W83L784R_05 Datasheet, PDF (19/52 Pages) Winbond – Monitoring IC

W83L784R/ W83L784G
6.5.1 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
15.
Determine the fan counter according to:
1.35 × 10⁶
Count = _{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.
1.35 × 10⁶
RPM = _{Count × Divisor}
The default divisor is 2 and defined at CR49.bit0~2, bit4~6 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.
Table 2
DIVISOR NOMINAL PRM
1
2 (default)
4
8
16
32
64
128
8800
4400
2200
1100
550
275
137
68
TIME PER
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
COUNTS
153
153
153
153
153
153
153
153
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: Jan. 2005
Revision 1.1