LPC47M14X datasheet(120/205 Pages) SMSC

120 / 205 page

SMSC DS – LPC47M14X

Page 120

Rev. 03/19/2001

The fan tachometer input signal and clock source is shown below.

The counter is reset by the rising edge of each pulse (and by writing the preload register). The counter does not

wrap; if it reaches 0xFF, it remains at 0xFF until it is reset by the next pulse.

The 2 MSBs of the count are sampled and a PME or SMI is generated (if enabled through the PME_EN1 enable

corresponds to a count value of 192.

The fan count is determined according to the following equation:

1.966 x 10

Count =

RPM x Divisor

+ Preload

(Equation 1)

(Term 1)

Term 1 in the equation above is 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 default divisor, located in the Fan Control

The divisor for each fan is programmable via the Fan Control Register, which is located in the Runtime Register block

at offset

0xFA. The choices for the divisor are 1, 2, 4 and 8. The default value is 2. The factor of ½ in Term 1

corresponds to two pulses per revolution.

The preload value is programmable via the FAN1 Preload Register and FAN2 Preload Register. The preload is the

initial value for the fan count which is used to adjust the count such that the value of 192 corresponds to the “lower

limit” of the RPM. By setting the preload value and divisor properly, the PME or SMI will be generated when the RPM

reaches the desired percentage of the nominal RPM to indicate a fan failure.

A PME or SMI is generated, if enabled through the PME or SMI enable register, at a count of 192, which corresponds

to the “upper limit” for the fan count. This value is made to correspond to the “lower limit” of the RPM for the fan by

programming the divisor and preload value accordingly. Typical practice is to consider 70% of normal RPM a fan

failure, at which point Term 1 in Equation 1 for the example above will be 160. Therefore, the preload value is

chosen to be 32 so that when the count reaches 192, this will correspond to 70% of the normal RPM for the

generation of a PME or SMI.

F = 32.786kHz

÷ Divisor

Fan

Tachometer

Input

Clock Source

for Counter

R = Revolution Time = 60/RPM (sec)

P = Pulse Time = TR/2

(Two Pulses Per Revolution)

LPC47M14X Datasheet(PDF) 120 Page - SMSC Corporation