21.1.6.4 High and Low Measurement Mode
This mode measures the high and low pulse time of the signal relative to the SMT clock. It begins incrementing the SMTxTMR on a rising edge on the input signal, then updates the SMTxCPW register with the value and resets the SMTxTMR on a falling edge, starting to increment again. Upon observing another rising edge, it updates the SMTxCPR register with its current value and once again resets the SMTxTMR value and begins incrementing again. See the figures below.