SAM4E MCU Tour - 5. LED Breathing and PWM

In high frequency conditions, a good use of PWM is to control the output power by controlling the duty cycle, such as controlling the fan speed, the brightness of the LED light, etc. This time, we use the interrupt function of PWM to dynamically change the duty cycle of the pulse to achieve the effect of a breathing light.

1. Implementation ideas

PWM can choose to have the counter generate an interrupt at the end of the period (when aligned at the center of the period, it may also choose to generate an interrupt at the center of the period), and can dynamically adjust the duty cycle, period, polarity and other properties during operation. So you can dynamically change the duty cycle in the interrupt handler to change the brightness of the LED light. 
This time, channel 0 and pin PA0 will also be used.

2. PWM Setting

A higher frequency clock is needed here, so the main clock is divided by 32 (12.5 kHz). The counter period is 400, that is, the output pulse frequency is 125000/400 = 312.5 Hz. At the same time, the corresponding interrupt needs to be enabled. 
The main configuration code of PWM is as follows:

3. PWM interrupt processing

After each cycle, an interrupt is generated. Then in the interrupt handling function, change the duty cycle. It should be noted that when PWM is enabled, the duty cycle needs to be changed by writing to the PWM duty cycle modification register (PWM_CDTYUPD). By default, the modification takes effect in the next cycle.  
For better results, you can set an interval between two breaths. 
Note that the interrupt generated needs to be pulled low by reading PWM_ISR1. The interrupt handling function is posted in the complete code below.