| 1 | Fan reading problems and solutions |
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| 2 | ---------------------------------- |
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| 3 | |
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| 4 | If you aren't getting the expected readings on your fans, |
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| 5 | try the following: |
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| 6 | |
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| 7 | - Do you get a fan reading in the BIOS or using a different OS? |
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| 8 | If not, you may not have a fan with a tachometer output. |
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| 9 | Look and see. Fans with tachometer outputs have at least 3 wires. |
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| 10 | Fans with only 2 wires cannot, in general, report their speed. |
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| 11 | |
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| 12 | - Try experimenting with the fan divisor settings: |
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| 13 | |
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| 14 | |
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| 15 | Fan Divisor Settings |
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| 16 | -------------------- |
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| 17 | |
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| 18 | Fan Divisor Minimum RPM Maximum RPM |
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| 19 | ----------- ----------- ----------- |
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| 20 | 1 5314 1350000 |
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| 21 | 2 2657 675000 default on most chips |
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| 22 | 4 1328 337500 |
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| 23 | 8 664 168750 |
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| 24 | 16 332 84375 not supported on most chips |
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| 25 | 32 166 42187 "" |
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| 26 | 64 83 21093 "" |
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| 27 | 128 41 10546 "" |
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| 28 | |
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| 29 | Pick a divisor so that the nominal RPM is about 50% |
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| 30 | above the minimum. This is a good compromise between |
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| 31 | margin and accuracy. Note that most chips only support |
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| 32 | fan divisors of 1, 2, 4, and 8. |
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| 33 | |
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| 34 | If you have a 0 RPM reading some or all of the time, |
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| 35 | increase the divisor until you get good readings. |
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| 36 | |
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| 37 | If you have a nominal reading less than 1.25 times the |
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| 38 | minimum, increase the divisor to give you margin |
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| 39 | so that you will not get spurious alarms. |
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| 40 | |
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| 41 | If you have a nominal reading more than 3 times the |
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| 42 | minimum, decrease the divisor to provide better |
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| 43 | accuracy. |
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| 44 | |
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| 45 | |
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| 46 | How to change fan divisors |
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| 47 | -------------------------- |
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| 48 | |
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| 49 | Put an entry "set fanN_div X" in the appropriate section of |
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| 50 | /etc/sensors3.conf and run 'sensors -s' |
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| 51 | (N is the number of the fan, and X is the divisor you want). |
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| 52 | |
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| 53 | |
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| 54 | Further details |
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| 55 | --------------- |
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| 56 | |
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| 57 | Fan divisors are quite confusing. |
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| 58 | |
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| 59 | Sensor chips count fan speed by using the fan signal |
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| 60 | to gate an 8-bit counter driven by a 22.5 kHz clock. |
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| 61 | So the _higher_ the counter value, the _slower_ the fan, |
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| 62 | and vice versa. |
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| 63 | |
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| 64 | The term 'fan divisor' is a misnomer because it doesn't divide |
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| 65 | the fan signal, it divides the 22.5 kHz clock. |
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| 66 | Thus you _increase_ the divisor if you have a slow fan. |
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| 67 | |
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| 68 | The drivers account for the 'fan divisor' in their calculation |
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| 69 | of RPM. So changing the fan divisor will NOT change the |
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| 70 | nominal RPM reading, it will only affect the minimum and maximum |
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| 71 | readings and the accuracy of the readings. |
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| 72 | |
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| 73 | The actual formula is RPM = (60 * 22500) / (count * divisor) |
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| 74 | |
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| 75 | The readings are most accurate when the fan speed is low |
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| 76 | (i.e., close to the minimum possible RPM reading). |
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| 77 | As fan speed gets closer to the maximum possible RPM reading, |
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| 78 | the reading becomes quite inaccurate. Fortunately, most computer |
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| 79 | fans do not approach 1,000,000 RPM ! |
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| 80 | |
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| 81 | |
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| 82 | Fan readings 2X too high |
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| 83 | ------------------------ |
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| 84 | |
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| 85 | This is rare, but can happen. This is typically caused by a fan which |
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| 86 | outputs 4 pulses by revolution, instead of the standard 2. |
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| 87 | |
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| 88 | As we learned above, you can _not_ fix this by changing |
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| 89 | the fan divisor. You must add entries into the appropriate |
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| 90 | section of /etc/sensors3.conf: |
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| 91 | |
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| 92 | compute fanN @/2, 2*@ |
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| 93 | |
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| 94 | (N is the number of the fan) |
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| 95 | |
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| 96 | |
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| 97 | |
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| 98 | ------------------ |
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| 99 | Copyright (c) 2000-2004 Mark D. Studebaker |
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