setup() and loop() execution model in Arduino - Step-by-Step Execution

Concept Flow - setup() and loop() execution model

Start Program

↓

Call setup() once

↓

Enter loop()

↓

Execute loop() body

↓

Repeat loop() forever

The Arduino program starts by running setup() once, then repeatedly runs loop() forever.

Execution Sample

Arduino

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

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

This code sets the built-in LED pin as output once, then turns the LED on and off every second repeatedly.

Execution Table

Step	Function Called	Action	State Change	Output/Effect
1	setup()	pinMode(LED_BUILTIN, OUTPUT)	LED pin set as output	No visible output
2	loop()	digitalWrite(LED_BUILTIN, HIGH)	LED turned ON	LED lights up
3	loop()	delay(1000)	Wait 1000 ms	LED stays ON for 1 second
4	loop()	digitalWrite(LED_BUILTIN, LOW)	LED turned OFF	LED turns off
5	loop()	delay(1000)	Wait 1000 ms	LED stays OFF for 1 second
6	loop()	Repeat steps 2-5	LED blinks repeatedly	LED blinks on and off every second

💡 loop() never exits; it repeats forever after setup() runs once

Variable Tracker

Variable	Start	After setup()	After loop() step 2	After loop() step 4	After loop() step 6 (repeat)
LED_BUILTIN mode	undefined	OUTPUT	OUTPUT	OUTPUT	OUTPUT
LED state	undefined	undefined	HIGH (ON)	LOW (OFF)	Repeats HIGH and LOW

Key Moments - 3 Insights

Why does setup() run only once but loop() runs forever?

setup() is designed to initialize hardware and runs once at program start (see execution_table step 1). loop() contains the main code and repeats endlessly (see steps 2-6).

What happens if loop() finishes its code?

loop() never actually finishes; after the last statement, Arduino calls loop() again automatically (see execution_table step 6).

Why do we use delay() inside loop()?

delay() pauses the program for a set time, controlling how long the LED stays ON or OFF (see steps 3 and 5). Without delay, LED would blink too fast to see.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the LED after step 4?

A LED is ON

B LED is blinking

C LED is OFF

D LED state is undefined

💡 Hint

Check the 'State Change' column at step 4 in execution_table

At which step does the program wait for 1 second with the LED ON?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for delay(1000) after turning LED ON in execution_table

If we remove delay() calls, how would the LED behave according to the execution_table?

A LED would stay OFF permanently

B LED would blink too fast to see

C LED would stay ON permanently

D LED would not change state

💡 Hint

Consider the role of delay() in timing the LED blinking in execution_table steps 3 and 5

Concept Snapshot

Arduino programs have two main functions:
setup() runs once at start to initialize hardware.
loop() runs repeatedly forever to keep the program running.
Use delay() inside loop() to control timing.
setup() prepares; loop() repeats the main actions.

Full Transcript

The Arduino program starts by running the setup() function once. This function is used to prepare the hardware, like setting pin modes. After setup() finishes, the program enters the loop() function. The loop() runs its code repeatedly forever. In the example, setup() sets the built-in LED pin as output. Then loop() turns the LED on, waits one second, turns it off, waits one second, and repeats this cycle forever. The delay() function pauses the program to make the LED blink visible. The key idea is setup() runs once, loop() runs forever, and delay() controls timing inside loop().

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