IoT Engineering Course using ESP32 Wifi Robots

📘 Structure of Arduino Program

🎯 Lesson Objective

By the end of this lesson, students will:

Understand how an Arduino/ESP32 program is structured
Learn what setup() and loop() functions are
Understand program execution flow
Learn how microcontrollers execute code continuously
Understand why loop() never stops
Understand how this structure applies to IoT systems

1️⃣ Why Arduino Has a Fixed Structure

Unlike normal C/C++ programs, Arduino programs follow a fixed template.

Every Arduino/ESP32 program must contain:

void setup() {
}

void loop() {
}

These two functions are mandatory.

Without them → Code will not run.

2️⃣ What is setup()?

🔹 Definition

setup() is a function that runs only once when the device starts.

It is executed:

When ESP32 is powered ON
When reset button is pressed
When code is uploaded

🔹 What Do We Put Inside setup()?

We initialize things here:

Serial communication
Pin modes
WiFi connection
Library initialization
Sensor startup

Example:

void setup() {
Serial.begin(115200);
pinMode(2, OUTPUT);
}

This code:

Starts Serial Monitor
Sets GPIO 2 as output

After this, setup() finishes.

It will not run again (unless reset).

3️⃣ What is loop()?

🔹 Definition

loop() is a function that runs repeatedly forever.

After setup() completes:

ESP32 enters loop()
And keeps repeating it continuously.

🔹 Why Does loop() Run Forever?

Microcontrollers are designed to:

Monitor sensors continuously
React to events
Run automation logic

They never “close” like a computer program.

They always stay active.

🔹 Example

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

Output:

Running…
Running…
Running…

This continues forever.

4️⃣ How Program Execution Works

Let’s understand the internal flow.

When ESP32 powers ON:

Step 1 → Load program from memory
Step 2 → Run setup() once
Step 3 → Enter loop()
Step 4 → Repeat loop() infinitely

Flow diagram:

Power ON
↓
setup()
↓
loop() → loop() → loop() → loop() → forever

5️⃣ Why setup() and loop() Are Separated

This separation keeps code organized.

setup() → Initialization

Only things that need to run once.

loop() → Repeated tasks

Sensor reading
Decision making
Communication
Control logic

This makes embedded systems efficient.

6️⃣ Real IoT Example

Let’s take Smart AC system.

setup()

Start Serial
Connect to WiFi
Initialize DHT sensor
Set relay pin mode

loop()

Read temperature
Compare with setpoint
Turn AC ON/OFF
Send data to Blynk

This pattern repeats in every IoT project.

7️⃣ What Happens If loop() Has No Delay?

Example:

void loop() {
Serial.println("Fast");
}

This will print extremely fast.

Thousands of lines per second.

Why?

Because loop() runs continuously with no pause.

8️⃣ What is delay()?

delay(milliseconds);

It pauses program execution for given time.

Example:

delay(1000);

Means wait 1 second.

This slows down loop execution.

9️⃣ Important Concept – Infinite Loop

In normal C programming, main() function runs once.

In Arduino, loop() acts like:

while(true) {
}

Which means:

Repeat forever.

This is required for real-time systems.

🔟 Where Do We Declare Variables?

Variables can be declared:

🔹 Outside functions (Global Variables)

Accessible everywhere.

Example:

int temperature;

🔹 Inside functions (Local Variables)

Accessible only inside that function.

Example:

void loop() {
int value = 10;
}

Understanding scope is important.

1️⃣1️⃣ What Happens If We Remove loop()?

Program will not compile.

Arduino requires both:

setup()
loop()

These are mandatory.

1️⃣2️⃣ What Happens During Reset?

When you press reset button:

ESP32 restarts
setup() runs again
loop() restarts

Reset clears runtime state.

1️⃣3️⃣ Advanced Insight – Hidden main()

Internally, Arduino has something like:

int main() {
setup();
while(true) {
loop();
}
}

This is hidden from user.

Arduino simplifies it.

1️⃣4️⃣ Common Beginner Mistakes

❌ Putting WiFi.begin() inside loop()
❌ Putting pinMode() inside loop()
❌ Forgetting Serial.begin()
❌ Using heavy code inside setup()
❌ Blocking loop with long delays

Always remember:

Initialization → setup()
Repeating logic → loop()

1️⃣5️⃣ Clean Program Example

int ledPin = 2;

void setup() {
Serial.begin(115200);
pinMode(ledPin, OUTPUT);
}

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

This follows correct structure.

📌 Lesson Summary

In this lesson, we learned:

Every Arduino program has setup() and loop()
setup() runs once
loop() runs forever
Program execution flow
Initialization vs repeating logic
Infinite loop concept
Where to declare variables

You now understand how microcontrollers execute programs.