21.3.3.1.6 Input Capture Frequency and Pulse-Width Measurement Mode

In the Input Capture Frequency and Pulse-Width Measurement mode, the timer will start counting when a positive edge is detected on the event input signal. The count value is captured on the following falling edge. The counter stops when the second rising edge of the event input signal is detected. This will set the interrupt flag.

The CAPT Interrupt flag is automatically cleared after the low byte of the Compare/Capture (TCBn.CCMP) register has been read, and the timer/counter is ready for a new capture sequence. Therefore, the Count (TCBn.CNT) register must be read before the Compare/Capture (TCBn.CCMP) register, since it is reset to BOTTOM at the next positive edge of the event input signal.