Nested interrupts in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are nested interrupts in embedded systems?

Nested interrupts allow an interrupt to be interrupted by another higher-priority interrupt before the first one finishes. This helps handle urgent tasks quickly.

intermediate

How does the processor handle nested interrupts?

The processor saves the current interrupt context, enables interrupts again, and then handles the new higher-priority interrupt. After finishing, it returns to the previous interrupt.

beginner

Why is it important to manage interrupt priorities in nested interrupts?

Managing priorities ensures that the most critical tasks interrupt less critical ones, preventing delays in urgent processing and avoiding conflicts.

intermediate

What can happen if nested interrupts are not handled properly?

Improper handling can cause stack overflow, data corruption, or system crashes because the processor may lose track of interrupt contexts.

intermediate

How can you enable nested interrupts in C for an embedded system?

You typically enable nested interrupts by setting interrupt priority levels and re-enabling global interrupts inside an interrupt service routine (ISR). For example, calling __enable_irq() inside an ISR in ARM Cortex-M.

What does nested interrupts allow in an embedded system?

A Only one interrupt can occur at a time

B Interrupts are disabled permanently

C Interrupts are ignored

D An interrupt can be interrupted by a higher-priority interrupt

Nested interrupts allow a higher-priority interrupt to interrupt a currently running interrupt.

What must be saved to handle nested interrupts correctly?

A Only global variables

B Current interrupt context (registers and state)

C Nothing needs to be saved

D Only the program counter

The processor saves the current interrupt context to resume correctly after handling nested interrupts.

Which of these is a risk if nested interrupts are not managed properly?

A Stack overflow

B Faster execution

C More memory available

D No effect

Improper nested interrupt handling can cause stack overflow due to too many saved contexts.

How do you typically enable nested interrupts inside an ISR?

A Re-enable global interrupts inside the ISR

B Disable all interrupts permanently

C Ignore interrupt priorities

D Use a delay function

Re-enabling global interrupts inside an ISR allows higher-priority interrupts to occur.

Why are interrupt priorities important in nested interrupts?

A They slow down the system

B They disable interrupts

C They decide which interrupt can interrupt another

D They are not important

Priorities determine which interrupts can preempt others in nested interrupt handling.

Explain how nested interrupts work and why they are useful in embedded systems.

Think about how urgent tasks can interrupt less urgent ones.

Describe the risks of not handling nested interrupts properly and how to avoid them.

Consider what happens if the system loses track of interrupt states.

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