W83627F Information Storage Devices, Inc, W83627F Datasheet - Page 35

• Manufacturer Part Number: W83627F
• Description: LPC Interface I/o Plus Kbc, Game Port, Midi Port
• Manufacturer: Information Storage Devices, Inc

2.4 FAN Speed Count and FAN Speed Control
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
2.4.
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.
2.4.1 Fan speed count
Pentium II
Bipolar Transistor
Temperature Sensor
Therminal
B
Diode
CPU
OR
C
E
2N3904
D+
D-
RPM
Count
Figure 2.3
Count
R=30K, 1%
RPM
1 35
- 31 -
1 35 10
.
.
R=30K, 1%
Divisor
C=3300pF
Divisor
10
C=3300pF
6
6
Publication Release Date:November 2002
VREF
VTINx
VTINx
W83627HF
W83627HF/F
Revision 2.0