W83627EHF Datasheet, PDF (35/141 Pages) Winbond – evolving product from Winbonds most popular I/O family

W83627EHF/EF, W83627EHG/EG
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 +3.3V. If the input signals from
the tachometer outputs are over the +3.3V, 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:
Count = ^{1.35 ×106}
RPM × Divisor
In other words, the fan speed counter has been read from register Bank0 Index 28h, 29h, 2Ah ,3Fh
and Bank5 53h, the fan speed can be evaluated by the following equation.
RPM = ^{1.35 × 106}
Count × Divisor
The default divisor is 2 and defined at Bank0 Index 47h.bit7~4, Index 4Bh.bit7~6, Index 4Ch.bit7,
Index 59h.bit7.bit3~2 and Index 5Dh.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, 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
Table 6.2
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: January 18, 2006
-29-
Revision 1.0