Timer overflow behavior in Embedded C - Mini Project: Build & Apply

Timer Overflow Behavior

📖 Scenario: You are working with a simple microcontroller timer that counts from 0 to 255 and then overflows back to 0. This timer is used to measure time intervals in milliseconds.

🎯 Goal: Build a small program that simulates the timer counting up, detects when it overflows, and counts how many times the overflow happens.

📋 What You'll Learn

Create a variable timer to hold the timer value starting at 0

Create a variable overflow_count to count how many times the timer overflows

Use a loop to increment the timer variable simulating timer ticks

Detect when the timer overflows from 255 back to 0 and increase overflow_count

Print the final overflow_count after simulating 300 timer ticks

💡 Why This Matters

🌍 Real World

Microcontrollers use timers to measure time intervals and generate delays. Understanding overflow helps avoid errors in timing calculations.

💼 Career

Embedded software engineers often work with hardware timers and must handle overflow behavior correctly to build reliable systems.

1

Create timer and overflow counter variables

Create an unsigned char variable called timer and set it to 0. Also create an unsigned int variable called overflow_count and set it to 0.

Embedded C

unsigned char timer = 0;
unsigned int overflow_count = 0;
// Your code here

Hint

Use unsigned char for the timer because it counts from 0 to 255.

2

Set the number of timer ticks to simulate

Create an int variable called ticks and set it to 300 to represent the number of timer increments to simulate.

Embedded C

unsigned char timer = 0;
unsigned int overflow_count = 0;
int ticks = 300;
// Your code here

Hint

This variable controls how many times the timer will increment in the simulation.

3

Simulate timer increments and detect overflow

Write a for loop using int i from 0 to less than ticks. Inside the loop, increment timer by 1. If timer becomes 0 after incrementing, increase overflow_count by 1.

Embedded C

unsigned char timer = 0;
unsigned int overflow_count = 0;
int ticks = 300;

// Your code here

Hint

Remember that when timer reaches 255 and increments, it wraps to 0 because it is an unsigned char.

4

Print the overflow count

Write a printf statement to print the text "Overflow count: " followed by the value of overflow_count and a newline.

Embedded C

unsigned char timer = 0;
unsigned int overflow_count = 0;
int ticks = 300;

for (int i = 0; i < ticks; i++) {
    timer++;
    if (timer == 0) {
        overflow_count++;
    }
}

// Your code here

Hint

The timer overflows once after 256 increments, so the overflow count should be 1 after 300 ticks.

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