Environmental Monitoring and Control Device
Datasheet
7.4.4 HOTTEST OPTION
If the “Hottest” option is chosen (101 or 110), then the fan is controlled by the limits and parameters
associated with the zone that requires the highest PWM duty cycle value, as calculated by the auto fan
algorithm.
7.5
Fan Speed Monitoring
The chip monitors the speed of the fans by utilizing fan tachometer input signals from fans equipped with
tachometer outputs. The fan tachometer inputs are monitored by using the fan tachometer registers.
These signals, as well as the Fan Tachometer registers, are described below.
7.5.1 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 7.3) 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. Reading registers reflect
a count value for one complete revolution (2 pulses).
TR
Fan Tachometer Input
TR = Revolution Time = 60/RPM (sec)
TP
TP = Pulse Time = TR/2
(Two Pulses Per Revolution)
Clock Source for Counter
F = 90kHz
FIGURE 7.3 - FAN TACHOMETER INPUT AND CLOCK SOURCE
The counter is used to determine the period of the Fan Tachometer input pulse. This counter is reset on
the rising edge of every other fan tachometer input pulse, and thus measures the number of clock pulses
generated by the clock source for the duration of one fan tachometer revolution. Since two fan tachometer
input pulses are generated per revolution of the fan rotor, the speed of the fan is easily calculated. The fan
tachometer input resets the counter on every other pulse and simultaneously loads the count into its
respective reading register. This value is used by the operating system to monitor the speed of the fan.
The fan tachometer reading registers contain the number of 11.111us periods (90kHz) between full fan
revolutions. Fans produce 2 pulses per revolution.
The Tachometer Reading registers are 16 bits. The value FFFFh indicates that the fan is not spinning, or
the tachometer input is not connected to a valid signal (this could be triggered by a counter overflow).
These registers are read only – a write to these registers has no effect.
The Fan Tachometer Reading registers contain the number of periods of a full tachometer revolution
(every two pulses). These registers are updated at least once every second. The frequency of the clock
source for the tachometer logic is 90kHz. This register is latched on the rising edge of every other fan
tachometer pulse and when the fan count reaches FFFFh. This latter condition is the stalled fan event.
SMSC EMC6D100/EMC6D101
Page 35
Rev. 09-09-04
DATASHEET
SMSC [ SMSC CORPORATION ]
