W83L785R Datasheet, PDF (15/38 Pages) Winbond – WINBOND H/W MONITORING IC FOR NOTEBOOK

W83L785R/W83L785G
6.6 FAN Speed Count and FAN Speed Control
6.6.1 Fan speed count
Inputs are provided by the signals from fans equipped with tachometer outputs. The level of these
signals should be set to TTL level, and maximum input voltage cannot be over VCC. 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 7.
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, 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.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, RPM, and count.
Table 2
DIVISOR NOMINAL RPM
1
2 (default)
4
8
16
32
64
128
+12V
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
Pull-up resister
4.7K Ohms
+12V
FAN Out
GND
22K~30K
Fan Input
Pin 1
Pin 2
10K
W83L785R
COUNTS
153
153
153
153
153
153
153
153
+12V
+12V
FAN Out
GND
FAN
Connector
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
Pull-up resister < 1K
or totem-pole output
> 1K
Fan Input
Pin 1
Pin 2
3.3V Zener
W83L785R
Figure 7-1. Fan with Tach Pull-Up to +12V, or Totern-Pole
Output and Register Attenuator
Figure 7-2. Fan with Tach Pull-Up to +12V, or
Totem-Pole Putput and Zener Clamp
- 11 -
Publication Release Date: Apr. 2006
Revision 1.1