Timer overflow behavior in Embedded C - Step-by-Step Execution

Concept Flow - Timer overflow behavior

Timer starts at 0

↓

Timer counts up each tick

↓

Timer reaches max value

↓

Timer overflows to 0

↓

Overflow interrupt triggers (optional)

↓

Timer continues counting

The timer counts up from zero to its maximum value, then resets to zero (overflows), optionally triggering an interrupt before continuing.

Execution Sample

Embedded C

uint8_t timer = 0;
for (int i = 0; i < 260; i++) {
  timer++;
  if (timer == 0) {
    // overflow occurred
  }
}

This code simulates an 8-bit timer incrementing and detecting when it overflows back to zero.

Execution Table

Iteration	timer before ++	timer after ++	Overflow?
1	0	1	No
2	1	2	No
3	2	3	No
254	253	254	No
255	254	255	No
256	255	0	Yes
257	0	1	No
258	1	2	No
259	2	3	No
260	3	4	No

💡 Loop ends after 260 increments; overflow detected at iteration 256 when timer resets from 255 to 0.

Variable Tracker

Variable	Start	After 1	After 2	After 255	After 256	After 260
timer	0	1	2	255	0	4

Key Moments - 3 Insights

Why does the timer reset to 0 after reaching 255?

Because the timer is an 8-bit variable, it can only hold values 0 to 255. When incremented at 255, it overflows and wraps back to 0, as shown in execution_table row 6.

How can we detect when an overflow happens?

By checking if the timer value becomes 0 immediately after incrementing, as done in the code and shown in execution_table at iteration 256.

Does the timer stop after overflow?

No, the timer continues counting from 0 upwards after overflow, as seen in execution_table rows after iteration 256.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of timer after iteration 256?

A 0

B 255

C 1

D 256

💡 Hint

Check the 'timer after ++' column at iteration 256 in the execution_table.

At which iteration does the timer overflow occur?

A 255

B 260

C 256

D 1

💡 Hint

Look for the row where 'Overflow?' is 'Yes' in the execution_table.

If the timer was 16-bit instead of 8-bit, how would the overflow iteration change?

A It would overflow at iteration 256

B It would overflow at iteration 65536

C It would never overflow

D It would overflow at iteration 260

💡 Hint

16-bit timer max value is 65535, so overflow happens after that many increments.

Concept Snapshot

Timer overflow behavior:
- Timer counts up from 0 to max value (e.g., 255 for 8-bit)
- When max reached, next increment resets timer to 0 (overflow)
- Overflow can trigger interrupt or flag
- Timer continues counting after overflow
- Detect overflow by checking if timer == 0 after increment

Full Transcript

This visual execution shows how a timer variable in embedded C counts up from zero, increments each step, and overflows when it reaches its maximum value. For an 8-bit timer, the max value is 255. When incremented at 255, it resets to zero, which is called overflow. The execution table traces each increment, showing timer values before and after increment, and marks when overflow happens. The variable tracker summarizes timer values at key steps. Key moments clarify why overflow happens, how to detect it, and that counting continues after overflow. The quiz tests understanding of timer values at specific iterations and how overflow changes with timer size. The snapshot summarizes the core behavior of timer overflow in embedded systems.