Timer overflow behavior in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when a 8-bit timer overflows?

Consider an 8-bit timer that increments every millisecond and resets to 0 after reaching 255. What will be the value of the timer after 260 milliseconds?

Embedded C

uint8_t timer = 0;
for (int i = 0; i < 260; i++) {
    timer++;
}
printf("%u", timer);

A 260

B 4

C 0

D 255

💡 Hint

Think about how an 8-bit unsigned integer behaves when it exceeds its max value.

An 8-bit unsigned integer can hold values from 0 to 255. After 255, it wraps around to 0. Incrementing 260 times means it overflows once (256 counts) and then counts 4 more, so the timer value is 4.

🧠 Conceptual

intermediate

Why does a timer overflow cause an interrupt?

In embedded systems, why is a timer overflow often used to trigger an interrupt?

A Because the timer overflow indicates the timer reached its maximum count and can be used to perform periodic tasks.

B Because the timer overflow resets the CPU clock speed automatically.

C Because the timer overflow disables all other interrupts.

D Because the timer overflow clears the memory registers.

💡 Hint

Think about what happens when the timer reaches its max count and why that event is useful.

Timer overflow signals that a fixed time period has passed. This event can trigger an interrupt to run code periodically without constant CPU checking.

🔧 Debug

advanced

Identify the cause of incorrect timer overflow handling

This code is intended to toggle an LED every time the 16-bit timer overflows. What is the bug causing the LED not to toggle?

Embedded C

volatile uint16_t timer = 0;
volatile uint8_t led_state = 0;

void timer_overflow_isr() {
    if (timer == 0xFFFF) {
        led_state = !led_state;
        timer = 0;
    }
    timer++;
}

A The ISR should not modify the timer variable.

B The led_state variable should be uint16_t, not uint8_t.

C The timer should be reset to 0xFFFF instead of 0 after overflow.

D The timer increments after checking for overflow, so the overflow condition is never true.

💡 Hint

Consider the order of increment and overflow check.

The timer is checked for 0xFFFF before incrementing, so it never reaches 0xFFFF inside the ISR. The increment should happen before the check or the condition adjusted.

📝 Syntax

advanced

Which code snippet correctly handles 8-bit timer overflow?

Select the code snippet that correctly detects an 8-bit timer overflow and toggles an LED.

A timer++; if (timer == 0) { led_state = !led_state; }

B if (timer++ == 255) { led_state = !led_state; timer = 0; }

C if (++timer == 255) { led_state = !led_state; timer = 0; }

D if (timer == 255) { led_state = !led_state; timer = 0; } timer++;

💡 Hint

Remember that an 8-bit unsigned integer overflows from 255 to 0.

Incrementing timer first and then checking if it is zero detects overflow correctly. Option A increments timer, then checks if it wrapped to zero.

🚀 Application

expert

Calculate timer overflow interval for a 16-bit timer

A 16-bit timer runs at a clock frequency of 1 MHz and increments every clock cycle. How long in milliseconds does it take for the timer to overflow?

A 0.065536 ms

B 65536 ms

C 65.536 ms

D 1 ms

💡 Hint

Calculate the time for 65536 counts at 1 MHz clock.

Timer counts from 0 to 65535 (65536 counts). At 1 MHz, each count is 1 microsecond. So overflow time = 65536 * 1 us = 65.536 ms.

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