External PWM Start mode is selected by programming TRGMODE = 2 in the PWM_ETRGx register.
In this mode, the internal PWM counter can only be reset once it has reached the CPRD value in the PWM Channel Period Register and when the correct level is detected on the corresponding external trigger input. Both conditions have to be met to start a new PWM period. The active detection level is defined by the bit TRGEDGE of the PWM_ETRGx register.
Note that this mode guarantees a constant tON time and a minimum tOFF time.
Figure 1. External PWM Start Mode
