, of course.
If it used to work without a problem, and now is
getting flagged in the BIOS, it must be running
slower for some reason.
If you need a program to monitor your fans, try
Speedfan from almico.com . Other than motherboard maker
provided programs, it is one of the few which is still
supported by its author. Speedfan has a nice autoranging
algorithm for the fan monitoring circuit, and will
adjust the scale register on the hardware chip, for
best measurement capability. BIOS codes tend to use
a fixed value, which is why they're so inflexible.
Paul
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