Timer overflow behavior in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a timer variable that counts up to overflow.

Embedded C

volatile unsigned int timer = [1];

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

A1

B65535

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting timer at max value causes immediate overflow.

Using negative values for unsigned timer.

The timer should start counting from 0 to overflow.

2fill in blank

medium

Complete the code to check if the timer has overflowed.

Embedded C

if (timer == [1]) {
    // Timer overflowed
}

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A0

B-1

C65536

D65535

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 65536 which is out of range for 16-bit.

Checking for zero which is the reset value after overflow.

The timer overflows when it reaches its maximum value 65535 for a 16-bit timer.

3fill in blank

hard

Fix the error in the overflow check condition.

Embedded C

if (timer [1] 65535) {
    // Handle overflow
}

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A==

B>

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' causes false positives.

Using '!=' triggers on all values except 65535.

The timer equals 65535 at overflow, so use '==' to check.

4fill in blank

hard

Fill both blanks to create a dictionary that maps timer values to overflow status.

Embedded C

const char* timer_status = (timer [1] 65535) ? "[2]" : "Running";

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A==

BOverflow

C!=

DComplete

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' causes wrong status.

Using wrong string for overflow status.

Check if timer equals 65535 to mark overflow status.

5fill in blank

hard

Fill all three blanks to implement a function that resets the timer on overflow.

Embedded C

void reset_timer_if_overflow(unsigned int* timer) {
    if (*timer [1] [2]) {
        *timer = [3];
    }
}

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

A==

B65535

C0

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' causes reset at wrong times.

Resetting timer to max value instead of zero.

The function checks if timer equals 65535 and resets it to 0.

Practice

1. What happens when a timer in embedded C reaches its maximum count value?

easy

A. It causes the microcontroller to reset.

B. It stops counting permanently.

C. It resets to zero and continues counting (overflow).

D. It triggers a syntax error in the program.

Solution

1. Step 1: Understand timer counting behavior

   Timers count up from zero to a maximum value based on their bit size.

2. Step 2: Identify what happens at max value

   When the timer reaches its max, it resets to zero and continues counting, called overflow.

3. Final Answer:

   It resets to zero and continues counting (overflow). -> Option C

4. Quick Check:

   Timer overflow means reset to zero [OK]

Hint: Timer overflow means count resets to zero [OK]

Common Mistakes:

• Thinking timer stops at max value
• Assuming overflow causes microcontroller reset
• Confusing overflow with syntax errors

2. Which of the following is the correct way to check a timer overflow flag in embedded C?

easy

A. if (TIMER_FLAG != 1) { /* handle overflow */ }

B. if (TIMER_FLAG == 1) { /* handle overflow */ }

C. if (TIMER_FLAG == 0) { /* handle overflow */ }

D. if TIMER_FLAG = 1 { /* handle overflow */ }

Solution

1. Step 1: Understand syntax for condition check

   In C, conditions use double equals '==' to compare values inside parentheses.

2. Step 2: Identify correct overflow flag check

   Checking if TIMER_FLAG equals 1 means overflow occurred, so use 'if (TIMER_FLAG == 1)'.

3. Final Answer:

   if (TIMER_FLAG == 1) { /* handle overflow */ } -> Option B

4. Quick Check:

   Use '==' for comparison in if conditions [OK]

Hint: Use '==' to compare flags in if statements [OK]

Common Mistakes:

• Using single '=' instead of '==' in if condition
• Checking for zero instead of one for overflow
• Missing parentheses around condition

3. Given this code snippet, what will be the value of overflow_count after the timer overflows twice?

volatile uint8_t timer = 254;
volatile uint8_t overflow_count = 0;

void timer_ISR() {
  timer++;
  if (timer == 0) {
    overflow_count++;
  }
}

medium

A. 2

B. 1

C. 0

D. 254

Solution

1. Step 1: Understand timer increment and overflow

   Timer starts at 254, increments to 255, then overflows to 0, triggering overflow_count++.

2. Step 2: Count overflows after two increments

   After two overflows (timer wraps to 0 twice), overflow_count increments twice, so value is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Two overflows increment count twice [OK]

Hint: Overflow increments count when timer resets to zero [OK]

Common Mistakes:

• Confusing timer value with overflow count
• Forgetting overflow increments counter
• Assuming overflow_count resets with timer

4. Identify the error in this timer overflow handling code:

volatile uint8_t timer = 0;
volatile uint8_t overflow_flag = 0;

void timer_ISR() {
  timer++;
  if (timer = 0) {
    overflow_flag = 1;
  }
}

medium

A. Overflow flag should be reset inside ISR.

B. Timer variable should be uint16_t, not uint8_t.

C. Missing semicolon after timer++.

D. Using assignment '=' instead of comparison '==' in if condition.

Solution

1. Step 1: Check if condition syntax

   The condition uses '=' which assigns 0 to timer instead of comparing it.

2. Step 2: Correct comparison operator

   It should use '==' to check if timer equals 0 for overflow detection.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition. -> Option D

4. Quick Check:

   Use '==' for comparisons, not '=' [OK]

Hint: Use '==' in if conditions, not '=' [OK]

Common Mistakes:

• Confusing '=' with '==' in conditions
• Assuming timer type causes error here
• Ignoring missing flag reset logic

5. You want to measure a time interval longer than your 8-bit timer max count (255). Which method correctly uses timer overflow to achieve this?

hard

A. Count the number of overflows in an interrupt and multiply by 256 plus current timer value.

B. Reset the timer to zero manually before it overflows.

C. Increase the timer frequency to count faster.

D. Ignore overflow and only read the timer value.

Solution

1. Step 1: Understand timer max count limitation

   An 8-bit timer counts from 0 to 255, then overflows back to 0.

2. Step 2: Use overflow count to extend timing range

   Counting overflows lets you track how many times timer wrapped, extending measurable time.

3. Step 3: Calculate total time

   Total count = (overflow_count * 256) + current timer value, giving full interval.

4. Final Answer:

   Count the number of overflows in an interrupt and multiply by 256 plus current timer value. -> Option A

5. Quick Check:

   Overflow count extends timer range [OK]

Hint: Use overflow count x max + timer for long intervals [OK]

Common Mistakes:

• Resetting timer manually loses overflow info
• Increasing frequency doesn't extend max count
• Ignoring overflow loses timing accuracy