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Problem of 3-Wire Fan Speed Measurement & Control

Question asked by NightSeas on May 13, 2012
Latest reply on May 14, 2012 by baird.hal.001
Recently I was designing a subsystem of closed loop fan control.  This diagram shows the method: using a channel of PWM to control the speed (GND method), and a channel of capture module to measure fan speed. The box on the left is a STM32 microcontroller.
FAN.jpg
The problem is the fan outputs speed wave only when PWM from STM32(GND of the fan) is high, so that I can't use PWM-Input method. Then how can I capture the speed wave at the right time and eliminate interferences.
WAVE.jpg
I wonder if there is a way to solve this problem.

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