Writing an ISR (Interrupt Service Routine) in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this ISR increment?

Consider this embedded C code snippet where an ISR increments a global counter each time an interrupt occurs. What will be the value of counter after the ISR is called 3 times?

Embedded C

volatile int counter = 0;

void __attribute__((interrupt)) Timer_ISR(void) {
    counter++;
}

int main() {
    // Simulate 3 interrupts
    Timer_ISR();
    Timer_ISR();
    Timer_ISR();
    return counter;
}

A 3

B 1

C 0

D Undefined behavior

💡 Hint

Each call to the ISR increases the counter by one.

The ISR increments the global counter variable by 1 each time it is called. After 3 calls, the value is 3.

🧠 Conceptual

intermediate

Which keyword ensures variable consistency in an ISR?

In embedded C, which keyword is essential to declare a variable that is shared between the main program and an ISR to prevent compiler optimization issues?

A register

B static

C volatile

D const

💡 Hint

This keyword tells the compiler the variable can change unexpectedly.

The volatile keyword tells the compiler not to optimize access to the variable because it can change at any time, such as inside an ISR.

🔧 Debug

advanced

What error occurs with this ISR declaration?

Examine this ISR declaration. What error will the compiler produce?

Embedded C

void Timer_ISR() {
    // ISR code
}

int main() {
    // Setup code
    return 0;
}

A No error, valid ISR declaration

B Linker error: ISR not linked to interrupt vector

C Runtime error: ISR causes stack overflow

D SyntaxError: Missing interrupt attribute

💡 Hint

ISRs usually need special attributes or linkage to connect to hardware interrupts.

Without the proper interrupt attribute or linkage, the function is not connected to the interrupt vector, causing a linker error.

📝 Syntax

advanced

Which ISR declaration is syntactically correct for GCC?

Choose the correct syntax to declare an ISR for a timer interrupt using GCC compiler extensions.

A

void Timer_ISR() __interrupt {
    // ISR code
}

B

void Timer_ISR(void) interrupt {
    // ISR code
}

C

interrupt void Timer_ISR(void) {
    // ISR code
}

D

void __attribute__((interrupt)) Timer_ISR(void) {
    // ISR code
}

💡 Hint

GCC uses __attribute__((interrupt)) to mark ISRs.

Option D uses the correct GCC syntax with __attribute__((interrupt)) before the function name.

🚀 Application

expert

What is the final state of the LED after 5 interrupts?

Given this embedded C code where an ISR toggles an LED state on each interrupt, what will be the final value of led_state after 5 interrupts? (1 = ON, 0 = OFF)

Embedded C

volatile int led_state = 0;

void __attribute__((interrupt)) Timer_ISR(void) {
    led_state = !led_state;
}

int main() {
    for (int i = 0; i < 5; i++) {
        Timer_ISR();
    }
    return led_state;
}

A 1 (LED ON)

B 0 (LED OFF)

C 5 (LED toggled 5 times, final state undefined)

D Compilation error due to volatile usage

💡 Hint

Each interrupt toggles the LED state from 0 to 1 or 1 to 0.

Starting from 0 (OFF), toggling 5 times results in final state 1 (ON).