In principle, it is the application of an adding counter, which is generally divided into sampling pulses and pulses to be measured. The sampling pulses are generally obtained by frequency division of the oscillator to ensure the stability of the sampling frequency. The sampling frequency can be set to 1Hz. There are two channels, 100Hz and 100Hz, which can be triggered by rising edge or falling edge. The frequency of 1Hz is used to measure frequencies above 50Hz.

The period of the 1Hz sampling pulse is exactly 1s. The frequency of the pulse to be measured is obtained by calculating the number of pulses to be measured within 1 second through the accumulator.

The pulse to be measured is a frequency waveform shaped by the circuit. For example, the AC current can be shaped into a square wave pulse with the same frequency using a zero-crossing trigger.

The 100Hz sampling pulse is used to measure frequencies below 50Hz. At this time, it is necessary to calculate the number of sampling pulses within one cycle of the pulse to be measured. The sampling pulse is 100Hz. If it is within one cycle of the pulse to be measured, Within, the number of sampling pulses is 200, then 100Hz/200=0.5Hz, which means the frequency to be measured is 0.5Hz, but this method is simpler to implement using a microcontroller.