ATmega328 Timer 1 & Timer 2 are programmed to count rising edge pulses of input rectangular waveform within a period of 1000 ms. The total count represents the frequency of the input signal.

Link to code:
https://akuzechie.blogspot.com/2021/0...

Contents:
0:00 Introduction
0:19 Theory of Frequency Counting
1:15 Block Diagram
1:39 Software Flowchart
2:53 C++ Sketch
5:56 Demonstration