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

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

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

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;
}

Hint

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

Practice

1. What is the main purpose of an ISR (Interrupt Service Routine) in embedded C?

easy

A. To allocate memory dynamically

B. To handle hardware events immediately when they occur

C. To run the main program loop continuously

D. To compile the program code

Solution

1. Step 1: Understand ISR role

   An ISR is a special function that runs automatically when a hardware event happens, like a button press or timer overflow.

2. Step 2: Compare options

   Only To handle hardware events immediately when they occur describes this immediate handling of hardware events. Other options describe unrelated tasks.

3. Final Answer:

   To handle hardware events immediately when they occur -> Option B

4. Quick Check:

   ISR = handle hardware events [OK]

Hint: ISRs respond instantly to hardware signals [OK]

Common Mistakes:

• Thinking ISR runs like normal functions
• Confusing ISR with main program loop
• Assuming ISR manages memory

2. Which of the following is the correct way to declare an ISR function in embedded C?

easy

A. void ISR_Handler() __interrupt { /* code */ }

B. void ISR_Handler() interrupt { /* code */ }

C. void ISR_Handler() __interrupt(0) { /* code */ }

D. void ISR_Handler() __isr { /* code */ }

Solution

1. Step 1: Recall ISR syntax in embedded C

   ISRs are declared with the __interrupt keyword followed by the interrupt vector number in parentheses.

2. Step 2: Check options

   void ISR_Handler() __interrupt(0) { /* code */ } uses __interrupt(0), which is the correct syntax. The others either miss the vector number, use the wrong keyword ('interrupt'), or a non-standard keyword ('__isr').

3. Final Answer:

   void ISR_Handler() __interrupt(0) { /* code */ } -> Option C

4. Quick Check:

   ISR syntax = __interrupt(vector) [OK]

Hint: Use __interrupt(vector_number) to declare ISR [OK]

Common Mistakes:

• Omitting interrupt vector number
• Using 'interrupt' instead of '__interrupt'
• Using non-standard keywords

3. Given the following ISR code snippet, what will be the value of flag after the interrupt occurs?

volatile int flag = 0;
void ISR_Timer() __interrupt(1) {
    flag = 1;
    // Interrupt flag cleared here
}

int main() {
    flag = 0;
    // Interrupt occurs here
    // ISR_Timer runs
    return flag;
}

medium

A. 1

B. 0

C. Undefined behavior

D. Compilation error

Solution

1. Step 1: Understand ISR effect on flag

   The ISR sets the variable 'flag' to 1 when the interrupt occurs.

2. Step 2: Check main function return

   Main sets flag to 0, then interrupt triggers ISR which sets flag to 1, so return value is 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   ISR sets flag = 1, so return = 1 [OK]

Hint: ISR changes flag to 1 before main returns [OK]

Common Mistakes:

• Ignoring ISR effect on flag
• Assuming flag stays 0
• Confusing volatile usage

4. Identify the error in this ISR code snippet:

void ISR_UART() __interrupt(4) {
    if (UART_Interrupt_Flag) {
        // Handle UART interrupt
        UART_Interrupt_Flag = 0;
    }
}

medium

A. ISR should not clear interrupt flags inside it

B. ISR function must return an int value

C. ISR must be declared with 'interrupt' keyword, not '__interrupt'

D. Missing volatile keyword for UART_Interrupt_Flag variable

Solution

1. Step 1: Check variable usage in ISR

   Interrupt flags accessed in ISR should be declared volatile to prevent compiler optimization issues.

2. Step 2: Analyze other options

   Clearing flags inside ISR is correct practice; __interrupt is correct keyword; ISR functions return void, not int.

3. Final Answer:

   Missing volatile keyword for UART_Interrupt_Flag variable -> Option D

4. Quick Check:

   Interrupt flags must be volatile [OK]

Hint: Declare interrupt flags volatile to avoid optimization bugs [OK]

Common Mistakes:

• Not declaring flags volatile
• Wrong ISR declaration keyword
• Expecting ISR to return a value

5. You want to write an ISR for a timer interrupt that increments a counter and clears the interrupt flag. Which of the following is the best practice to ensure correct ISR behavior?

hard

A. Increment counter, then clear interrupt flag inside ISR

B. Clear interrupt flag, then increment counter inside ISR

C. Increment counter outside ISR and clear flag inside ISR

D. Do not clear interrupt flag; let main program clear it

Solution

1. Step 1: Understand ISR order for flag and counter

   Best practice is to handle the event (increment counter) first, then clear the interrupt flag to acknowledge the interrupt.

2. Step 2: Evaluate options

   Increment counter, then clear interrupt flag inside ISR follows this order. Clear interrupt flag, then increment counter inside ISR clears flag before handling event, which may cause missed interrupts. Increment counter outside ISR and clear flag inside ISR splits logic incorrectly. Do not clear interrupt flag; let main program clear it risks repeated interrupts.

3. Final Answer:

   Increment counter, then clear interrupt flag inside ISR -> Option A

4. Quick Check:

   Handle event first, then clear flag [OK]

Hint: Process event before clearing interrupt flag [OK]

Common Mistakes:

• Clearing flag before handling event
• Leaving flag uncleared causing repeated interrupts
• Handling event outside ISR