Winbond W83627DHG, Manual

Page: 51 / 268

W83627DHG
Publication Release Date: Aug, 22, 2007
-39- Version 1.4
7.6 Fan Speed Measurement and Control
This section is divided into two parts, one to measure the speed and one to control the speed.
7.6.1 Fan Speed Measurement
The W83627DHG can measure fan speed for fans equipped with tachometer outputs. The tachometer
signals should be set to TTL-level, and the maximum input voltage cannot 3.3 V. If the
tachometer signal e3.3 V, an external trimming circuit should be added to reduce the voltage
accordingly.
The fan speed counter is read from Bank0 Index 28h, 29h, 2Ah, and 3Fh and Bank5 Index 53h. The fan
speed can then be evaluated by the following equation:
RPM
Count Divisor
=
×
×
1 35 10
6
.
The default divisor is 2 and is specified at Bank0 Index 47h, bits 7 ~ 4; Index 4Bh, bits 7 ~ 6; Index 4Ch,
bit 7; Index 59h, bit 7 and bits 3 ~ 2; and Index 5Dh, bits 5 ~ 7. There are three bits for each divisor, and
the corresponding divisor is listed in the table below.
BIT 2 BIT 1 BIT 0 FAN DIVISOR BIT 2 BIT 1 BIT 0 FAN DIVISOR
0 0 0
1
1 0 0
16
0 0 1
2
1 0 1
32
0 1 0
4
1 1 0
64
0 1 1
8
1 1 1
128
The following table provides some examples of the relationship between divisor, RPM, and count.
DIVISOR
NOMINAL
RPM
TIME PER
REVOLUTION
COUNTS 70% RPM TIME FOR 70%
1 8800 6.82 ms
153
6160 9.84 ms
2 (default)
4400 13.64 ms
153
3080 19.48 ms
4 2200 27.27 ms
153
1540 38.96 ms
8 1100 54.54 ms
153
770 77.92 ms
16 550 109.08 ms
153
385 155.84 ms
32 275 218.16 ms
153
192 311.68 ms
64 137 436.32 ms
153
96 623.36 ms
128 68 872.64 ms
153
48 1246.72 ms