The PCA channel 0 ISR is a little more complicated as it is measuring the PRX low time by capturing the falling and rising edges and is entered twice. The ISR first determines if the event was from a falling edge by measuring the pin value. If low, the ISR then stores the captured timer value and exits and waits for the next edge. On the next trigger event, which would be a rising edge, the ISR once again stores the captured timer value and then determines the time interval between edges by subtracting the falling edge capture from the rising edge capture. This calculation provides the measure of the reflected energy. In the case of the three LED example, once the value of the reflected energy is measured for the current LED the ISR also moves the external gate enable to the next LED in the sequence and the cycle repeats.