Nested interrupts in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable global interrupts.

Embedded C

#include <avr/interrupt.h>

int main() {
    sei(); // Enable global interrupts
    [1];
    while(1) {}
    return 0;
}

Drag options to blanks, or click blank then click option'

Aenable_interrupts()

Bcli()

Cinterrupt_enable()

Dsei()

Attempts:

3 left

💡 Hint

Common Mistakes

Using cli() which disables interrupts instead of enabling them.

Using non-standard function names.

The sei() function enables global interrupts in AVR microcontrollers.

2fill in blank

medium

Complete the code to define an interrupt service routine (ISR) for TIMER1 overflow.

Embedded C

#include <avr/interrupt.h>

ISR([1]) {
    // ISR code here
}

Drag options to blanks, or click blank then click option'

AINT0_vect

BTIMER0_OVF_vect

CTIMER1_OVF_vect

DADC_vect

Attempts:

3 left

💡 Hint

Common Mistakes

Using TIMER0_OVF_vect which is for TIMER0.

Using unrelated interrupt vectors.

The ISR for TIMER1 overflow uses the vector TIMER1_OVF_vect.

3fill in blank

hard

Fix the error in the ISR declaration to allow nested interrupts by enabling global interrupts inside the ISR.

Embedded C

ISR(TIMER2_OVF_vect) {
    [1](); // Enable nested interrupts
    // ISR code
}

Drag options to blanks, or click blank then click option'

Acli

Bsei

Cenable_global_interrupts

Dinterrupt_enable

Attempts:

3 left

💡 Hint

Common Mistakes

Using cli() which disables interrupts.

Using non-standard function names.

Calling sei() inside an ISR enables nested interrupts by setting the global interrupt flag.

4fill in blank

hard

Fill both blanks to create a nested interrupt safe ISR that enables global interrupts and clears the interrupt flag.

Embedded C

ISR(ADC_vect) {
    [1](); // Enable nested interrupts
    ADCSRA [2] (1 << ADIF); // Clear interrupt flag
}

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Asei

B|=

C&=

Dcli

Attempts:

3 left

💡 Hint

Common Mistakes

Using cli() instead of sei() to enable interrupts.

Using &= which clears bits instead of setting them.

Inside the ISR, sei() enables nested interrupts. The interrupt flag is cleared by setting the ADIF bit using |=.

5fill in blank

hard

Fill all three blanks to implement a nested interrupt safe ISR that enables global interrupts, clears the interrupt flag, and increments a counter.

Embedded C

volatile uint8_t count = 0;

ISR(INT0_vect) {
    [1](); // Enable nested interrupts
    EIFR [2] (1 << INTF0); // Clear interrupt flag
    [3]++; // Increment counter
}

Drag options to blanks, or click blank then click option'

Asei

B|=

Ccount

Dcli

Attempts:

3 left

💡 Hint

Common Mistakes

Using cli() instead of sei() to enable interrupts.

Using &= instead of |= to clear the flag.

Incrementing a wrong or undefined variable.

Inside the ISR, sei() enables nested interrupts, EIFR |= (1 << INTF0) clears the interrupt flag, and count++ increments the counter.

Practice

1.

What is the main purpose of enabling nested interrupts in embedded C?

easy

A. Allow higher priority interrupts to interrupt lower priority ISRs

B. Disable all interrupts during ISR execution

C. Prevent any interrupt from occurring

D. Increase the execution time of all ISRs

Solution

1. Step 1: Understand interrupt priority

   Interrupts have priorities; higher priority interrupts can preempt lower priority ones if nesting is enabled.

2. Step 2: Role of nested interrupts

   Nested interrupts allow a higher priority interrupt to interrupt an ISR of a lower priority interrupt, improving responsiveness.

3. Final Answer:

   Allow higher priority interrupts to interrupt lower priority ISRs -> Option A

4. Quick Check:

   Nested interrupts = higher priority preempts lower priority [OK]

Hint: Nested interrupts let urgent tasks interrupt less urgent ones [OK]

Common Mistakes:

• Thinking nested interrupts disable all interrupts
• Assuming nested interrupts increase ISR execution time
• Believing nested interrupts prevent interrupts

2.

Which of the following code snippets correctly enables nested interrupts inside an ISR in embedded C?

void ISR_Handler(void) {
    // Enable global interrupts
    __enable_irq();
    // ISR code
}

easy

A. __disable_irq(); // disables global interrupts inside ISR

B. sei(); // disables interrupts globally

C. cli(); // disables interrupts globally

D. __enable_irq(); // enables global interrupts inside ISR

Solution

1. Step 1: Identify function to enable interrupts

   In embedded C, __enable_irq() enables global interrupts, allowing nesting.

2. Step 2: Confirm correct usage inside ISR

   Calling __enable_irq() inside ISR allows higher priority interrupts to nest.

3. Final Answer:

   __enable_irq(); // enables global interrupts inside ISR -> Option D

4. Quick Check:

   __enable_irq() enables interrupts inside ISR [OK]

Hint: Use __enable_irq() inside ISR to allow nesting [OK]

Common Mistakes:

• Using __disable_irq() which disables interrupts
• Confusing cli() and sei() functions
• Not enabling interrupts inside ISR

3.

Consider the following code snippet in embedded C where nested interrupts are enabled:

volatile int count = 0;

void ISR_LowPriority(void) {
    __enable_irq();
    for(int i = 0; i < 5; i++) {
        count++;
    }
}

void ISR_HighPriority(void) {
    count += 10;
}

int main() {
    // Assume interrupts are configured and enabled
    while(1) {}
}

If ISR_HighPriority interrupts ISR_LowPriority exactly once during the loop, what is the final value of count after ISR_LowPriority finishes?

medium

A. 10

B. 5

C. 15

D. 20

Solution

1. Step 1: Analyze ISR_LowPriority increments

   ISR_LowPriority increments count 5 times, so count increases by 5.

2. Step 2: Account for ISR_HighPriority interrupt

   ISR_HighPriority adds 10 to count during ISR_LowPriority execution.

3. Step 3: Calculate final count

   Count = 5 (from low priority) + 10 (from high priority) = 15.

4. Final Answer:

   15 -> Option C

5. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add increments from both ISRs considering nesting [OK]

Common Mistakes:

• Ignoring the interrupt during the loop
• Counting only one ISR's increments
• Assuming interrupts don't nest

4.

Given the following ISR code, identify the error that prevents nested interrupts from working correctly:

void ISR_Handler(void) {
    // ISR code
    __disable_irq();
    // Critical section
    __enable_irq();
}

medium

A. __disable_irq() disables interrupts inside ISR, preventing nesting

B. Missing __enable_irq() call to enable interrupts inside ISR

C. ISR_Handler should not contain any interrupt control calls

D. __enable_irq() disables interrupts instead of enabling

Solution

1. Step 1: Understand effect of __disable_irq()

   Calling __disable_irq() inside ISR disables global interrupts, blocking nesting.

2. Step 2: Check interrupt enabling

   Although __enable_irq() is called later, interrupts remain disabled during critical section, so nesting is blocked.

3. Final Answer:

   __disable_irq() disables interrupts inside ISR, preventing nesting -> Option A

4. Quick Check:

   Disabling interrupts inside ISR blocks nesting [OK]

Hint: Avoid disabling interrupts inside ISR to allow nesting [OK]

Common Mistakes:

• Thinking __disable_irq() enables interrupts
• Assuming __enable_irq() disables interrupts
• Believing ISR should never control interrupts

5.

You have two interrupts: ISR_A with priority 1 and ISR_B with priority 2 (2 is higher). You want ISR_B to interrupt ISR_A when it occurs. Which of the following is the correct approach to implement nested interrupts in embedded C?

void ISR_A(void) {
    // Enable nested interrupts
    __enable_irq();
    // ISR_A code
}

void ISR_B(void) {
    // ISR_B code
}

hard

A. Set ISR_A priority higher than ISR_B to allow nesting

B. Enable global interrupts inside ISR_A using __enable_irq() to allow ISR_B nesting

C. Call ISR_B manually inside ISR_A to simulate nesting

D. Disable global interrupts inside ISR_A to prevent ISR_B nesting

Solution

1. Step 1: Understand priority and nesting

   Higher priority ISR_B (priority 2) should interrupt lower priority ISR_A (priority 1).

2. Step 2: Enable nested interrupts inside ISR_A

   Calling __enable_irq() inside ISR_A allows ISR_B to interrupt ISR_A.

3. Step 3: Avoid disabling interrupts or wrong priority settings

   Disabling interrupts or reversing priorities prevents correct nesting.

4. Final Answer:

   Enable global interrupts inside ISR_A using __enable_irq() to allow ISR_B nesting -> Option B

5. Quick Check:

   Enable interrupts inside lower priority ISR for nesting [OK]

Hint: Enable interrupts inside lower priority ISR to allow higher priority nesting [OK]

Common Mistakes:

• Disabling interrupts inside ISR_A
• Manually calling ISR_B instead of nesting
• Setting wrong priority order