The setup for Measure Input Period to measure the input period can also be used to measure the frequency of an unknown clock. Set up the timer as per Table 2-6. But instead of selecting a system oscillator as a timer clock, connect the unknown signal from the I/O pin (PPS input) or CLC. Then select the ERS input as a pulse train or clock of known period TSAMPLE (for example, the output of a crystal-clocked timer or a Pulse-Width Modulator (PWM)). The CIF interrupt will occur with each cycle of ERS, and the frequency of the input is then the captured value in TUxyCR/TSAMPLE.
As an example, suppose the ERS-input timer is set to overflow 100 times every second, making TSAMPLE = 10 ms. Let’s say that when CIF occurs, the value of TUxyCR = 7300. Upon calculation, the input frequency comes out to be 7300/10 ms = 730 kHz.