W83791G Datasheet, PDF (27/99 Pages) Winbond – Winbond H/W Monitoring IC

W83791D/G
6.7 FAN Speed Count and FAN Speed Control
6.7.1 Fan speed count
W83791D/G support 5 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:
Count = ^1.35×106
RPM × Divisor
In other words, if the fan speed counter has been read from register CR [28] or CR [29] or CR [2A] or
CR [BA] or CR [BB] , then the fan speed can be evaluated by the following equation.
RPM = ^{1.35 ×106}
Count × Divisor
The default divisor is 2 and defined at CR47.bit7~4, CR4B.bit7~6, and Bank0 CR5D.bit5~7 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
1
2 (default)
4
8
16
32
64
128
NOMINAL RPM
8800
4400
2200
1100
550
275
137
68
TIME PER
REVOLUTION
COUNTS
6.82 ms
153
13.64 ms
153
27.27 ms
153
54.54 ms
153
109.08 ms
153
218.16 ms
153
436.32 ms
153
872.64 ms
153
Table 2
70% RPM
6160
3080
1540
770
385
192
96
48
TIME FOR
70%
9.74 ms
19.48 ms
38.96 ms
77.92 ms
155.84 ms
311.68 ms
623.36 ms
1246.72 ms
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Publication Release Date: April 14, 2006
Revision 1.1