Enabling and disabling interrupts in Embedded C - Mini Project: Build & Apply

Enabling and Disabling Interrupts in Embedded C

📖 Scenario: You are working on a simple embedded system that needs to control when interrupts are allowed to happen. Interrupts are signals that tell the processor to stop what it is doing and handle something important. Sometimes, you want to turn interrupts off to finish a task without interruption, and then turn them back on.

🎯 Goal: Build a small program that shows how to enable and disable interrupts using variables and functions in Embedded C.

📋 What You'll Learn

Create a variable to represent the interrupt status

Create a configuration variable to hold the interrupt mask

Write functions to enable and disable interrupts

Print the interrupt status after enabling and disabling

💡 Why This Matters

🌍 Real World

Embedded systems often need to control when interrupts can happen to avoid errors during critical tasks.

💼 Career

Understanding how to enable and disable interrupts is essential for embedded software developers working on microcontrollers and real-time systems.

1

Create the interrupt status variable

Create an unsigned int variable called interrupt_status and set it to 0 to represent that interrupts are initially disabled.

Embedded C

// Create an unsigned int variable called interrupt_status and set it to 0
// Your code here

Hint

Use unsigned int interrupt_status = 0; to create the variable.

2

Create the interrupt mask configuration

Create an unsigned int variable called interrupt_mask and set it to 1 to represent the mask used to enable interrupts.

Embedded C

unsigned int interrupt_status = 0;
// Create an unsigned int variable called interrupt_mask and set it to 1
// Your code here

Hint

Use unsigned int interrupt_mask = 1; to create the mask variable.

3

Write functions to enable and disable interrupts

Write two functions: enable_interrupts() and disable_interrupts(). In enable_interrupts(), set interrupt_status to interrupt_mask. In disable_interrupts(), set interrupt_status to 0.

Embedded C

unsigned int interrupt_status = 0;
unsigned int interrupt_mask = 1;

// Write the function enable_interrupts() to set interrupt_status to interrupt_mask
// Write the function disable_interrupts() to set interrupt_status to 0
// Your code here

Hint

Define two functions that change the value of interrupt_status accordingly.

4

Print the interrupt status after enabling and disabling

Call enable_interrupts() and then print interrupt_status. Then call disable_interrupts() and print interrupt_status again. Use printf to display the values.

Embedded C

unsigned int interrupt_status = 0;
unsigned int interrupt_mask = 1;

void enable_interrupts() {
    interrupt_status = interrupt_mask;
}

void disable_interrupts() {
    interrupt_status = 0;
}

int main() {
    // Call enable_interrupts and print interrupt_status
    // Call disable_interrupts and print interrupt_status
    // Your code here
    return 0;
}

Hint

Use printf("Interrupt status after enabling: %u\n", interrupt_status); to print the status.

Practice

1. What is the main purpose of using __disable_irq() in embedded C programming?

easy

A. To permanently disable the CPU

B. To temporarily stop all interrupts from occurring

C. To enable all interrupts

D. To reset the microcontroller

Solution

1. Step 1: Understand the function of __disable_irq()

   This function is used to stop interrupts temporarily so that critical code can run without interruption.

2. Step 2: Differentiate from other options

   It does not permanently disable the CPU, enable interrupts, or reset the microcontroller.

3. Final Answer:

   To temporarily stop all interrupts from occurring -> Option B

4. Quick Check:

   __disable_irq() stops interrupts temporarily [OK]

Hint: Disable interrupts to protect critical code sections [OK]

Common Mistakes:

• Thinking it permanently disables the CPU
• Confusing disable with enable interrupts
• Assuming it resets the device

2. Which of the following is the correct syntax to enable interrupts in embedded C?

easy

A. __enableInterrupts();

B. enable_irq();

C. __enable_irq();

D. enableInterrupt();

Solution

1. Step 1: Recall the standard function name

   The correct function to enable interrupts is __enable_irq() with double underscores and parentheses.

2. Step 2: Check other options for syntax errors

   Other options miss underscores or have incorrect function names.

3. Final Answer:

   __enable_irq(); -> Option C

4. Quick Check:

   Correct function syntax = __enable_irq(); [OK]

Hint: Look for double underscores and exact function name [OK]

Common Mistakes:

• Omitting underscores
• Using incorrect function names
• Missing parentheses

3. Consider the following code snippet:

__disable_irq();
// Critical section code
__enable_irq();
int x = 5;

What is the effect of this code on interrupts?

medium

A. Interrupts are disabled during the critical section and enabled afterward

B. Interrupts remain disabled after the code runs

C. Interrupts are enabled before and after the critical section

D. Interrupts are never disabled

Solution

1. Step 1: Analyze the order of function calls

   The code disables interrupts first, runs critical code, then enables interrupts again.

2. Step 2: Understand the effect on interrupts

   Interrupts are stopped during the critical section and allowed again afterward.

3. Final Answer:

   Interrupts are disabled during the critical section and enabled afterward -> Option A

4. Quick Check:

   Disable then enable interrupts around critical code [OK]

Hint: Disable before critical code, enable after [OK]

Common Mistakes:

• Assuming interrupts stay disabled
• Thinking interrupts are enabled all the time
• Ignoring the order of calls

4. Identify the error in this code snippet:

void process() {
  __enable_irq();
  // critical code
  __disable_irq();
}

medium

A. Interrupts are enabled before critical code and disabled after, which is incorrect

B. The function names are misspelled

C. Missing semicolons after function calls

D. No error, code is correct

Solution

1. Step 1: Check the order of enabling/disabling interrupts

   Interrupts should be disabled before critical code and enabled after, but here it is reversed.

2. Step 2: Confirm function names and syntax

   Function names and semicolons are correct, so no syntax errors.

3. Final Answer:

   Interrupts are enabled before critical code and disabled after, which is incorrect -> Option A

4. Quick Check:

   Disable before critical code, enable after [OK]

Hint: Disable interrupts before critical code, not after [OK]

Common Mistakes:

• Reversing enable and disable calls
• Assuming order does not matter
• Ignoring critical section protection

5. You want to protect a shared variable counter from being changed by interrupts during update. Which code snippet correctly uses interrupt control to do this safely?

hard

A. __disable_irq();
__enable_irq();
counter++;

B. __enable_irq();
counter++;
__disable_irq();

C. counter++;
__disable_irq();
__enable_irq();

D. __disable_irq();
counter++;
__enable_irq();

Solution

1. Step 1: Understand the need to protect the update

   The shared variable update must happen without interruption to avoid inconsistent data.

2. Step 2: Check which snippet disables interrupts before update and enables after

   __disable_irq();
   counter++;
   __enable_irq();

   disables interrupts before incrementing and enables them after, protecting the update.

3. Final Answer:

   __disable_irq(); counter++; __enable_irq(); -> Option D

4. Quick Check:

   Disable interrupts before critical update, enable after [OK]

Hint: Disable interrupts before update, enable after [OK]

Common Mistakes:

• Enabling interrupts before update
• Updating variable outside disabled section
• Disabling and enabling interrupts without protecting update