Writing an ISR (Interrupt Service Routine) in Embedded C - Mini Project: Build & Apply

Writing an ISR (Interrupt Service Routine)

📖 Scenario: You are programming a simple embedded system that controls an LED. The LED should toggle its state (on/off) every time a button is pressed. The button press triggers an interrupt, and you need to write an Interrupt Service Routine (ISR) to handle this event.

🎯 Goal: Build a program that sets up an ISR to toggle an LED when a button press interrupt occurs.

📋 What You'll Learn

Create a variable to hold the LED state

Create a flag variable to indicate the button press

Write an ISR function named button_ISR that sets the flag

Write a main loop that checks the flag and toggles the LED state

Print the LED state changes

💡 Why This Matters

🌍 Real World

Embedded systems often use ISRs to respond quickly to hardware events like button presses, sensors, or communication signals.

💼 Career

Understanding ISRs is essential for embedded software engineers working on microcontrollers, IoT devices, and real-time systems.

1

DATA SETUP: Create variables for LED state and button flag

Create an integer variable called led_state and set it to 0. Create another integer variable called button_pressed and set it to 0.

Embedded C

# Create variables led_state and button_pressed
# Your code here

Need a hint?

Use int led_state = 0; and int button_pressed = 0; to create the variables.

2

CONFIGURATION: Write the ISR function to set the button flag

Write a function called button_ISR with no parameters and void return type. Inside it, set button_pressed to 1.

Embedded C

int led_state = 0;
int button_pressed = 0;

// Write ISR function button_ISR
// Your code here

Need a hint?

Define void button_ISR(void) and set button_pressed = 1; inside it.

3

CORE LOGIC: Write the main loop to toggle LED on button press

Write a main function with an infinite loop. Inside the loop, check if button_pressed is 1. If yes, toggle led_state between 0 and 1, reset button_pressed to 0.

Embedded C

int led_state = 0;
int button_pressed = 0;

void button_ISR(void) {
    button_pressed = 1;
}

// Write main function with infinite loop
// Your code here

Need a hint?

Use while (1) for infinite loop, check button_pressed == 1, toggle led_state with led_state = !led_state;, then reset button_pressed.

4

OUTPUT: Print the LED state when toggled

Inside the if block in main, add a printf statement to print "LED is ON" if led_state is 1, else print "LED is OFF".

Embedded C

int led_state = 0;
int button_pressed = 0;

void button_ISR(void) {
    button_pressed = 1;
}

int main(void) {
    while (1) {
        if (button_pressed == 1) {
            led_state = !led_state;
            button_pressed = 0;
            // Print LED state
            // Your code here
        }
    }
    return 0;
}

Need a hint?

Use printf("LED is ON\n"); when led_state == 1, else printf("LED is OFF\n");.