how does a microcontroller work!

I'm doing some study on PID AC motor controller.
I have a commercial PID motor controller (AC/single phase/220V/50Hz) in hand. It is based on 80535A. I just traced it and drawn the schematic. I noticed an interesting thing that the RPM sensor is not connected to an INT port. It is just connected to a standard IO0 port. Seems RPM signal is converted to analog signal. My question is, how it is then counting RPM?

I wanted to attach the sensor part here. Anybody can explain?

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