Enabling and disabling interrupts in Embedded C - Step-by-Step Execution

Concept Flow - Enabling and disabling interrupts

Start

↓

Check interrupt flag

↓

Interrupt enabled?

No→Skip interrupt handling

Yes↓

Handle interrupt

↓

Return to main program

↓

Enable or disable interrupts as needed

↓

Loop back to Start

This flow shows how the program checks if interrupts are enabled, handles them if yes, and allows enabling or disabling interrupts to control when they occur.

Execution Sample

Embedded C

sei();
// Interrupts enabled
cli();
// Interrupts disabled

This code enables interrupts with sei() and disables them with cli() in embedded C.

Execution Table

Step	Action	Interrupt Flag	Interrupts Enabled?	Result
1	Call sei()	N/A	Yes	Interrupts enabled
2	Interrupt occurs	Set	Yes	Interrupt handled
3	Call cli()	N/A	No	Interrupts disabled
4	Interrupt occurs	Set	No	Interrupt ignored
5	End of demonstration	N/A	No	Program continues without interrupts

💡 Interrupts disabled at step 3, so further interrupts are ignored.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
Interrupts Enabled	No	Yes	No	No
Interrupt Flag	Clear	Clear	Clear	Set

Key Moments - 2 Insights

Why does the interrupt get ignored after calling cli()?

Because cli() disables interrupts, so even if the interrupt flag is set (see step 4 in execution_table), the CPU does not handle the interrupt.

Does calling sei() immediately handle interrupts?

No, calling sei() only enables interrupts. The interrupt must occur (flag set) to be handled, as shown between steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'Interrupts Enabled?' after step 3?

A Yes

B No

C Unknown

D Depends on interrupt flag

💡 Hint

Check the 'Interrupts Enabled?' column at step 3 in the execution_table.

At which step does the interrupt get handled?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for 'Interrupt handled' in the Result column of the execution_table.

If we remove the cli() call, what would happen at step 4?

A Interrupt handled

B Interrupt ignored

C Program crashes

D No interrupt flag set

💡 Hint

Refer to the 'Interrupts Enabled?' status and Result at step 4 in the execution_table.

Concept Snapshot

Enabling and disabling interrupts in embedded C:
- Use sei() to enable interrupts.
- Use cli() to disable interrupts.
- Interrupts only handled if enabled and interrupt flag is set.
- Control interrupts to manage when CPU responds to events.

Full Transcript

This visual execution shows how interrupts are enabled and disabled in embedded C using sei() and cli(). Initially, interrupts are disabled. Calling sei() enables them, allowing the CPU to handle interrupts when they occur. When cli() is called, interrupts are disabled again, so even if an interrupt occurs, it is ignored. The execution table traces these steps, showing the interrupt flag and enabled status. Key moments clarify why interrupts are ignored after disabling and that enabling interrupts does not immediately handle them without an interrupt event. The quiz tests understanding of interrupt states at different steps.

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