Fade LED

The LED starts off increases its brightness until it is fully or nearly fully on. The brightness then decreases until the LED is off, completing on fade cycle. Each cycle takes 2.5 seconds, and the cycles repeat forever. The PWM period is taken from Devicetree. It should be fast enough to be above the flicker fusion threshold.

cd ~/zephyrproject/zephyr
west build -b rpi_pico samples/basic/fade_led

Demo

Code

/*
 * Copyright (c) 2016 Intel Corporation
 * Copyright (c) 2020 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: Apache-2.0
 */

/**
 * @file Sample app to demonstrate PWM.
 */

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

static const struct pwm_dt_spec pwm_led0 = PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0));

#define MIN_PERIOD PWM_SEC(1U) / 128U
#define MAX_PERIOD PWM_SEC(1U)

int main(void)
{
	uint32_t max_period;
	uint32_t period;
	uint8_t dir = 0U;
	int ret;

	printk("PWM-based blinky\n");

	if (!pwm_is_ready_dt(&pwm_led0)) {
		printk("Error: PWM device %s is not ready\n",
		       pwm_led0.dev->name);
		return 0;
	}

	/*
	 * In case the default MAX_PERIOD value cannot be set for
	 * some PWM hardware, decrease its value until it can.
	 *
	 * Keep its value at least MIN_PERIOD * 4 to make sure
	 * the sample changes frequency at least once.
	 */
	printk("Calibrating for channel %d...\n", pwm_led0.channel);
	max_period = MAX_PERIOD;
	while (pwm_set_dt(&pwm_led0, max_period, max_period / 2U)) {
		max_period /= 2U;
		if (max_period < (4U * MIN_PERIOD)) {
			printk("Error: PWM device "
			       "does not support a period at least %lu\n",
			       4U * MIN_PERIOD);
			return 0;
		}
	}

	printk("Done calibrating; maximum/minimum periods %u/%lu nsec\n",
	       max_period, MIN_PERIOD);

	period = max_period;
	while (1) {
		ret = pwm_set_dt(&pwm_led0, period, period / 2U);
		if (ret) {
			printk("Error %d: failed to set pulse width\n", ret);
			return 0;
		}

		period = dir ? (period * 2U) : (period / 2U);
		if (period > max_period) {
			period = max_period / 2U;
			dir = 0U;
		} else if (period < MIN_PERIOD) {
			period = MIN_PERIOD * 2U;
			dir = 1U;
		}

		k_sleep(K_SECONDS(4U));
	}
	return 0;
}