Nested interrupts in Embedded C - Mini Project: Build & Apply

Nested Interrupts in Embedded C

📖 Scenario: You are working on a simple embedded system that handles two types of interrupts: a timer interrupt and an external button interrupt. The system should allow the button interrupt to interrupt the timer interrupt handler, demonstrating nested interrupts.

🎯 Goal: Build a program in Embedded C that sets up two interrupt service routines (ISRs) with nested interrupts enabled. The timer ISR will increment a counter, and the button ISR will increment a separate counter. The button ISR should be able to interrupt the timer ISR.

📋 What You'll Learn

Create two counters: timer_count and button_count initialized to 0

Create a configuration variable nested_interrupts_enabled set to 1 to enable nested interrupts

Write two ISR functions: Timer_ISR() and Button_ISR()

In Timer_ISR(), increment timer_count and simulate enabling nested interrupts

In Button_ISR(), increment button_count

Simulate calling Timer_ISR() and inside it call Button_ISR() to show nesting

Print the final values of timer_count and button_count

💡 Why This Matters

🌍 Real World

Embedded systems often need to handle multiple interrupts where higher priority interrupts can interrupt lower priority ones. This project shows how nested interrupts work in a simple way.

💼 Career

Understanding nested interrupts is important for embedded software engineers working on microcontrollers, real-time systems, and hardware interfacing.

1

Create counters for interrupts

Create two integer variables called timer_count and button_count and set both to 0.

Embedded C

// Create two integer counters timer_count and button_count and set them to 0
// Your code here

Hint

Use int to declare the counters and set them to zero.

2

Enable nested interrupts configuration

Create an integer variable called nested_interrupts_enabled and set it to 1 to represent that nested interrupts are enabled.

Embedded C

int timer_count = 0;
int button_count = 0;

// Create nested_interrupts_enabled variable and set it to 1
// Your code here

Hint

This variable will simulate enabling nested interrupts.

3

Write the interrupt service routines

Write two functions: void Timer_ISR() and void Button_ISR(). In Timer_ISR(), increment timer_count. If nested_interrupts_enabled is 1, call Button_ISR() inside Timer_ISR(). In Button_ISR(), increment button_count.

Embedded C

int timer_count = 0;
int button_count = 0;

int nested_interrupts_enabled = 1;

// Write Timer_ISR and Button_ISR functions
// Your code here

Hint

Define the two functions and use the nested_interrupts_enabled variable to decide when to call Button_ISR() inside Timer_ISR().

4

Simulate interrupts and print results

Call Timer_ISR() once to simulate the timer interrupt. Then print the values of timer_count and button_count using printf in the format: "Timer count: %d\nButton count: %d\n".

Embedded C

int timer_count = 0;
int button_count = 0;

int nested_interrupts_enabled = 1;

void Button_ISR() {
    button_count++;
}

void Timer_ISR() {
    timer_count++;
    if (nested_interrupts_enabled == 1) {
        Button_ISR();
    }
}

// Call Timer_ISR and print the counts
// Your code here

Hint

Call Timer_ISR() once and then use printf to show the counts.

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