Arduino setup() and loop() explained

Every Arduino sketch has two functions. setup() runs once, when the board powers up or resets: it gets things ready, like setting pin modes and starting the serial port. loop() runs next, then again and again, for as long as the board has power: it's where the sketch reads inputs and drives outputs.

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);   // L, on pin 13
  Serial.begin(9600);             // start the serial monitor
  Serial.println("Ready");        // said once
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(100);                     // on for 100 ms
  digitalWrite(LED_BUILTIN, LOW);
  delay(900);                     // off for 900 ms
}

That sketch says Ready once, then flashes L once a second, forever.

What happens behind them

Arduino writes the rest of the program for you. Stripped down, it's this:

int main() {
  init();          // set up the chip's timers
  setup();         // your code, once
  for (;;) {
    loop();        // your code, forever
  }
}

So loop() isn't magic: it's an ordinary function called inside a loop that never ends.

Live experiment

Meet the board

The board starts unplugged and asleep. Tap the USB socket to plug it in, and the starter program blinks L. Then tap round the board to find its six parts: the USB socket, the microcontroller, L, the digital pins, the analog pins and 5V and GND.

Try it yourself: free lesson

“Meet the board” is lesson 1 of 6 in Blink Bay, Embedded Systems World 1. Free with an account. It’s open as soon as you sign up.

What goes where

In setup(): anything that only needs doing once. pinMode(), Serial.begin(), a welcome message, starting a sensor.

In loop(): anything that should keep happening. Reading buttons and sensors, deciding what to do, writing to LEDs and motors. Read a sensor only in setup() and you get one reading, at power-on, and never another.

Above setup(): variables that must remember something between passes, like a count or the time of the last blink. A variable declared inside loop() is made fresh on every pass, so it can't keep a running total.

The syntax rules

• Each statement ends with a semicolon, ;
• A block of statements sits between { and }
• // starts a comment, a note the board ignores
• A function call passes its arguments in brackets: delay(100)

The compiler is strict: Delay and delay are different names, and a missing ; stops the upload with a message naming the line.

Once the sketch is uploaded, the board runs it by itself, even with the computer unplugged, starting from setup() every time it powers up.