LPC47M192 datasheet(123/228 Pages) SMSC

123 / 228 page

SMSC DS – LPC47M192

Page 123

Rev. 03/30/05

DATASHEET

are monitored by using the Fan Tachometer registers. These signals, as well as the Fan Tachometer registers, are

described below (see also section 8

− RUNTIME REGISTERS).

Fan Tachometer Inputs

A fan tachometer input is used to measure the speed at which a fan is rotating. The fan tachometer input is a train of

square pulses with a 50% duty cycle (see FIGURE 8) that are derived from the magnetic fields generated by the

rotating rotor of the fan. The speed of the fan can be determined by calculating the period of the Fan Tachometer

input pulse.

Note:

All calculations are based on fans that emit 2 square pulses per revolution.

Fan Tachometer Input

Clock Source for Counter

R = Revolution Time = 60/RPM (sec)

P = Pulse Time = TR/2

(Two Pulses Per Revolution)

F = 32.786kHz / Divisor

FIGURE 8

− FAN TACHOMETER INPUT AND CLOCK SOURCE

The counter is used to determine the period of the Fan Tachometer input pulse. This counter, if reset on the rising

edge of the fan tachometer input pulse, measures the number of clock pulses generated by the clock source for the

duration of one fan tachometer input pulse. Since two fan tachometer input pulses are generated per revolution of

the fan rotor, the speed of the fan is easily calculated. Everytime the fan tachometer input resets the counter, it also

latches the value of the counter into the Fan Tachometer register. This value is used by the operating system to

monitor the speed of the fan.

Fan Monitoring Logic

The clock source for the counter is the output of the programmable divider whose input is the 32.768kHz oscillator

(see figure below).

The counter is reset by the rising edge of each fan tachometer input pulse, by itself if it reaches its’ maximum count of

0xFF, or by writing to the preload register.

If the fan is functioning, a value is preloaded into the counter on the rising edge of the Fan Tachometer input. The

counter counts the number of clock pulses generated for the duration of one Fan Tachometer input pulse. This value

(Count) is then stored into the Fan Tachometer register. If the fan is operating at 70% (or less) of the maximum

capacity, the Count value will be greater than or equal to 192 and thereby generate a PME# signal. To ensure the

Fan Tachometer register has been updated if the fan suddenly seizes, a second PME is generated when the counter

reaches 0xFF and the Fan Tachometer register is latched again. The Fan Tachometer register will remain at this

value until another Fan Tachometer input pulse is received.

A representation of the logic for the fan tachometer implementation is shown below.

LPC47M192 Datasheet(PDF) 123 Page - SMSC Corporation