ADC interrupt-driven reading in Embedded C - Step-by-Step Execution

Concept Flow - ADC interrupt-driven reading

Start ADC Conversion

↓

ADC Conversion Complete?

No→Wait

Yes↓

ADC Interrupt Triggered

↓

Interrupt Service Routine (ISR)

↓

Read ADC Result

↓

Process or Store Result

↓

Restart ADC Conversion

↩Back to Start

The ADC starts conversion, waits until done, triggers an interrupt, ISR reads and processes the result, then restarts conversion.

Execution Sample

Embedded C

void ADC_ISR(void) {
  int adc_value = ADC_Read();
  process(adc_value);
  ADC_StartConversion();
}

This ISR reads the ADC value when conversion completes, processes it, and starts a new conversion.

Execution Table

Step	Event	ADC Conversion Status	ISR Called	ADC Value Read	Action Taken
1	ADC conversion started	In Progress	No	N/A	Waiting for conversion
2	ADC conversion complete	Complete	Yes	Value read from ADC	Process value and restart conversion
3	New ADC conversion started	In Progress	No	N/A	Waiting for next conversion
4	ADC conversion complete	Complete	Yes	New value read	Process value and restart conversion
Exit	Stop or power down	N/A	No	N/A	ADC conversions stopped

💡 ADC conversions stop when device powers down or ADC is disabled

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
adc_value	undefined	Value from first conversion	Value from second conversion	Last read value
ADC Conversion Status	Idle	Complete	Complete	Stopped or Idle

Key Moments - 3 Insights

Why does the ADC conversion start again inside the ISR?

Because after reading the ADC value in the ISR (see Step 2 in execution_table), we need to start a new conversion to keep reading fresh data continuously.

What triggers the ADC ISR to run?

The ADC hardware triggers the ISR automatically when the conversion completes (Step 2 and 4 in execution_table), so the CPU can read the result immediately.

Why don't we read the ADC value outside the ISR?

Reading inside the ISR ensures we get the value right after conversion finishes, avoiding delays or missed data, as shown in the execution_table steps.

Visual Quiz - 3 Questions

Look at the execution_table, what is the ADC Conversion Status at Step 1?

A Complete

B Idle

C In Progress

D Stopped

💡 Hint

Check the 'ADC Conversion Status' column at Step 1 in the execution_table.

At which step does the ISR read the ADC value?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look for 'ISR Called' and 'ADC Value Read' columns in execution_table.

If the ADC conversion never completes, what happens to the ISR calls?

A ISR is called repeatedly

B ISR is never called

C ISR is called once

D ISR calls happen randomly

💡 Hint

Refer to the 'ADC Conversion Complete?' decision in concept_flow and ISR trigger in execution_table.

Concept Snapshot

ADC interrupt-driven reading:
- Start ADC conversion
- Wait for conversion complete interrupt
- ISR reads ADC value
- Process value inside ISR
- Restart conversion for continuous reading

Full Transcript

This visual execution trace shows how ADC interrupt-driven reading works. The ADC starts conversion and waits until it finishes. When done, it triggers an interrupt. The interrupt service routine (ISR) runs, reads the ADC value, processes it, and starts a new conversion. Variables like adc_value update after each ISR call. This cycle repeats until ADC is stopped or powered down. Key points include starting conversion inside ISR and ISR triggering only after 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