Tasks

Let's upgrade our blink LED code. Instead of blinking the LED directly inside app_main(), let's create a function blink_led() and call it from app_main():

#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

// More on `void *pvParameter` later (hang tight)
void blink_led(void *pvParameter) {
    while (1) {
        gpio_set_level(LED_GPIO, 1);
        ESP_LOGI("LED", "LED State: ON");
        vTaskDelay(pdMS_TO_TICKS(1000));

        gpio_set_level(LED_GPIO, 0);
        ESP_LOGI("LED", "LED State: OFF");
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

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

    blink_led();
}

Now, imagine we need to log "LED started blinking" after calling blink_led(). But this is not possible because blink_led() runs an infinite loop and doesn't return.

Even if we add ESP_LOGI("LED", "LED started blinking") after blink_led() call, it won't reach there.

The fix

Instead of calling blink_led() directly, what if we let it run parallel to app_main()? app_main() only need to start blink_led(). It will run on it's own from then, and app_main() can do other stuff.

To do this, we create a Task using the blink_led() function. Once we create a task, it starts running parallel to app_main().

We can use the xTaskCreate() function to create tasks. Update app_main() like this:

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

  // Create the task
  xTaskCreate(blink_led,    // 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 blink_led (NULL since we aren't giving any parameters)
              1,            // Priority of the task (we'll discuss this later)
              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.
  // Let's log something!
  ESP_LOGI("LED", "LED started blinking");
}

Now you can test your code & ensure it's working.

A few improvements

Since blink_led() is a task, it's better to rename it to blink_led_task() or blink_task(). But it's optional.

Task priority

In bigger firmware, we might have multiple tasks running in parallel. In those cases, task priority matters a lot.

Here are the key points:

• 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 will be creating programs with multiple tasks later in the course!