W83637HF Winbond Electronics Corp America, W83637HF Datasheet - Page 34

• Manufacturer Part Number: W83637HF
• Description: Description = LPC I/o Plus H/w Monitor, Smart Fan Control, Smart Card (PC/SC) Reader, Memory Stick Reader I/F, Sd/mmc Card Reader I/f ;; Package = QFP 128
• Manufacturer: Winbond Electronics Corp America

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
6.4.
Determine the fan counter according to:
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.
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.
Pentium II/III
Bipolar Transistor
Temperature Sensor
B
CPU
OR
Therminal
Diode
C
E
2N3904
D+
D-
RPM
Count
R=30K,1%
=
Figure 6.3
=
Count
C=3300pF
RPM Divisor
1 35 10
1 35 10
.
.
- 29 -
R=30K,1%
×
×
×
×
C=3300pF
Divisor
6
6
VREF
SYSTIN
CPUTIN
CPUD-
Publication Release Date: June 25, 2003
W83637HF
W83637HF
Revision 1.3