Nested interrupts in Embedded C

Introduction

Nested interrupts let a higher priority interrupt stop a lower priority one. This helps the system respond faster to important events.

When a critical sensor needs immediate attention even if another interrupt is running.

In real-time systems where some tasks must preempt others quickly.

When handling multiple devices with different urgency levels.

To improve responsiveness in embedded control systems.

When you want to avoid missing important interrupts during long interrupt processing.

Syntax

Embedded C

void ISR_LowPriority(void) {
    // Enable global interrupts to allow nesting
    __enable_irq();
    // Low priority interrupt code
    __disable_irq();
}

void ISR_HighPriority(void) {
    // High priority interrupt code
}

Enabling global interrupts inside an ISR allows higher priority interrupts to interrupt the current ISR.

Make sure to re-disable interrupts before exiting the ISR to avoid unwanted nesting.

Examples

This example shows enabling interrupts inside a low priority ISR to allow higher priority interrupts to occur.

Embedded C

void ISR_LowPriority(void) {
    __enable_irq(); // Allow nesting
    // Handle low priority interrupt
    __disable_irq();
}

High priority ISR runs immediately if nested interrupts are enabled.

Embedded C

void ISR_HighPriority(void) {
    // Handle high priority interrupt
}

Sample Program

This program simulates nested interrupts by calling a high priority ISR inside a low priority ISR after enabling interrupts. It prints the order of execution to show nesting.

Embedded C

#include <stdio.h>
#include <stdbool.h>

volatile bool high_interrupt_triggered = false;
volatile bool low_interrupt_triggered = false;

void __enable_irq() {
    // Simulate enabling interrupts
}

void __disable_irq() {
    // Simulate disabling interrupts
}

void ISR_HighPriority(void) {
    printf("High priority interrupt started\n");
    high_interrupt_triggered = true;
    printf("High priority interrupt finished\n");
}

void ISR_LowPriority(void) {
    printf("Low priority interrupt started\n");
    __enable_irq(); // Allow nesting
    if (!high_interrupt_triggered) {
        ISR_HighPriority(); // Simulate nested high priority interrupt
    }
    __disable_irq();
    low_interrupt_triggered = true;
    printf("Low priority interrupt finished\n");
}

int main() {
    ISR_LowPriority();
    if (high_interrupt_triggered && low_interrupt_triggered) {
        printf("Both interrupts handled with nesting.\n");
    } else {
        printf("Interrupts not handled as expected.\n");
    }
    return 0;
}

Important Notes

Nested interrupts can cause complexity; use them carefully to avoid bugs.

Always save and restore processor state properly in real embedded systems.

Check your microcontroller's documentation for how to enable nested interrupts.

Summary

Nested interrupts let higher priority interrupts interrupt lower priority ones.

Enable global interrupts inside an ISR to allow nesting.

Use nested interrupts to improve responsiveness in embedded systems.

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