Tasks

In the previous project, we blinked an LED in GPIO22 in app_main() function. But what if we need to keep the LED blinking, while allowing app_main() to do other stuff?

We do this by creating a "Task" that blinks the LED indefinitely. Tasks are functions that run in parallel to app_main(). They can be started from app_main().

Now, app_main() can start the blink LED task, let it blink the LED, and focus on other things.

Task priority

This time, we are only using a single task, so task priority doesn't matter that much. But it matters when multiple tasks are running in parallel.

Task priority is a number assigned to a task that represents the priority of the task. Higher priority tasks block lower priority tasks. Lower priority tasks can only execute if higher priority tasks stop (temporarily or permanently). If both tasks have the same priority, they run in parallel. We'll learn more about these later in the course.

Code

We have to first define the task (let's call it blink_task), and start the task from app_main() using the xTaskCreate() function:

#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "driver/gpio.h"
#include "esp_log.h"

#define LED_GPIO GPIO_NUM_22

// Define the task
void blink_task(void *pvParameter) {
    while (1) {
        gpio_set_level(LED_GPIO, 1);
        ESP_LOGI("LED", "LED turned ON!");
        vTaskDelay(pdMS_TO_TICKS(1000));

        gpio_set_level(LED_GPIO, 0);
        ESP_LOGI("LED", "LED turned OFF!");
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void app_main(void) {
    gpio_reset_pin(LED_GPIO);
    gpio_set_direction(LED_GPIO, GPIO_MODE_OUTPUT);

    // Start the task
    xTaskCreate(
        blink_task,    /* The task function */
        "blink_task",  /* Name of the task (used for debugging) */
        2048,          /* RAM allocated to the task (in bytes) */
        NULL,          /* Pointer to the parameters passed to the task function (NULL since we aren't giving any parameters) */
        1,             /* Priority of the task (higher number = higher priority) */
        NULL           /* Task handle to track or control the task later (we aren't using this as of now, so NULL) */
    );

    // Now you can do anything else here.
    // blink_task() will run in parallel.
}

Now, you can build and flash the code to ensure it's working.

Tip

app_main() is also a task. It just starts automatically. The priority of app_main() task is 1. The lowest priority value that can be given to a task is 0.

Challege (optional)

Connect a new LED to GPIO23 (don't forget to add the resistor), and create a new task blink_task_fast() to blink that second LED faster than the first LED. Start both tasks from app_main() (one after the other) with the same priority so that both LEDs are blinking but in different speeds.