Frequency Counter with 2.5 GHz Rating

The project utilizes an AVR controller that can be used as a counter by being more involved with a PIC which has synchronous counter input.

The PIC input can handle frequencies up to 40MHz. frequencies over half the clock frequency cannot be measured by AVR’s because they have a synchronous counter input which is sampled with the clock frequency. For instance using 4MHz, the input frequencies must be lower than 2MHz and to be on the safe side, 40% of the clock frequency should be used.

By starting an AVR counter, the input frequency can be measured and stopping the counter requires some predetermined time. The frequency will be determined by reading the counter register and accounting for the measurement time. To control the counter, a few logic circuits will be needed. This circuit will open or close the gate to the counter and will let through the extra counts to determine the end value of counter.

The gates are controlled by the AVR and extra pulses are fed into the second gate to determine the value of the 4020 counter. A FET switch was added to enable AVR to powerdown prescalers.

Rest of the story and avr/rfcounter/index.html