LEDC duty resolution / freq pair causing watchdog errors

[ant000]

I have some code using LEDC_TIMER_10_BIT with a frequency of 1kHz that runs but causes watchdog errors. The errors continue if I drop the freq to 500Hz so I tried 50Hz which worked. My understanding is that LEDC_TIMER_10_BIT should support up to about 78kHz, but I also understand that some of the PWM channels do not support LEDC_HIGH_SPEED_MODE, assuming I even need that.

Assuming this is correct, my question is, where can I find the information that shows the device PWM channel capabilities? I have an old ESP32 and a newer S3 and the datasheets do not appear to have this information.

Also, is there a definition of LEDC_HIGH_SPEED_MODE etc, as to their capabilities?

[MicroController]

I have some code using LEDC_TIMER_10_BIT with a frequency of 1kHz that runs but causes watchdog errors.

The LEDC itself runs independent of any watchdog timer. The problem is likely in your code doing some busy-waiting where it shouldn't.

[ant000]

That is entirely probable, I am a beginner. This is the code in case it is something obvious. Still wouldn't mind knowing the other information if anyone knows.

Code: Select all

#include <stdio.h>
#include <math.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/queue.h"
#include "driver/ledc.h"


#define B_LEDC_PIN 18                         // LED connected to GPIO 18
#define B_LEDC_CHANNEL LEDC_CHANNEL_0
#define B_LEDC_TIMER LEDC_TIMER_0
#define B_LEDC_MODE LEDC_HIGH_SPEED_MODE
#define B_LEDC_DUTY_RES LEDC_TIMER_10_BIT     // 10-bit resolution (0-1023)
#define B_LEDC_FREQUENCY (1000)               // 1 kHz PWM frequency (1000)
#define B_LEDC_PERIOD 1                       // 1ms period
#define B_MIN_DUTY 10
#define B_MAX_DUTY 300
#define B_PI_STEP 0.01

void led_mgmt(void *pvParameter){

	printf("LED on core: %d \n", xPortGetCoreID());
	

    ledc_timer_config_t ledc_timer = {
        .speed_mode = B_LEDC_MODE,
        .duty_resolution = B_LEDC_DUTY_RES,
        .timer_num = B_LEDC_TIMER,
        .freq_hz = B_LEDC_FREQUENCY
    };
	
	
    ledc_timer_config(&ledc_timer);

    ledc_channel_config_t ledc_channel = {
        .gpio_num = B_LEDC_PIN,
        .speed_mode = B_LEDC_MODE,
        .channel = B_LEDC_CHANNEL,
        .timer_sel = B_LEDC_TIMER,
        .duty = 0
    };
	
	
	
    ledc_channel_config(&ledc_channel);

	uint16_t led_duty;
	float pi_angle;

    while (1) {

		for (	pi_angle = M_PI / 2;
				pi_angle >= 0;
				pi_angle -= B_PI_STEP) {
					
			led_duty = sin(pi_angle) * (B_MAX_DUTY - B_MIN_DUTY) + B_MIN_DUTY;
            ledc_set_duty(B_LEDC_MODE, B_LEDC_CHANNEL, led_duty);
            ledc_update_duty(B_LEDC_MODE, B_LEDC_CHANNEL);
            vTaskDelay(B_LEDC_PERIOD / portTICK_PERIOD_MS);
        }

		for (	pi_angle = 0;
				pi_angle <= M_PI / 2;
				pi_angle += B_PI_STEP) {
					
			led_duty = sin(pi_angle) * (B_MAX_DUTY - B_MIN_DUTY) + B_MIN_DUTY;
            ledc_set_duty(B_LEDC_MODE, B_LEDC_CHANNEL, led_duty);
            ledc_update_duty(B_LEDC_MODE, B_LEDC_CHANNEL);
            vTaskDelay(B_LEDC_PERIOD / portTICK_PERIOD_MS);
        }
    }
}

[MicroController]

Code: Select all

#define B_LEDC_PERIOD 1                       // 1ms period
...
vTaskDelay(B_LEDC_PERIOD / portTICK_PERIOD_MS);

There's your problem. By default, FreeRTOS's portTICK_PERIOD_MS is 10 ("tick rate"=100Hz). This results in your vTaskDelay(...) "delaying" for (1/10)=0, so no delay at all. This in turn makes your task use 100% of the CPU and the watchdog angry. Easiest solution is probably to just reduce the update rate of the LEDC to, e.g., 100Hz, i.e. change both B_PI_STEP and B_LEDC_PERIOD to 10x their current values (0.1f and 10 respectively).

Another option is to use a "High Resolution Timer" set to 1ms/1kHz.

[ant000]

Thanks, that make perfect sense. Does that mean I would have to remove vTaskDelay and block the core for that single task or probably better still, use an ISR timer with call back to run LEDC for higher frequencies?

[MicroController]

Not sure if you actually want/need to change an LED's brightness 1000x per second. Also, changing the duty cycle of a 1kHz PWM at 1kHz is bound to result in tons of glitches at the output.

[ant000]

Noted. Thanks for your help. I'll use the low frequency for now and mess around with the higher frequencies when I get around to production quality video lighting tests. Might need a different MCU or go analogue.

[MicroController]

Other options for fast "analog" output include the GPIOs' PDM ("Sigma Delta Modulator") (8 bits resolution, no DMA) and I2S in PDM mode (16 bits resolution).

[MicroController]

where can I find the information that shows the device PWM channel capabilities? I have an old ESP32 and a newer S3 and the datasheets do not appear to have this information.

You'll probably find the information in the respective "Technical Reference Manuals" (TRMs) (vs. in the datasheets).