ISR best practices (keep it short) in Embedded C - Mini Project: Build & Apply

ISR Best Practices in Embedded C

📖 Scenario: You are programming a microcontroller that reads a button press using an Interrupt Service Routine (ISR). To keep the system responsive and safe, you need to follow best practices for writing ISRs.

🎯 Goal: Build a simple program that sets up a button press ISR with best practices: keep ISR short, use a flag variable, and handle the main logic outside the ISR.

📋 What You'll Learn

Create a volatile flag variable to indicate button press

Write a short ISR that sets the flag

In the main loop, check the flag and clear it

Print a message when the button press is handled

💡 Why This Matters

🌍 Real World

Microcontrollers use ISRs to respond quickly to hardware events like button presses or sensor signals.

💼 Career

Embedded developers must write efficient ISRs to keep devices responsive and avoid bugs.

1

Create a volatile flag variable

Create a volatile int variable called button_pressed and set it to 0.

Embedded C

# Create a volatile int variable called button_pressed and set it to 0
# Your code here

Need a hint?

Use the keyword volatile to tell the compiler this variable can change unexpectedly.

2

Write a short ISR to set the flag

Write an ISR function called button_isr that sets button_pressed to 1. Keep the ISR short and do not add any other logic.

Embedded C

volatile int button_pressed = 0;

// Write ISR function button_isr that sets button_pressed to 1
# Your code here

Need a hint?

ISR should only set the flag and return quickly.

3

Check and clear the flag in main loop

In the main function, write a loop that checks if button_pressed is 1. If yes, print "Button pressed!" and reset button_pressed to 0.

Embedded C

volatile int button_pressed = 0;

void button_isr(void) {
    button_pressed = 1;
}

int main(void) {
    while (1) {
        // Check if button_pressed is 1
        // Print "Button pressed!"
        // Reset button_pressed to 0
        # Your code here
    }
}

Need a hint?

Use an if statement to check the flag and printf to show the message.

4

Run and observe the output

Run the program and simulate the ISR by calling button_isr() once before the loop. The program should print "Button pressed!" once.

Embedded C

volatile int button_pressed = 0;

void button_isr(void) {
    button_pressed = 1;
}

#include 

int main(void) {
    // Simulate ISR call
    button_isr();

    while (1) {
        if (button_pressed == 1) {
            printf("Button pressed!\n");
            button_pressed = 0;
            break;
        }
    }
    return 0;
}

Need a hint?

Calling button_isr() simulates the interrupt. The program prints the message once and exits.