setup() and loop() execution model in Arduino

Introduction

The setup() and loop() functions control how an Arduino program starts and runs repeatedly. This helps the board do tasks over and over without stopping.

When you want to initialize sensors or devices once at the start.

When you want your Arduino to keep checking buttons or sensors continuously.

When you want to blink an LED repeatedly.

When you want to read data and send it over serial repeatedly.

When you want to run a program that never stops unless the board is turned off.

Syntax

Arduino

void setup() {
  // code here runs once
}

void loop() {
  // code here runs repeatedly
}

setup() runs only once when the Arduino starts or resets.

loop() runs over and over forever after setup() finishes.

Examples

This example blinks the built-in LED on pin 13 on and off every second.

Arduino

void setup() {
  pinMode(13, OUTPUT); // set pin 13 as output
}

void loop() {
  digitalWrite(13, HIGH); // turn LED on
  delay(1000); // wait 1 second
  digitalWrite(13, LOW); // turn LED off
  delay(1000); // wait 1 second
}

This example prints a message to the serial monitor every 2 seconds.

Arduino

void setup() {
  Serial.begin(9600); // start serial communication
}

void loop() {
  Serial.println("Hello Arduino!"); // print message
  delay(2000); // wait 2 seconds
}

Sample Program

This program sets up pin 13 as an output and starts serial communication. It prints "Setup done" once. Then it turns the LED on and off every half second, printing the LED status each time.

Arduino

void setup() {
  pinMode(13, OUTPUT); // initialize pin 13 as output
  Serial.begin(9600); // start serial communication
  Serial.println("Setup done");
}

void loop() {
  digitalWrite(13, HIGH); // turn LED on
  Serial.println("LED ON");
  delay(500); // wait half a second
  digitalWrite(13, LOW); // turn LED off
  Serial.println("LED OFF");
  delay(500); // wait half a second
}

Important Notes

Do not put long delays in setup() because it delays starting the repeated loop().

Use loop() to keep your program running tasks continuously.

Arduino automatically calls setup() once and then calls loop() repeatedly.

Summary

setup() runs once at the start to prepare your Arduino.

loop() runs forever to keep your program working.

This model helps Arduino do repeated tasks easily.

Practice

1. What is the main purpose of the setup() function in an Arduino program?

easy

A. To stop the Arduino program

B. To run code repeatedly forever

C. To reset the Arduino board

D. To run code once at the start to prepare the Arduino

Solution

1. Step 1: Understand the role of setup()

   The setup() function runs only once when the Arduino starts. It is used to prepare things like pin modes or initial settings.

2. Step 2: Compare with other options

   loop() runs repeatedly, so To run code repeatedly forever is incorrect. Options A and D describe actions not done by setup().

3. Final Answer:

   To run code once at the start to prepare the Arduino -> Option D

4. Quick Check:

   setup() runs once = C [OK]

Hint: Remember: setup() runs once, loop() runs forever [OK]

Common Mistakes:

• Thinking setup() runs repeatedly
• Confusing setup() with loop()
• Believing setup() resets the board

2. Which of the following is the correct way to declare the loop() function in Arduino?

easy

A. int loop() {}

B. void loop() {}

C. void loop(void) {}

D. loop() void {}

Solution

1. Step 1: Recall Arduino function syntax

   Arduino functions like loop() are declared with return type void and empty parentheses: void loop() {}.

2. Step 2: Check each option

   void loop() {} matches correct syntax. int loop() {} wrongly uses int return type. void loop(void) {} is valid C++ but less common in Arduino examples. loop() void {} has incorrect order.

3. Final Answer:

   void loop() {} -> Option B

4. Quick Check:

   Standard Arduino loop syntax = A [OK]

Hint: Use 'void loop()' exactly as Arduino expects [OK]

Common Mistakes:

• Using wrong return type like int
• Swapping order of function name and return type
• Adding parameters inside loop()

3. What will be the output on the Serial Monitor when running this Arduino code?

void setup() {
  Serial.begin(9600);
  Serial.println("Start");
}

void loop() {
  Serial.println("Looping");
  delay(1000);
}

medium

A. Start and Looping printed once each

B. Only Looping printed repeatedly

C. Start printed once, then Looping printed every second

D. No output because Serial.begin() is missing

Solution

1. Step 1: Analyze setup() output

   Serial.begin(9600) starts serial communication. Serial.println("Start") prints "Start" once at the beginning.

2. Step 2: Analyze loop() output

   loop() prints "Looping" every 1000 milliseconds (1 second) repeatedly.

3. Final Answer:

   Start printed once, then Looping printed every second -> Option C

4. Quick Check:

   setup() once, loop() repeats = B [OK]

Hint: setup() prints once, loop() repeats output [OK]

Common Mistakes:

• Thinking loop() runs once
• Missing Serial.begin() call
• Ignoring delay causing fast prints

4. Identify the error in this Arduino code:

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

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000)
}

medium

A. Missing semicolon after delay(1000) in loop()

B. pinMode should be in loop(), not setup()

C. digitalWrite cannot be used with pin 13

D. delay() cannot be used inside loop()

Solution

1. Step 1: Check syntax line by line

   In loop(), the line delay(1000) is missing a semicolon at the end.

2. Step 2: Verify other parts

   pinMode is correctly placed in setup(). digitalWrite works with pin 13. delay() is allowed in loop().

3. Final Answer:

   Missing semicolon after delay(1000) in loop() -> Option A

4. Quick Check:

   Syntax error = missing semicolon [OK]

Hint: Check every line ends with a semicolon [OK]

Common Mistakes:

• Placing pinMode in loop() unnecessarily
• Assuming digitalWrite can't use pin 13
• Thinking delay() is disallowed in loop()

5. You want to blink an LED connected to pin 9 exactly 5 times when the Arduino starts, then stop blinking. Which code correctly uses setup() and loop() to do this?

hard

A. Use a counter in setup() to blink 5 times; leave loop() empty

B. Blink 5 times inside loop() without a counter

C. Blink 5 times inside loop() using a counter, then stop blinking

D. Blink 5 times inside setup(), then keep blinking in loop()

Solution

1. Step 1: Understand blinking 5 times only once

   Since blinking 5 times is a one-time task, it should be done in setup() which runs once.

2. Step 2: Keep loop() empty to stop further blinking

   Leaving loop() empty prevents repeated blinking after the initial 5 times.

3. Final Answer:

   Use a counter in setup() to blink 5 times; leave loop() empty -> Option A

4. Quick Check:

   One-time task in setup() = A [OK]

Hint: One-time actions go in setup(), repeated in loop() [OK]

Common Mistakes:

• Blinking in loop() without stopping
• Not using a counter to limit blinks
• Putting one-time code in loop() causing repeats