Something you might have noticed about the PWM code I posted is that the sinusoidal waveform is generated by a busy-wait loop in the main function. It sets the PWM to the right level, sleeps, then does it again. This is fine for a demo, but obviously inappropriate if the controller needs to be doing other things at the same time: communications, measurements, calculations, receiving and processing user commands, anything. Zephyr, being a thorough RTOS, has excellent support for asynchronous stuff. For instance, using a timer to update the PWM would be an improvement. Let’s do that:

The timer generates an interrupt when it expires, but it’s best to not do too much in the handler. The example code in the timer docs submits to the system workqueue, which seems like good practice. So we’re getting into threads as well.

The actual code is fairly straightforward. A timer gets defined with a handler, and is started with the desired period during initialization. We also define a work to be able to put into the system workqueue. The timer handler just submits the work and exits. The work callback that actually does the calculations and PWM setting executes out of the interrupt context. That callback gets all the code we had in the main loop before. Finally, the main loop is now empty, and just sleeps forever. Simple, better, and seems to work.

#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>
#include <zephyr/device.h>
#include <zephyr/drivers/pwm.h>
#include <math.h>

static const struct pwm_dt_spec custompwm0 = PWM_DT_SPEC_GET(DT_ALIAS(mycustompwm));


#define PWM_FREQ 10000 // Hz
#define CYCLE_FREQ 0.3 // Hz

#define STEPS 180
float levels[STEPS]; // holds duty cycles, duty = what fraction of time HS switch is on
#define DUTY_AVG 0.5f // 0. to 1.
#define DUTY_RANGE 0.90f // 0. to 1.
#define DEADTIME_NS 500U

#define TWO_PI 6.28318530718f

void step_handler(struct k_work *work)
{
	static uint32_t count = 0;
	static uint32_t oldpulsewidth_ns = 0;

	uint32_t pulsewidth_ns = PWM_HZ(PWM_FREQ)*levels[count++ % STEPS]; 
	uint32_t ret = pwm_set_dt(&custompwm0, 
		PWM_HZ(PWM_FREQ), 
		pulsewidth_ns);
	if (pulsewidth_ns > oldpulsewidth_ns) {
		ret |= pwm_set(custompwm0.dev, 1,
			PWM_HZ(PWM_FREQ), 
			pulsewidth_ns + DEADTIME_NS, PWM_POLARITY_INVERTED);
		ret |= pwm_set(custompwm0.dev, 2,
			PWM_HZ(PWM_FREQ), 
			pulsewidth_ns - DEADTIME_NS, PWM_POLARITY_NORMAL);
	} else {
		ret |= pwm_set(custompwm0.dev, 2,
			PWM_HZ(PWM_FREQ), 
			pulsewidth_ns - DEADTIME_NS, PWM_POLARITY_NORMAL);
		ret |= pwm_set(custompwm0.dev, 1,
			PWM_HZ(PWM_FREQ), 
			pulsewidth_ns + DEADTIME_NS, PWM_POLARITY_INVERTED);
	}
	oldpulsewidth_ns = pulsewidth_ns;
	if (ret) {
		printk("Error %d: failed to set pulse width\n", ret);
	}
}

K_WORK_DEFINE(step_work, step_handler);

void step_timer_handler(struct k_timer *dummy)
{
    k_work_submit(&step_work);
}

K_TIMER_DEFINE(step_timer, step_timer_handler, NULL);

int main(void)
{
	uint32_t ret;

	if (!device_is_ready(custompwm0.dev)) {
		printk("Error: PWM device %s is not ready\n",
		       custompwm0.dev->name);
		return 0;
	}

	for (int i =0; i < STEPS; i++) {
		levels[i] = DUTY_AVG*(1 + DUTY_RANGE*sin(TWO_PI*i/STEPS));
	}

	k_timer_start(&step_timer, K_USEC(0U), K_USEC(1000000U/(CYCLE_FREQ*STEPS)));

	k_sleep(K_FOREVER);
	return 0;
}