One-shot vs auto-reload timers in FreeRTOS - Performance Comparison

Time Complexity: One-shot vs auto-reload timers

O(n)

Understanding Time Complexity

When using timers in FreeRTOS, it is important to understand how their execution time changes as the number of timers grows.

We want to know how the timer type affects the cost of running timer callbacks repeatedly.

Scenario Under Consideration

Analyze the time complexity of these timer callback executions.

// Create a one-shot timer
TimerHandle_t oneShotTimer = xTimerCreate("OneShot", pdMS_TO_TICKS(1000), pdFALSE, NULL, callback);

// Create an auto-reload timer
TimerHandle_t autoReloadTimer = xTimerCreate("AutoReload", pdMS_TO_TICKS(1000), pdTRUE, NULL, callback);

// Start timers
xTimerStart(oneShotTimer, 0);
xTimerStart(autoReloadTimer, 0);

// Timer callback function
void callback(TimerHandle_t xTimer) {
    // Code to run when timer expires
}
    

This code creates two timers: one that runs once and one that runs repeatedly every interval.

Identify Repeating Operations

Look at what repeats when timers run.

• Primary operation: The callback function execution each time a timer expires.
• How many times: One-shot timer runs once; auto-reload timer runs repeatedly at fixed intervals.

How Execution Grows With Input

Imagine you have more timers running at once.

Number of Timers (n)	Approx. Callback Executions per Interval
10	10 (if all auto-reload)
100	100
1000	1000

Each timer callback runs once per interval, so total executions grow directly with the number of timers.

Final Time Complexity

Time Complexity: O(n)

This means the total work grows linearly with the number of timers running their callbacks.

Common Mistake

[X] Wrong: "One-shot timers are always faster because they run only once."

[OK] Correct: While one-shot timers run once, if you have many timers or restart them often, the total callback executions can still add up and affect performance.

Interview Connect

Understanding how timer callbacks scale helps you design responsive and efficient real-time systems, a key skill in embedded programming.

Self-Check

What if we changed all timers from auto-reload to one-shot timers that restart themselves manually? How would the time complexity change?

Practice

1. What is the main difference between a one-shot timer and an auto-reload timer in FreeRTOS?

easy

A. One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals.

B. One-shot timers run repeatedly; auto-reload timers run only once.

C. One-shot timers are faster than auto-reload timers.

D. Auto-reload timers cannot be stopped once started.

Solution

1. Step 1: Understand timer behavior

   One-shot timers execute their callback function only once after the specified delay.

2. Step 2: Compare with auto-reload timers

   Auto-reload timers automatically restart after each expiration, running repeatedly at fixed intervals.

3. Final Answer:

   One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals. -> Option A

4. Quick Check:

   One-shot = single run, Auto-reload = repeated runs [OK]

Hint: One-shot = once, Auto-reload = repeat [OK]

Common Mistakes:

• Confusing which timer runs repeatedly
• Thinking one-shot timers repeat automatically
• Assuming auto-reload timers cannot be stopped

2. Which of the following is the correct way to create an auto-reload timer in FreeRTOS using xTimerCreate?

easy

A. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);

B. xTimerCreate("Timer", 1000, pdTRUE, NULL, TimerCallback);

C. xTimerCreate("Timer", 1000, pdFALSE, NULL, TimerCallback);

D. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);

Solution

1. Step 1: Identify auto-reload parameter

   The third parameter of xTimerCreate is uxAutoReload, which should be pdTRUE for auto-reload timers.

2. Step 2: Check time parameter format

   Use pdMS_TO_TICKS(1000) to convert milliseconds to ticks correctly.

3. Final Answer:

   xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback); -> Option D

4. Quick Check:

   Auto-reload needs pdTRUE and tick conversion [OK]

Hint: Auto-reload = pdTRUE, use pdMS_TO_TICKS for time [OK]

Common Mistakes:

• Using pdFALSE for auto-reload timers
• Passing raw milliseconds instead of ticks
• Confusing parameter order

3. Consider this FreeRTOS timer code snippet:

TimerHandle_t timer = xTimerCreate("MyTimer", pdMS_TO_TICKS(500), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What will happen when this timer expires?

medium

A. The TimerCallback function will be called once after 500 ms.

B. The TimerCallback function will be called repeatedly every 500 ms.

C. The timer will never start because pdFALSE is invalid.

D. The TimerCallback function will be called immediately and repeatedly.

Solution

1. Step 1: Analyze timer creation parameters

   The timer is created with pdFALSE for auto-reload, meaning it is a one-shot timer.

2. Step 2: Understand timer start and expiration

   After starting, the timer waits 500 ms, then calls TimerCallback once and stops.

3. Final Answer:

   The TimerCallback function will be called once after 500 ms. -> Option A

4. Quick Check:

   pdFALSE means one-shot = single callback [OK]

Hint: pdFALSE means one-shot, callback runs once [OK]

Common Mistakes:

• Thinking pdFALSE means repeated calls
• Assuming timer starts immediately
• Confusing pdFALSE with invalid parameter

4. You wrote this code to create an auto-reload timer but it only runs once:

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What is the likely error?

medium

A. The timer period is too short to see repeated calls.

B. xTimerStart was called with zero block time.

C. The auto-reload parameter is set to pdFALSE instead of pdTRUE.

D. The timer callback function is missing.

Solution

1. Step 1: Check auto-reload parameter

   The timer was created with pdFALSE, which means it is a one-shot timer.

2. Step 2: Understand effect on timer behavior

   Because of pdFALSE, the timer runs only once and does not auto-reload.

3. Final Answer:

   The auto-reload parameter is set to pdFALSE instead of pdTRUE. -> Option C

4. Quick Check:

   pdFALSE disables auto-reload, causing single run [OK]

Hint: Auto-reload needs pdTRUE, not pdFALSE [OK]

Common Mistakes:

• Ignoring auto-reload parameter
• Thinking block time affects reload
• Assuming callback absence causes single run

5. You want to toggle an LED every 2 seconds indefinitely using a FreeRTOS timer. Which timer type and setup should you use?

hard

A. Create a one-shot timer with 2000 ms period and restart it manually each time.

B. Create an auto-reload timer with 2000 ms period to toggle LED automatically.

C. Create a one-shot timer with 1000 ms period and toggle LED twice per cycle.

D. Create an auto-reload timer with 1000 ms period and toggle LED every 2 cycles.

Solution

1. Step 1: Identify requirement for repeated action

   Toggling LED every 2 seconds indefinitely means the action repeats automatically.

2. Step 2: Choose timer type and period

   An auto-reload timer with 2000 ms period will call the toggle function every 2 seconds without manual restart.

3. Final Answer:

   Create an auto-reload timer with 2000 ms period to toggle LED automatically. -> Option B

4. Quick Check:

   Repeated action = auto-reload timer with correct period [OK]

Hint: Repeated tasks use auto-reload timers with correct period [OK]

Common Mistakes:

• Using one-shot timer and forgetting to restart
• Setting wrong timer period
• Confusing toggle frequency with timer period