W83627HF/ F/ HG/ G
VREF
R=30K, 1%
Bipolar Transistor
Temperature Sensor
VTINx
C=3300pF
C
B
2N3904
W83627HF
E
R=30K, 1%
OR
Pentium II
CPU
D+
VTINx
Therminal
Diode
C=3300pF
D-
Figure. 8.3
7.4
FAN Speed Count and FAN Speed Control
7.4.1 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
8.4.
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
Count × Divisor
RPM =
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 fol-
lowed table is an example for the relation of divisor, PRM, and count.
Publication Release Date: June 09, 2006
- 35 -
Revision 2.27
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