In order to achieve the maximum possible resolution, the input signal is probed for 0.125 seconds and the prescaler value is computed accordingly. I checked with two different PIC16F628A and they easily go over 200Mhz barrier. So the maximum frequency can be between 100Mhz and 256MHz. On the other hand, in the datasheet of 16F628A there is a requirement for the input pulse at RA4 to be with minimum width of 10ns which is 100MHz frequency. In theory this can allow the input signal to be up to 256MHz. The max frequency of Timer0 is 1/4 of the CPU clock which is 1MHz, but there is internal prescaler and it can be set from 1 to 256. Timer0 is used to count the input signal at pin RA4. Timer1 uses an external crystal resonator (watch crystal) with 32768Hz frequency for setting the 1 second time base. The microcontroller uses its internal 4MHz oscillator for the CPU clock. 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. 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.
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