68.6.6 PWM External Trigger Mode
The PWM channels 1 and 2 can be configured to use an external trigger for generating specific PWM signals to provide functions such as DC/DC converters, etc.
The external trigger source can be selected through the bit TRGSRC of the PWM External Trigger Register (see the table below).
| Channel | Trigger Source Selection | Trigger Source |
|---|---|---|
| 1 | PWM_ETRG1.TRGSRC = 0 | From PWMEXTRG1 input |
| PWM_ETRG1.TRGSRC = 1 | From Analog Comparator Controller | |
| 2 | PWM_ETRG2.TRGSRC = 0 | From PWMEXTRG2 input |
| PWM_ETRG2.TRGSRC = 1 | From Analog Comparator Controller |
Each external trigger source can be filtered by writing a one to PWM_ETRGx.TRGFILT.
When the external trigger event is detected, the internal counter of the PWM channel can be modified when conditions are met, depending on the value of the PWM_ETRGx.TRGMODE field.
Each time an external trigger event is detected, the corresponding PWM channel counter value is stored in PWM_ETRGx.MAXCNT if it is greater than the previously stored value. Reading PWM_ETRGx clears the MAXCNT value.
To adapt to different use cases, three different external trigger mode modes (ETM) are available for channels 1 and 2 depending on the value of the PWM_ETRGx.TRGMODE field:
- TRGMODE = 1: External PWM Reset Mode (see External PWM Reset Mode)
- TRGMODE = 2: External PWM Start Mode (see External PWM Start Mode)
- TRGMODE = 3: Cycle-By-Cycle Duty Mode (see Cycle-By-Cycle Duty Mode)
The ETM feature is disabled when PWM_ETRGx.TRGMODE = 0.
The ETM mode can be associated with the recoverable fault mode (PWM_ETRGx.RFEN=1) to manage specific use case (see Cycle-By-Cycle Duty Mode: LED String Control).
The use cases described in the figures External PWM Start Mode: Buck DC/DC Converter and Cycle-By-Cycle Duty Mode are managed by PWM_ETRGx.RFEN=0.
The ETM must be enabled only if the corresponding channel is left-aligned (CALG = 0 in PWM Channel Mode Register of channel 1 or 2) and not managed as a synchronous channel (SYNCx = 0 in PWM Sync Channels Mode Register where x = 1 or 2). Programming the channel to be center-aligned or synchronous while TRGMODE is not 0 is forbidden.