ADC interrupt-driven reading in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main advantage of using ADC interrupt-driven reading over polling?

Interrupt-driven reading allows the CPU to perform other tasks and only respond when the ADC conversion is complete, improving efficiency and responsiveness.

beginner

In ADC interrupt-driven reading, what triggers the interrupt?

The ADC hardware triggers an interrupt when the analog-to-digital conversion is finished and the result is ready to be read.

intermediate

What is the typical role of the Interrupt Service Routine (ISR) in ADC interrupt-driven reading?

The ISR reads the ADC conversion result, processes or stores it, and clears the interrupt flag to prepare for the next conversion.

intermediate

Why is it important to clear the ADC interrupt flag inside the ISR?

Clearing the interrupt flag prevents the ISR from being called repeatedly for the same event and allows new ADC interrupts to be detected.

intermediate

How do you start a new ADC conversion in an interrupt-driven ADC reading system?

You typically start a new ADC conversion either at the end of the ISR or by a timer or event that triggers the ADC hardware to begin conversion again.

What happens when the ADC conversion completes in an interrupt-driven system?

A An interrupt is triggered to run the ISR

B The CPU stops all tasks

C The ADC conversion restarts automatically

D The system resets

When ADC conversion finishes, it triggers an interrupt to run the ISR where the result is handled.

Where should the ADC result be read in an interrupt-driven ADC system?

A In the main loop before starting conversion

B In a separate thread

C After system reset

D Inside the Interrupt Service Routine (ISR)

The ADC result is read inside the ISR to immediately process the conversion result.

Why is polling less efficient than interrupt-driven ADC reading?

A Polling is faster

B Polling wastes CPU time checking repeatedly

C Polling uses less power

D Polling requires no code

Polling wastes CPU cycles by constantly checking if conversion is done instead of reacting only when ready.

What must be done inside the ISR to allow future ADC interrupts?

A Reset the microcontroller

B Disable ADC

C Clear the ADC interrupt flag

D Start a new ADC conversion

Clearing the interrupt flag inside the ISR allows the system to detect the next ADC interrupt.

How can you trigger ADC conversions repeatedly in an interrupt-driven system?

A Start a new conversion at the end of the ISR or use a timer

B Only once at system start

C By polling continuously

D By resetting the CPU

Starting a new conversion inside the ISR or using a timer ensures continuous ADC readings.

Explain how ADC interrupt-driven reading works and why it is useful.

Think about how the CPU reacts only when ADC finishes.

Describe the key steps you must do inside the ADC ISR.

Focus on what happens right after ADC conversion completes.

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