What is the difference between setup() and loop() functions?

 These functions are fundamental to Arduino programming (and similar embedded frameworks), serving distinct purposes:

setup() Function

• Runs once when the microcontroller starts or after a reset

• Primary purposes:

  • Initialize hardware (pins, sensors, displays)

  • Configure communication interfaces (Serial, I2C, SPI)

  • Set initial values for variables

  • Perform one-time initialization tasks

• Example uses:

  cpp

  void setup() {
    pinMode(LED_PIN, OUTPUT);     // Set pin as output
    Serial.begin(9600);           // Initialize serial communication
    display.begin();              // Initialize OLED display
  }

loop() Function

• Runs continuously after setup() completes

• Contains the main program logic that repeats indefinitely

• Primary purposes:

  • Read sensors and inputs

  • Process data

  • Control outputs

  • Implement state machines

• Example uses:

  cpp

  void loop() {
    int sensorValue = analogRead(A0);  // Read sensor
    Serial.println(sensorValue);      // Send data to PC
    digitalWrite(LED_PIN, HIGH);      // Turn on LED
    delay(1000);                      // Wait 1 second
    digitalWrite(LED_PIN, LOW);       // Turn off LED
    delay(1000);
  }

Key Differences

Characteristic	setup()	loop()
Execution	Runs once at startup	Runs repeatedly forever
Timing	First function called	Called after setup()
Typical contents	Initialization code	Main program logic
Blocking allowed?	Yes (during setup)	Avoid long blocks

Best Practices

• Keep setup() clean and focused on initialization

• Make loop() as efficient as possible (avoid long delays when possible)

• For complex programs, call separate functions from loop() rather than putting all code directly in it

• Remember that loop() must never exit - it should run continuously

This structure (setup() + loop()) provides a simple framework for embedded programming while abstracting away low-level details of the microcontroller's operation.