This project shows how to build a very simple yet very useful tool that every DIY enthusiast should have in his lab: a 100MHz+ frequency counter.
The schematic is fairly simple and straightforward and uses a PIC16F628A microcontroller for measuring frequency and a high speed comparator for signal amplification and conditioning.
The microcontroller uses its internal 4MHz oscillator for the CPU clock. Timer1 uses an external crystal resonator (watch crystal) with 32768Hz frequency for setting the 1 second time base.
Timer0 is used to count the input signal at pin RA4.
100MHZ Frequency Counter with PIC16F628A – [Link]