Enabling and disabling interrupts in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable global interrupts.

Embedded C

sei(); // Call the function to [1] interrupts

Drag options to blanks, or click blank then click option'

Adisable

Benable

Ccheck

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing enable with disable

Using functions that clear flags instead

The sei() function enables global interrupts in embedded C.

2fill in blank

medium

Complete the code to disable global interrupts.

Embedded C

cli(); // Call the function to [1] interrupts

Drag options to blanks, or click blank then click option'

Acheck

Benable

Cset

Ddisable

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing disable with enable

Using functions that set flags instead

The cli() function disables global interrupts in embedded C.

3fill in blank

hard

Fix the error in the code to enable interrupts.

Embedded C

void enable_interrupts() {
    [1]();
}

Drag options to blanks, or click blank then click option'

Asei

Bcli

Cint_enable

Dinterrupt_on

Attempts:

3 left

💡 Hint

Common Mistakes

Using cli() instead of sei()

Using non-standard function names

The correct function to enable interrupts is sei(). Using cli() disables them.

4fill in blank

hard

Fill both blanks to disable interrupts and then enable them again.

Embedded C

void toggle_interrupts() {
    [1]();
    // some code
    [2]();
}

Drag options to blanks, or click blank then click option'

Acli

Bsei

Cenable

Ddisable

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of enable and disable

Using incorrect function names

First, cli() disables interrupts, then sei() enables them again.

5fill in blank

hard

Fill all three blanks to check if interrupts are enabled, and disable them if yes.

Embedded C

if ([1] & (1 << [2])) {
    [3]();
}

Drag options to blanks, or click blank then click option'

ASREG

BI

Ccli

Dsei

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong register or bit names

Confusing enable and disable functions

The SREG register holds the interrupt flag I. If set, interrupts are enabled. Then cli() disables them.

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