Blink: your first program

Every embedded journey starts with one pin going high and low. Blink is not a toy: it proves that your toolchain works, that the board is alive, and that you understand the three things every program on a microcontroller does: configure, act, wait.

The circuit

An LED is a diode: current flows one way only, and it needs a resistor to limit that current. Without one, the LED (and possibly the pin) burns out in a moment.

How big should the resistor be? Ohm's law, with the LED dropping about 2 V:

R = (Vpin - Vled) / I = (5 V - 2 V) / 0.015 A = 200 Ω

The nearest standard value is 220 Ω. That gives about 13.6 mA: bright enough, safely under the limit.

The program

void setup() {
  pinMode(13, OUTPUT);     // configure: pin 13 drives current out
}

void loop() {
  digitalWrite(13, HIGH);  // act: pin 13 to 5 V, LED on
  delay(500);              // wait
  digitalWrite(13, LOW);   // act: pin 13 to 0 V, LED off
  delay(500);              // wait
}

setup() runs once at power-up. loop() runs forever. That is the whole execution model.

Try it

Change the delay, press Run, and watch the pin.

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(500);
  digitalWrite(13, LOW);
  delay(500);
}

What delay() really does

delay(500) makes the CPU spin in a loop for 500 ms. Nothing else can happen during that time: no button reads, no sensor polling. For Blink that is fine. Later in this course you will replace it with millis(), which lets you keep several timers running at once.