W83783S Datasheet, PDF (19/57 Pages) Winbond – MONITORING IC

W83783S
Preliminary
6.4 FAN Speed Count and FAN Speed Control
6.4.1 Fan speed count
Inputs are provides for signals from fans equipped with tachometer outputs. The level of these signals
should be set to TTL level, and maximum input voltage can 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 obtain the input specification. The normal circuit and trimming circuits are
shown as Figure 13.
Determine the fan counter according to:
Count = ^{1.35 × 106}
RPM × Divisor
In other words, the fan speed counter has been read from register CR28 or CR29 or CR2A, the fan
speed can be evaluated by the following equation.
R P M = ^{1.35 × 10 6}
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. That provides very low speed fan counter such as power supply fan. The
followed table is an example for the relation of divisor, PRM, and count.
Divisor
1
2 (default)
4
8
16
32
64
128
Nominal
PRM
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 1.
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 1999
Revision 0.55