E15. FreeRTOS Tasks and Dual-Core Basics

What you will do

On the Uno, delay() stops everything, so you learned millis() to do two things at once. The ESP32 has another way: tasks.

The sketch starts a task that blinks the LED on GPIO 4 once a second, while loop() blinks the LED on GPIO 2 four times faster. Both use plain delay().

What is a task?

The ESP32 runs a small operating system called FreeRTOS. It shares the processor between tasks, switching between them many times a second. Your loop() is already one task.

A task you write looks like a function with a while (true) loop inside, so it never ends. When a task calls delay(), only that task waits - the others keep running. A classic ESP32 also has two processor cores, so two tasks can really run at the same moment.

Task or millis()?

millis() in one loop() is still a good choice for simple jobs, and you already know it. Tasks help when one job is slow - such as waiting for a website - and another must keep going.

Be careful when two tasks use the same variable or the same Serial port at once. Bigger projects pass data between tasks with a FreeRTOS queue - a topic for after this course.

Wiring and safe build sequence

1. GPIO 2 -> 220-330 ohm -> LED -> GND (or the on-board LED)
2. GPIO 4 -> 220-330 ohm -> LED -> GND

What this sketch is for: The GPIO 2 LED blinks fast and the GPIO 4 LED blinks slowly, and neither one waits for the other.

const int fastLed = 2;                    // blinked by loop()
const int slowLed = 4;                    // blinked by the task

void blinkTask(void* parameter) {         // the task (copy this first line as it is)
  pinMode(slowLed, OUTPUT);               // the slow LED pin is an output
  while (true) {                          // a task never ends, so it loops for ever
    digitalWrite(slowLed, HIGH);          // slow LED on
    delay(1000);                          // wait 1 second - only this task waits
    digitalWrite(slowLed, LOW);           // slow LED off
    delay(1000);                          // wait 1 second
  }
}

void setup() {                            // runs once
  pinMode(fastLed, OUTPUT);               // the fast LED pin is an output
  xTaskCreate(blinkTask, "blink", 2048, NULL, 1, NULL); // start the task running
}

void loop() {                             // loop() is a task too
  digitalWrite(fastLed, HIGH);            // fast LED on
  delay(250);                             // wait a quarter of a second
  digitalWrite(fastLed, LOW);             // fast LED off
  delay(250);                             // wait a quarter of a second
}