LPC47M14X View Datasheet(PDF) - SMSC -> Microchip
Part Name
Description
MFG CO.
'LPC47M14X' PDF : 205 Pages View PDF
A representation of the logic for the fan tachometer implementation is shown below.
32 kHz
Preload
Programmable
Divider
1, 2, 4, 8
Counter
MSB
Latch on Read
Sync
To nPME
Logic
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.
RPM
4400
3080
2640
2204
TIME PER
REVOLUTION
13.64 ms
19.48 ms
22.73 ms
27.22 ms
TERM 1 FOR “DIVIDE
BY 2” (DEFAULT) IN
DECIMAL
112 counts
160 counts
186 counts
223 counts
PRELOAD
32
32
32
32
COUNT =
(TERM 1) +
PRELOAD
144
192
218
255
(maximum count)
COMMENTS
Typical RPM
70% RPM
60% RPM
50% RPM
MODE
SELECT
Divide by 1
Divide by 2
Divide by 4
Divide by 8
NOMINAL
RPM
8800
4400
2200
1100
TIME PER
REVOLUTION
6.82 ms
13.64 ms
27.27 ms
54.54 ms
PRELOAD
32
32
32
32
COUNTS FOR
THE GIVEN
SPEED IN
DECIMAL
144
144
144
144
70% RPM
6160
3080
1540
770
TIME PER
REVOLUTION
FOR 70% RPM
9.74 ms
19.48 ms
38.96 ms
77.92 ms
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
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