W83791D Datasheet, PDF (28/46 Pages) Winbond – W83791D Winbond H/W Monitoring IC

W83791D
6.7
FAN Speed Count and FAN Speed Control
Preliminary
6.7.1 Fan speed count
W83791D 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
Revolution
6.82 ms
13.64 ms
27.27 ms
54.54 ms
109.08 ms
218.16 ms
436.32 ms
872.64 ms
per Counts
153
153
153
153
153
153
153
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
21
Publication Release Date: Aug, 2001
Revision 0.41