Input capture mode in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the input capture mode on Timer1.

Embedded C

TCCR1B = (1 << ICES1) | (1 << [1]);

Drag options to blanks, or click blank then click option'

ACS12

BCS11

CCS10

DICNC1

Attempts:

3 left

💡 Hint

Common Mistakes

Using ICNC1 instead of a clock select bit.

Choosing CS11 or CS12 which set different prescalers.

The CS10 bit sets the clock prescaler to start the timer in input capture mode.

2fill in blank

medium

Complete the code to enable the input capture interrupt.

Embedded C

TIMSK1 |= (1 << [1]);

Drag options to blanks, or click blank then click option'

AICIE1

BOCIE1A

COCIE1B

DTOIE1

Attempts:

3 left

💡 Hint

Common Mistakes

Using TOIE1 which is for overflow interrupt.

Using output compare interrupt enables OCIE1A or OCIE1B.

ICIE1 enables the input capture interrupt for Timer1.

3fill in blank

hard

Fix the error in the input capture ISR declaration.

Embedded C

ISR(TIMER1_[1]_vect) {
    // Handle input capture event
}

Drag options to blanks, or click blank then click option'

ACAPT

BCOMPA

COVF

DCAPTURE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'CAPTURE' instead of 'CAPT'.

Using overflow (OVF) or compare (COMPA) vectors by mistake.

The correct ISR vector name for input capture on Timer1 is TIMER1_CAPT_vect.

4fill in blank

hard

Fill both blanks to configure input capture to trigger on falling edge and enable noise canceler.

Embedded C

TCCR1B = (0 << [1]) | (1 << [2]);

Drag options to blanks, or click blank then click option'

AICES1

BICNC1

CCS10

DCS11

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing ICES1 and ICNC1 bits.

Using clock select bits instead of these control bits.

Setting ICES1 to 0 triggers on falling edge; setting ICNC1 to 1 enables noise canceler.

5fill in blank

hard

Fill all three blanks to read the input capture value, clear the flag, and start the timer with no prescaling.

Embedded C

uint16_t capture = ICR1;
TIFR1 |= (1 << [1]);
TCCR1B = (1 << [2]) | (0 << [3]);

Drag options to blanks, or click blank then click option'

AICF1

BCS10

CICES1

DTOIE1

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to clear the overflow flag instead of input capture flag.

Using wrong bits to start the timer.

Confusing edge select bit with timer start bits.

ICF1 is the input capture flag to clear; CS10 starts timer with no prescaling; ICES1 selects edge.

Practice

1. What is the main purpose of Input Capture Mode in embedded C timer modules?

easy

A. To record the timer value when a specific input event occurs

B. To generate PWM signals automatically

C. To reset the timer periodically

D. To output a fixed frequency signal

Solution

1. Step 1: Understand Input Capture Mode function

   Input Capture Mode records the timer count when an input event (like a signal edge) happens.

2. Step 2: Compare options with this function

   Only To record the timer value when a specific input event occurs describes recording the timer value on an input event, which matches the purpose.

3. Final Answer:

   To record the timer value when a specific input event occurs -> Option A

4. Quick Check:

   Input Capture = Record timer on event [OK]

Hint: Input Capture records timer on input events [OK]

Common Mistakes:

• Confusing Input Capture with PWM generation
• Thinking it resets the timer automatically
• Assuming it outputs signals instead of capturing

2. Which of the following is the correct way to configure an input capture to trigger on a rising edge in embedded C?

easy

A. Clear the timer counter register

B. Set the capture edge select bit to 0

C. Set the capture edge select bit to 1

D. Enable the PWM output mode

Solution

1. Step 1: Identify capture edge select bit meaning

   Usually, setting the capture edge select bit to 1 configures capture on rising edge; 0 is for falling edge.

2. Step 2: Match options with rising edge setting

   Set the capture edge select bit to 1 sets the bit to 1, which is correct for rising edge trigger.

3. Final Answer:

   Set the capture edge select bit to 1 -> Option C

4. Quick Check:

   Edge select = 1 for rising edge [OK]

Hint: Rising edge capture usually means edge bit = 1 [OK]

Common Mistakes:

• Confusing edge select bit values
• Trying to clear timer instead of setting edge
• Enabling unrelated PWM mode

3. Given this code snippet for input capture:

volatile uint16_t captured_value = 0;
void ISR_InputCapture() {
  captured_value = TIMER_CAPTURE_REG;
  CLEAR_CAPTURE_INTERRUPT_FLAG();
}

// Assume timer increments every 1us
// Input event occurs at 1500us

What will be the value of captured_value after the input event?

medium

A. 1500

B. 0

C. Timer overflow value

D. Undefined

Solution

1. Step 1: Understand timer and capture behavior

   The timer increments every 1 microsecond, so at 1500us the timer count is 1500.

2. Step 2: ISR captures timer value on input event

   The ISR stores the current timer count (1500) into captured_value.

3. Final Answer:

   1500 -> Option A

4. Quick Check:

   Timer count at event = captured value = 1500 [OK]

Hint: Captured value equals timer count at event time [OK]

Common Mistakes:

• Assuming captured value resets to zero
• Confusing overflow with current count
• Ignoring ISR capture assignment

4. Identify the error in this input capture interrupt service routine:

void ISR_InputCapture() {
  uint16_t value = TIMER_CAPTURE_REG;
  process(value);
  CLEAR_CAPTURE_INTERRUPT_FLAG();
}

What is the main problem?

medium

A. The process function should not be called inside ISR

B. The interrupt flag is cleared before reading the capture register

C. The ISR does not enable the timer

D. The captured value is stored in a local variable and lost after ISR ends

Solution

1. Step 1: Analyze variable scope in ISR

   The variable value is local to ISR and lost after ISR finishes.

2. Step 2: Consider data usage outside ISR

   If the captured value is needed elsewhere, it should be stored in a global or volatile variable.

3. Final Answer:

   The captured value is stored in a local variable and lost after ISR ends -> Option D

4. Quick Check:

   Local variable in ISR = lost data [OK]

Hint: Store captured data in global variable, not local [OK]

Common Mistakes:

• Clearing interrupt flag too early
• Not enabling timer before capture
• Avoiding function calls in ISR (not always wrong)

5. You want to measure the pulse width of a signal using input capture mode on a timer. Which approach correctly uses input capture to find the pulse width?

hard

A. Reset timer on every input event and read timer after fixed delay

B. Capture timer value on rising edge, then capture again on falling edge; subtract first from second

C. Capture timer value only on rising edges and multiply by 2

D. Use input capture only on falling edges and ignore rising edges

Solution

1. Step 1: Understand pulse width measurement

   Pulse width is the time between rising and falling edges of a signal.

2. Step 2: Use input capture on both edges

   Capture timer value at rising edge (start) and falling edge (end), then subtract to get pulse width.

3. Final Answer:

   Capture timer value on rising edge, then capture again on falling edge; subtract first from second -> Option B

4. Quick Check:

   Pulse width = falling edge time - rising edge time [OK]

Hint: Measure pulse by capturing rising and falling edges [OK]

Common Mistakes:

• Capturing only one edge and guessing width
• Resetting timer incorrectly
• Ignoring one edge of the pulse