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.