Enabling and disabling interrupts in Embedded C

Introduction

Interrupts let a microcontroller stop what it's doing to handle important tasks quickly. Enabling and disabling interrupts controls when these quick responses can happen.

When you want the microcontroller to react immediately to a button press.

When you need to pause interrupts to safely update shared data.

When setting up hardware timers that trigger actions automatically.

When you want to avoid interruptions during critical code sections.

When managing communication protocols that rely on interrupts.

Syntax

Embedded C

/* Enable interrupts */
__enable_irq();

/* Disable interrupts */
__disable_irq();

These functions are often provided by the microcontroller's library or CMSIS.

Disabling interrupts stops all maskable interrupts until they are enabled again.

Examples

This turns on all maskable interrupts so the CPU can respond to them.

Embedded C

/* Enable interrupts globally */
__enable_irq();

This stops all maskable interrupts temporarily.

Embedded C

/* Disable interrupts globally */
__disable_irq();

Use this to protect important code from being interrupted.

Embedded C

/* Disable interrupts, do critical work, then enable again */
__disable_irq();
// critical code here
__enable_irq();

Sample Program

This program shows disabling interrupts before a critical task and enabling them after. The print statements show the flow.

Embedded C

#include <stdio.h>
#include "cmsis_gcc.h"  // Assume this header provides __enable_irq and __disable_irq

int main() {
    printf("Interrupts disabled.\n");
    __disable_irq();

    // Critical section: no interrupts allowed
    printf("Performing critical task...\n");

    __enable_irq();
    printf("Interrupts enabled.\n");

    return 0;
}

Important Notes

Always re-enable interrupts after disabling them to avoid missing important events.

Disabling interrupts for too long can cause missed signals or slow system response.

Some microcontrollers have different ways to enable/disable specific interrupts instead of all.

Summary

Interrupts let the CPU handle urgent tasks quickly.

Use __enable_irq() to allow interrupts and __disable_irq() to stop them temporarily.

Disable interrupts only when necessary and keep the disabled time short.

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