Enabling and disabling interrupts in Embedded C - Time & Space Complexity

Time Complexity: Enabling and disabling interrupts

O(n)

Understanding Time Complexity

When working with interrupts in embedded C, it's important to understand how enabling and disabling them affects program flow.

We want to see how the time cost changes as the program runs these interrupt control commands.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void critical_section() {
    __disable_irq();  // Disable interrupts
    // Critical code here
    __enable_irq();   // Enable interrupts
}

int main() {
    int n = 10; // Example value for n
    for (int i = 0; i < n; i++) {
        critical_section();
    }
}
    

This code disables interrupts before a critical section and enables them after, repeating this process n times.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop in main() calls critical_section() n times.
• How many times: The interrupt enable/disable commands run once per loop iteration, so n times total.

How Execution Grows With Input

Each time the loop runs, it disables and then enables interrupts once.

Input Size (n)	Approx. Operations
10	20 (10 disables + 10 enables)
100	200 (100 disables + 100 enables)
1000	2000 (1000 disables + 1000 enables)

Pattern observation: The number of interrupt control operations grows directly with n.

Final Time Complexity

Time Complexity: O(n)

This means the time spent enabling and disabling interrupts grows in a straight line as the number of loop iterations increases.

Common Mistake

[X] Wrong: "Enabling and disabling interrupts inside a loop takes constant time no matter how many times it runs."

[OK] Correct: Each enable/disable call happens every loop iteration, so total time grows with the number of iterations.

Interview Connect

Understanding how interrupt control affects program timing helps you write efficient embedded code and explain your reasoning clearly in technical discussions.

Self-Check

"What if we moved the interrupt enable/disable calls outside the loop? How would the time complexity change?"

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