setup() and loop() execution model in Arduino - Mini Project: Build & Apply

Understanding Arduino setup() and loop() Execution Model

📖 Scenario: You are creating a simple Arduino program to control an LED light. The LED should turn on once when the Arduino starts and then blink repeatedly.

🎯 Goal: Build an Arduino sketch that uses the setup() function to turn on an LED once, and the loop() function to blink the LED on and off repeatedly.

📋 What You'll Learn

Create a variable called ledPin and set it to 13

Use pinMode(ledPin, OUTPUT); inside setup()

Turn the LED on once inside setup() using digitalWrite(ledPin, HIGH);

In loop(), blink the LED by turning it off and on with a delay of 500 milliseconds

Use digitalWrite(ledPin, LOW);, delay(500);, digitalWrite(ledPin, HIGH);, and delay(500); inside loop()

💡 Why This Matters

🌍 Real World

Controlling lights, motors, and sensors in simple electronics projects.

💼 Career

Understanding Arduino's execution model is essential for embedded systems and hardware programming jobs.

1

Set up the LED pin variable

Create an integer variable called ledPin and set it to 13.

Arduino

# Your code here

Hint

Pin 13 is usually connected to the built-in LED on many Arduino boards.

2

Initialize the LED pin in setup()

Write the setup() function. Inside it, use pinMode(ledPin, OUTPUT); to set the LED pin as an output. Then turn the LED on once using digitalWrite(ledPin, HIGH);.

Arduino

int ledPin = 13;

void setup() {
  // Your code here
}

void loop() {
  // loop code will go here
}

Hint

The setup() function runs once when the Arduino starts.

3

Write the loop() function to blink the LED

Write the loop() function. Inside it, turn the LED off with digitalWrite(ledPin, LOW);, wait 500 milliseconds with delay(500);, then turn the LED on with digitalWrite(ledPin, HIGH);, and wait another 500 milliseconds with delay(500);.

Arduino

int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);
}

void loop() {
  // Your code here
}

Hint

The loop() function runs over and over again after setup() finishes.

4

Test the program by printing a message

Add a Serial.begin(9600); line inside setup() to start serial communication. Then add Serial.println("LED is blinking"); inside loop() to print the message each time the LED blinks.

Arduino

int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);
  // Your code here
}

void loop() {
  digitalWrite(ledPin, LOW);
  delay(500);
  digitalWrite(ledPin, HIGH);
  delay(500);
  // Your code here
}

Hint

Open the Serial Monitor in the Arduino IDE to see the printed messages.

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