ADC interrupt-driven reading in Embedded C

Introduction

ADC interrupt-driven reading lets your microcontroller get analog data without waiting. It helps your program do other things while the ADC works.

You want to read sensor data without stopping your main program.

You need to process analog signals continuously and quickly.

You want to save power by letting the CPU sleep until data is ready.

You have multiple tasks and want ADC readings to happen automatically.

You want to avoid missing fast-changing analog signals.

Syntax

Embedded C

void ADC_Interrupt_Handler(void) {
    // Read ADC result
    uint16_t adc_value = ADC_ReadResult();
    // Process or store adc_value
    // Clear ADC interrupt flag
    ADC_ClearInterruptFlag();
}

// Setup function
void ADC_Init(void) {
    // Configure ADC settings
    ADC_EnableInterrupt();
    ADC_StartConversion();
}

The interrupt handler runs automatically when ADC finishes conversion.

Always clear the interrupt flag inside the handler to avoid repeated triggers.

Examples

Simple interrupt handler that reads ADC value and saves it.

Embedded C

void ADC_IRQHandler(void) {
    uint16_t value = ADC_ReadResult();
    // Store value for main program
    adc_data = value;
    ADC_ClearInterruptFlag();
}

Initialize ADC on channel 0 and enable interrupt-driven reading.

Embedded C

void ADC_Init(void) {
    ADC_SetChannel(0);
    ADC_EnableInterrupt();
    ADC_StartConversion();
}

Sample Program

This program sets up ADC interrupt reading. When ADC finishes, it triggers the interrupt handler, which saves the value and sets a flag. The main program then prints the ADC value.

Embedded C

#include <stdint.h>
#include <stdbool.h>

volatile uint16_t adc_data = 0;
volatile bool adc_ready = false;

// Mock functions to simulate hardware
uint16_t ADC_ReadResult(void) {
    return 1234; // example ADC value
}
void ADC_ClearInterruptFlag(void) {
    // Clear interrupt flag (hardware specific)
}
void ADC_EnableInterrupt(void) {
    // Enable ADC interrupt (hardware specific)
}
void ADC_StartConversion(void) {
    // Start ADC conversion (hardware specific)
}

void ADC_IRQHandler(void) {
    adc_data = ADC_ReadResult();
    adc_ready = true;
    ADC_ClearInterruptFlag();
}

void ADC_Init(void) {
    ADC_EnableInterrupt();
    ADC_StartConversion();
}

int main(void) {
    ADC_Init();

    // Simulate ADC interrupt call
    ADC_IRQHandler();

    if (adc_ready) {
        // Print ADC value
        // In embedded, replace with actual output method
        // Here we use printf for demonstration
        printf("ADC value: %u\n", adc_data);
    }

    return 0;
}

Important Notes

Interrupt handlers should be short and fast to avoid blocking other tasks.

Use volatile variables for data shared between interrupt and main code.

Always clear the interrupt flag inside the handler to prevent repeated interrupts.

Summary

ADC interrupt-driven reading lets your program get analog data without waiting.

Use an interrupt handler to read and store ADC results automatically.

Remember to clear the interrupt flag and use volatile variables for shared data.

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