A representation of the logic for the fan tachometer implementation is shown below.
Preload
Programmable
Divider
32 kHz
Counter
1, 2, 4, 8
MSB
To nPME
Logic
Sync
Latch on Read
The following tables show examples of the desired functionality. Counts are based on 2 pulses per revolution
tachometer outputs with a default divisor of 2.
TERM 1 FOR “DIVIDE
BY 2” (DEFAULT) IN
DECIMAL
COUNT =
TIME PER
(TERM 1) +
REVOLUTION
PRELOAD
RPM
4400
3080
2640
2204
PRELOAD
COMMENTS
Typical RPM
70% RPM
60% RPM
50% RPM
13.64 ms
19.48 ms
22.73 ms
27.22 ms
112 counts
160 counts
186 counts
223 counts
32
32
32
32
144
192
218
255
(maximum count)
COUNTS FOR
THE GIVEN
SPEED IN
DECIMAL
TIME PER
REVOLUTION
FOR 70% RPM
MODE
NOMINAL
RPM
TIME PER
SELECT
REVOLUTION
PRELOAD
70% RPM
6160
Divide by 1
Divide by 2
Divide by 4
Divide by 8
8800
4400
2200
1100
6.82 ms
13.64 ms
27.27 ms
54.54 ms
32
32
32
32
144
144
144
144
9.74 ms
19.48 ms
38.96 ms
77.92 ms
3080
1540
770
Pins 51 and 52 are the fan tachometer inputs, FAN_TACH2 and FAN_TACH1, respectively.
The configuration registers for the fan tachometer inputs are defined in the “Runtime Registers” section.
SMSC DS – LPC47M14X
Page 121
Rev. 03/19/2001
SMSC [ SMSC CORPORATION ]
