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Mark D. Studebaker
08422ec5f1
git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@1256 7894878c-1315-0410-8ee3-d5d059ff63e0
100 lines
2.8 KiB
Plaintext
100 lines
2.8 KiB
Plaintext
Fan reading problems and solutions
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If you aren't getting the expected readings on your fans,
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try the following:
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- Do you get a fan reading in the BIOS or in DOS?
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If not, you may not have a fan with a tachometer output.
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Look and see. Fans with tachometer outputs have THREE wires.
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- Try experimenting with the fan divisor settings:
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Fan Divisor Settings
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--------------------
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Fan Divisor Minimum RPM Maximum RPM
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----------- ----------- -----------
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1 5314 1350000
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2 2657 675000 default on most chips
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4 1328 337500
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8 664 168750
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16 332 84375 not supported on most chips
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32 166 42187 ""
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64 83 21093 ""
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128 41 10546 ""
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Pick a divisor so that the nominal RPM is about 50%
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above the minimum. This is a good compromise between
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margin and accuracy. Note that most chips only support
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fan divisors of 1, 2, 4, and 8.
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If you have a 0 RPM reading some or all of the time,
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increase the divisor until you get good readings.
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If you have a nominal reading less than 1.25 times the
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minimum, increase the divisor to give you margin
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so that you will not get spurious alarms.
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If you have a nominal reading more than 3 times the
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minimum, decrease the divisor to provide better
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accuracy.
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How to change fan divisors
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--------------------------
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Method 1: "echo x y z > /proc/sys/dev/sensors/...../fan_div"
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where 'x y z' are the fan divisors for the three fans and '...'
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is the sensor in question.
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Method 2: Put an entry "set fanx_div y" in the appropriate section
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of /etc/sensors.conf and rerun 'sensors -s'.
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(x is the number of the fan 1-3 and y is the divisor).
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Further details
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---------------
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Fan divisors are quite confusing.
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Sensor chips count fan speed by using the fan signal
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to gate an 8-bit counter driven by a 22.5 kHz clock.
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So the _higher_ the counter value, the _slower_ the fan,
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and vice versa.
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The term 'fan divisor' is a misnomer because it doesn't divide
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the fan signal, it divides the 22.5 kHz clock.
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Thus you _increase_ the divisor if you have a slow fan.
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The drivers account for the 'fan divisor' in its calculation
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of RPM. So changing the fan divisor will NOT change the
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nominal RPM reading, it will only affect the minimum and maximum
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readings and the accuracy of the readings.
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The actual formula is RPM = (60 * 22500) / (count * divisor)
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Fan readings 2X too high
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------------------------
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This is caused by a fan clock which is actually half the expected
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22.5 kHz, or by a fan which outputs twice as many pulses per revolution
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as expected.
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As we learned above, you can _not_ fix this by changing
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the fan divisor. You must add entries into the appropriate
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section of /etc/sensors.conf:
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compute fan1 @/2, 2*@
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compute fan2 @/2, 2*@
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compute fan3 @/2, 2*@
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------------------
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Copyright (c) 2000,2001 Mark D. Studebaker <mdsxyz123@yahoo.com>
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