ADC interrupt-driven reading in Embedded C - Time & Space Complexity

Time Complexity: ADC interrupt-driven reading

O(n)

Understanding Time Complexity

We want to understand how the time cost changes when reading analog data using interrupts.

How does the program's work grow as more ADC readings happen?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

volatile int adc_value = 0;

void ADC_IRQHandler(void) {
    adc_value = ADC_ReadData();
    ADC_ClearInterruptFlag();
}

int main(void) {
    ADC_EnableInterrupt();
    while(1) {
        // main loop does other work
    }
}
    

This code reads ADC values using an interrupt handler that runs when a conversion finishes.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The ADC interrupt handler runs each time a conversion completes.
• How many times: It runs once per ADC conversion, which depends on how often the ADC triggers.

How Execution Grows With Input

Each ADC conversion triggers the interrupt once, so the work grows directly with the number of conversions.

Input Size (n)	Approx. Operations
10	10 interrupt calls
100	100 interrupt calls
1000	1000 interrupt calls

Pattern observation: The total work increases in a straight line as the number of ADC readings increases.

Final Time Complexity

Time Complexity: O(n)

This means the time spent grows directly with the number of ADC readings taken.

Common Mistake

[X] Wrong: "The interrupt handler runs only once, so time cost is constant."

[OK] Correct: The interrupt runs every time the ADC finishes a conversion, so the total time grows with the number of conversions.

Interview Connect

Understanding how interrupt-driven code scales helps you write efficient embedded programs that handle real-time data smoothly.

Self-Check

"What if we changed the ADC to trigger interrupts only every 10th conversion? How would the time complexity change?"

Practice

1. What is the main advantage of using ADC interrupt-driven reading in embedded C?

easy

A. The CPU can perform other tasks while ADC conversion happens

B. It makes the ADC conversion faster

C. It disables other interrupts during ADC conversion

D. It reduces the ADC resolution automatically

Solution

1. Step 1: Understand ADC interrupt-driven reading

   ADC interrupt-driven reading allows the CPU to continue other work while waiting for ADC conversion to complete.

2. Step 2: Identify the main benefit

   This method improves efficiency by not blocking the CPU during ADC conversion.

3. Final Answer:

   The CPU can perform other tasks while ADC conversion happens -> Option A

4. Quick Check:

   Interrupt-driven ADC = CPU multitasking [OK]

Hint: Interrupts let CPU work during ADC conversion [OK]

Common Mistakes:

• Thinking ADC conversion speed increases
• Assuming interrupts disable other interrupts
• Believing ADC resolution changes automatically

2. Which of the following is the correct way to declare a variable shared between main code and ADC interrupt handler?

easy

A. const int adc_value;

B. int adc_value;

C. static int adc_value;

D. volatile int adc_value;

Solution

1. Step 1: Identify variable sharing between ISR and main code

   Variables shared between interrupt handlers and main code must be declared volatile to prevent compiler optimization issues.

2. Step 2: Choose the correct declaration

   Using 'volatile int adc_value;' ensures the variable is read fresh each time and not cached.

3. Final Answer:

   volatile int adc_value; -> Option D

4. Quick Check:

   Shared variable = volatile [OK]

Hint: Use volatile for variables shared with interrupts [OK]

Common Mistakes:

• Omitting volatile causing stale reads
• Using const which prevents modification
• Using static which limits scope but not volatility

3. Given this ADC interrupt handler code snippet, what will be the value of adc_result after the interrupt?

volatile int adc_result = 0;
void ADC_ISR(void) {
    adc_result = ADC_Read();
    ADC_ClearInterruptFlag();
}

medium

A. The previous ADC conversion value

B. Zero, because adc_result is volatile

C. The latest ADC conversion value

D. An undefined value due to missing initialization

Solution

1. Step 1: Analyze the interrupt handler code

   The handler reads the ADC value using ADC_Read() and stores it in adc_result, then clears the interrupt flag.

2. Step 2: Understand the effect on adc_result

   After the interrupt, adc_result holds the latest ADC conversion value read by ADC_Read().

3. Final Answer:

   The latest ADC conversion value -> Option C

4. Quick Check:

   ISR updates adc_result with new ADC value [OK]

Hint: ISR updates adc_result with fresh ADC reading [OK]

Common Mistakes:

• Confusing volatile with zero initialization
• Assuming adc_result remains unchanged
• Ignoring ADC_ClearInterruptFlag() role

4. Identify the error in this ADC interrupt-driven code snippet:

volatile int adc_val;
void ADC_ISR(void) {
    adc_val = ADC_Read();
    // Missing ADC_ClearInterruptFlag();
}

medium

A. adc_val should not be volatile

B. ADC_ClearInterruptFlag() is missing, causing repeated interrupts

C. ADC_Read() should not be called inside ISR

D. ISR function must return a value

Solution

1. Step 1: Review the ISR code

   The ISR reads ADC value but does not clear the interrupt flag.

2. Step 2: Understand interrupt flag importance

   Not clearing the interrupt flag causes the interrupt to trigger repeatedly, leading to an infinite loop of interrupts.

3. Final Answer:

   ADC_ClearInterruptFlag() is missing, causing repeated interrupts -> Option B

4. Quick Check:

   Clear interrupt flag to stop repeated ISR calls [OK]

Hint: Always clear interrupt flag inside ISR [OK]

Common Mistakes:

• Forgetting to clear interrupt flag
• Thinking volatile is incorrect here
• Expecting ISR to return a value

5. You want to implement ADC interrupt-driven reading that updates a global variable only when the ADC value exceeds 500. Which code snippet correctly implements this logic inside the ISR?

hard

A. void ADC_ISR(void) {
int val = ADC_Read();
if (val > 500) {
adc_value = val;
}
ADC_ClearInterruptFlag();
}

B. void ADC_ISR(void) {
if (adc_value > 500) {
adc_value = ADC_Read();
}
ADC_ClearInterruptFlag();
}

C. void ADC_ISR(void) {
adc_value = ADC_Read();
if (adc_value > 500) {
ADC_ClearInterruptFlag();
}
}

D. void ADC_ISR(void) {
ADC_ClearInterruptFlag();
int val = ADC_Read();
if (val > 500) {
adc_value = val;
}
}

Solution

1. Step 1: Check ADC reading and condition

   void ADC_ISR(void) {
       int val = ADC_Read();
       if (val > 500) {
           adc_value = val;
       }
       ADC_ClearInterruptFlag();
   }

   reads ADC value into a local variable, then updates global adc_value only if val > 500.

2. Step 2: Verify interrupt flag clearing

   void ADC_ISR(void) {
       int val = ADC_Read();
       if (val > 500) {
           adc_value = val;
       }
       ADC_ClearInterruptFlag();
   }

   clears the interrupt flag after processing, which is correct to prevent repeated interrupts.

3. Final Answer:

   Updates adc_value only if ADC reading exceeds 500 and clears interrupt flag -> Option A

4. Quick Check:

   Read ADC, check >500, update, clear flag [OK]

Hint: Read ADC first, then check condition, clear flag last [OK]

Common Mistakes:

• Clearing interrupt flag before reading ADC
• Checking old adc_value instead of new ADC reading
• Not clearing interrupt flag causing repeated interrupts