Volatile variables in ISR context in Embedded C - Mini Project: Build & Apply

Volatile Variables in ISR Context

📖 Scenario: You are programming a simple embedded system that counts button presses using an interrupt service routine (ISR). The main program reads the count and displays it.

🎯 Goal: Learn how to declare and use a volatile variable to safely share data between the ISR and the main program.

📋 What You'll Learn

Create a global variable button_press_count to store the number of button presses.

Declare button_press_count as volatile.

Write an ISR function button_isr that increments button_press_count.

In the main function, read and print the value of button_press_count.

💡 Why This Matters

🌍 Real World

Embedded systems often use interrupts to respond quickly to hardware events like button presses. Using <code>volatile</code> ensures the main program sees the latest data changed by interrupts.

💼 Career

Understanding <code>volatile</code> and ISR interaction is essential for embedded software engineers working on microcontrollers and real-time systems.

1

Create a global variable for button press count

Create a global variable called button_press_count of type int and initialize it to 0.

Embedded C

# Create a global variable button_press_count initialized to 0
# Your code here

Need a hint?

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

2

Write the ISR to increment the count

Write a function called button_isr with no parameters and void return type. Inside it, increment the button_press_count variable by 1.

Embedded C

volatile int button_press_count = 0;

// Write ISR function button_isr to increment button_press_count
// Your code here

Need a hint?

The ISR function should have void return type and no parameters. Use button_press_count++; to add one.

3

Read the volatile variable in main

Write the main function with int return type and no parameters. Inside it, declare an int variable called count_snapshot and assign it the value of button_press_count.

Embedded C

volatile int button_press_count = 0;

void button_isr(void) {
    button_press_count++;
}

// Write main function to read button_press_count into count_snapshot
// Your code here

Need a hint?

In main, create count_snapshot and set it equal to button_press_count.

4

Print the button press count

Add a printf statement inside main to print the text "Button presses: " followed by the value of count_snapshot and a newline.

Embedded C

volatile int button_press_count = 0;

void button_isr(void) {
    button_press_count++;
}

int main(void) {
    int count_snapshot = button_press_count;
    // Print the button press count
    // Your code here
    return 0;
}

Need a hint?

Use printf("Button presses: %d\n", count_snapshot); to display the count.