LPC47S457-NC SMSC [SMSC Corporation], LPC47S457-NC Datasheet - Page 127

• Manufacturer Part Number: LPC47S457-NC
• Description: Advanced I/O with X-Bus Interface
• Manufacturer: SMSC [SMSC Corporation]

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.
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).
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.
SMSC LPC47S45x
Divide by 1
Divide by 2
Divide by 4
Divide by 8
RPM
4400
3080
2640
2204
SELECT
MODE
13.64 ms
19.48 ms
22.73 ms
27.22 ms
REVOLUTION
TIME PER
8800
4400
2200
1100
NOMINAL
RPM
112 counts
160 counts
186 counts
223 counts
6.82 ms
13.64 ms
27.27 ms
54.54 ms
TERM 1 FOR “DIVIDE BY
REVOLUTION
TIME PER
2” (DEFAULT) IN
DECIMAL
DATASHEET
Page 127 of 259
32
32
32
32
PRELOAD
32
32
32
32
PRELOAD
144
144
144
144
SPEED IN
DECIMAL
FOR THE
COUNTS
GIVEN
The following tables show examples of the
144
192
218
255
(maximum count)
(TERM 1) +
PRELOAD
COUNT =
6160
3080
1540
770
70% RPM
9.74 ms
19.48 ms
38.96 ms
77.92 ms
FOR 70% RPM
REVOLUTION
Typical RPM
70% RPM
60% RPM
50% RPM
COMMENTS
TIME PER
Rev. 06-01-06