W83627DHG Datasheet, PDF (51/268 Pages) Winbond – WINBOND LPC I/O

W83627DHG
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 exceed +3.3 V. If the
tachometer signal exceeds +3.3 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 = ^{1.35 × 106}
Count × Divisor
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
0
0
0
0
BIT 1
0
0
1
1
BIT 0
0
1
0
1
FAN DIVISOR
1
2
4
8
BIT 2
1
1
1
1
BIT 1
0
0
1
1
BIT 0
0
1
0
1
FAN DIVISOR
16
32
64
128
The following table provides some examples of the relationship between divisor, RPM, and count.
DIVISOR
1
2 (default)
4
8
16
32
64
128
NOMINAL
RPM
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
COUNTS
153
153
153
153
153
153
153
153
70% RPM
6160
3080
1540
770
385
192
96
48
TIME FOR 70%
9.84 ms
19.48 ms
38.96 ms
77.92 ms
155.84 ms
311.68 ms
623.36 ms
1246.72 ms
Publication Release Date: Aug, 22, 2007
-39-
Version 1.4