root/lm-sensors/trunk/doc/fan-divisors

Revision 6071, 4.1 KB (checked in by khali, 19 months ago)

Update fan-divisors: fan divisors are optional, recent monitoring chips
don't have them so the document doesn't apply to these chips.

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1Fan reading problems and solutions
2----------------------------------
3
4If you aren't getting the expected readings on your fans,
5try the following:
6
7- Do you get a fan reading in the BIOS or using a different OS?
8  If not, you may not have a fan with a tachometer output.
9  Look and see. Fans with tachometer outputs have at least 3 wires.
10  Fans with only 2 wires cannot, in general, report their speed.
11
12- If your monitoring chip supports fan divisors, try experimenting
13  with the fan divisor settings.
14
15
16Does my monitoring chip use fan divisors?
17-----------------------------------------
18
19  Recent monitoring chips tend to store fan speed values on 12 or even
20  16 bits, so divisors are no longer required. In that case, you won't
21  see "div" values in the output of sensors, and nothing needs to be
22  configured:
23 
24  CPU1 Fan:    2160 RPM  (min =  800 RPM)
25  CPU2 Fan:       0 RPM  (min =  799 RPM)  ALARM
26  Front3 Fan:     0 RPM  (min =    0 RPM)
27  Front4 Fan:   779 RPM  (min =  600 RPM)
28
29  If your "sensors" output looks like the above, stop reading here,
30  the explanations below do not apply to your case.
31
32  Older chips stored the fan speed values on 8 bits, which wasn't
33  sufficient to cover the full range from very slow fans to very
34  fast fans with good accuracy. This is where fan divisors came
35  into play:
36
37  Case Fan:       0 RPM  (min =  998 RPM, div = 8)  ALARM
38  CPU Fan:     2500 RPM  (min = 1500 RPM, div = 4)
39
40  If your "sensors" output looks like the above, read below.
41
42  As a notable exception, the w83627ehf driver exposes fan divisor
43  values, but they are adjusted automatically by the driver, so you
44  shouldn't have to care.
45
46
47Fan Divisor Settings
48--------------------
49
50Fan Divisor     Minimum RPM     Maximum RPM
51-----------     -----------     -----------
52  1             5314            1350000
53  2             2657             675000         default on most chips
54  4             1328             337500
55  8              664             168750
56 16              332              84375         not supported on most chips
57 32              166              42187         ""
58 64               83              21093         ""
59128               41              10546         ""
60
61Pick a divisor so that the nominal RPM is about 50%
62above the minimum. This is a good compromise between
63margin and accuracy. Note that most chips only support
64fan divisors of 1, 2, 4, and 8.
65
66If you have a 0 RPM reading some or all of the time,
67increase the divisor until you get good readings.
68
69If you have a nominal reading less than 1.25 times the
70minimum, increase the divisor to give you margin
71so that you will not get spurious alarms.
72
73If you have a nominal reading more than 3 times the
74minimum, decrease the divisor to provide better
75accuracy.
76
77
78How to change fan divisors
79--------------------------
80
81Put an entry "set fanN_div X" in the appropriate section of
82/etc/sensors3.conf and run 'sensors -s'
83(N is the number of the fan, and X is the divisor you want).
84
85
86Further details
87---------------
88
89Fan divisors are quite confusing.
90
91Sensor chips count fan speed by using the fan signal
92to gate an 8-bit counter driven by a 22.5 kHz clock.
93So the _higher_ the counter value, the _slower_ the fan,
94and vice versa.
95
96The term 'fan divisor' is a misnomer because it doesn't divide
97the fan signal, it divides the 22.5 kHz clock.
98Thus you _increase_ the divisor if you have a slow fan.
99
100The drivers account for the 'fan divisor' in their calculation
101of RPM. So changing the fan divisor will NOT change the
102nominal RPM reading, it will only affect the minimum and maximum
103readings and the accuracy of the readings.
104
105The actual formula is RPM = (60 * 22500) / (count * divisor)
106
107The readings are most accurate when the fan speed is low
108(i.e., close to the minimum possible RPM reading).
109As fan speed gets closer to the maximum possible RPM reading,
110the reading becomes quite inaccurate. Fortunately, most computer
111fans do not approach 1,000,000 RPM !
112
113
114Fan readings 2X too high
115------------------------
116
117This is rare, but can happen. This is typically caused by a fan which
118outputs 4 pulses by revolution, instead of the standard 2.
119
120As we learned above, you can _not_ fix this by changing
121the fan divisor. You must add entries into the appropriate
122section of /etc/sensors3.conf:
123
124        compute fanN  @/2,  2*@
125
126(N is the number of the fan)
127
128
129
130------------------
131Copyright (c) 2000-2004 Mark D. Studebaker
132Copyright (c) 2006-2012 Jean Delvare
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