Notice in the examples shown below the Preload value was determined to be 32 for a fan that normally operates at
4400 RPMs. This preload value was determined for the case that a PME should be initiated at 70% of normal
operation. A PME is generated when the counter reaches 192, but at 70% in this example only 160 counts are
generated during one Fan Tachometer input pulse leaving a difference of 32.
Examples
Term 1 in the equation above is ½ of the value determined by multiplying the clock source of 32.768kHz by
60sec/min and dividing by the product of the revolutions per minute times the divisor. The factor of ½ in Term 1
corresponds to two pulses per revolution. The default divisor, located in the Fan Control Register, is two. This results
in a value for Term 1 in Equation 1 of 111.70 (~112) for a 4400 RPM. The following tables show examples of the
desired functionality.
Note: All calculations of Count were based on tachometer inputs with two pulses per revolution and on the clock
source input divided down by two (default settings).
COUNT =
TERM 1 FOR “DIVIDE BY
2” (DEFAULT) IN
DECIMAL
TIME PER
REVOLUTION
RPM
PRELOAD
COMMENTS
(TERM 1) +
PRELOAD
4400
3080
2640
2204
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
Typical RPM
70% RPM
60% RPM
50% RPM
192
218
255
(maximum count)
The divisor for each fan is programmable via the Fan Control Register, Logical Device 8, 0xFA. The choices for the
divisor are 1, 2, 4 and 8. The default value is 2.
COUNTS
FOR THE
GIVEN
SPEED IN
DECIMAL
TIME PER
REVOLUTION
FOR 70% RPM
MODE
SELECT
NOMINAL
RPM
TIME PER
REVOLUTION
PRELOAD
70% RPM
Divide by 1 8800
Divide by 2 4400
Divide by 4 2200
Divide by 8 1100
6.82 ms
32
32
32
32
144
6160
3080
1540
770
9.74 ms
13.64 ms
27.27 ms
54.54 ms
144
144
144
19.48 ms
38.96 ms
77.92 ms
SMSC LPC47S45x
Page 127 of 259
Rev. 06-01-06
DATASHEET
SMSC [ SMSC CORPORATION ]
