F71869A
T OVT
T Hhys
T HIGH
T Hhys
PME#
(pulse mode)
*
*
*
*
*Interrupt Reset when Interrupt Status Registers are written 1
Figure 4 Hysteresis mode illustration
5.2.9. Fan Speed Count
Inputs are provided by the signals from fans equipped with tachometer outputs. The level of
these signals should be set to TTL level, and maximum input voltage cannot be over 5V. If the
input signals from the tachometer outputs are over the 5V, the external trimming circuit should
be added to reduce the voltage to obtain the input specification.
Determine the fan counter according to:
1.5×106
Count =
RPM
In other words, the fan speed counter has been read from register, the fan speed can be
evaluated by the following equation. As for fan, it would be best to use 2 pulses tachometer
output per round.
1.5×106
RPM =
Count
As the register description of datasheet, the parameter “Count” register provides 12-bit
resolution for RPM counting. In Fintek design, the value of parameter “Count” is from 4096 ~
64 (5 bit filter). Therefore the RPM measure capability is from 366 ~ 23438 rpm.
Above example is for 2 pulses tachometer (Normal 4 Phases fan) output per round. If you
use 8 Phases fan, means output 4 pulses per round. The RPM measure capability is from 183
~ 11719 rpm.
5.2.10. Fan Speed Control
The F71869A provides 2 fan speed control methods: one is DAC FAN control and the other
is PWM duty cycle.
1. DAC Fan Control
26
Oct., 2011
V0.19P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
