这个可以用GATE计数功能。手册(https://ww1.microchip.com/downloads/en/DeviceDoc/40001609E.pdf)章节:
19.6.3 TIMER1 GATE TOGGLE MODE
When Timer1 Gate Toggle mode is enabled, it is possi-
ble to measure the full-cycle length of a Timer1 gate
signal, as opposed to the duration of a single level
pulse.
19.6.4 TIMER1 GATE SINGLE-PULSE
MODE
When Timer1 Gate Single-Pulse mode is enabled, it is
possible to capture a single pulse gate event.