W83792AD/AG/D/G
7.7 FAN Speed Count and FAN Speed Control
7.7.1 Fan speed count
W83792D/G support 6 sets of fan counting. Fan inputs are provided for signals from fans equipped
with tachometer outputs. The level of these signals should be set to TTL level, and the maximum input
voltage should not be over +5.5V. If the input signals from the tachometer outputs are over the VCC,
the external trimming circuit should be added to reduce the voltage to meet the input specification. The
normal circuit and trimming circuits are shown as Figure 8.
Determine the fan counter according to:
1.35×106
Count =
RPM × Divisor
In other words, if the fan speed counter has been read from register CR [28], CR [29], CR [2A], CR
[B8], CR [B9] or CR [BA] , then the fan speed can be evaluated by the following equation.
1.35×106
RPM =
Count × Divisor
The default divisor is 2 and defined at CR47, CR5B, and CR5C which are three bits for divisor. This
provides very low speed fan counter such as power supply fan. The followed table is an example for
the relation of divisor, RPM, and count.
DIVISOR
NOMINAL RPM
TIME PER
COUNTS
REVOLUTION
1
8800
4400
2200
1100
550
6.82 ms
13.64 ms
27.27 ms
54.54 ms
109.08 ms
218.16 ms
436.32 ms
872.64 ms
153
153
153
153
153
153
153
153
2 (default)
4
8
16
32
64
128
275
137
68
Table 2.
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