Timer callback execution context in FreeRTOS

Introduction

Timer callbacks run code when a timer finishes. They help do tasks regularly without blocking other work.

You want to blink an LED every second without stopping other code.

You need to check a sensor value every 500 milliseconds.

You want to send a message after a delay without pausing the whole program.

You want to perform cleanup tasks periodically in the background.

Syntax

FreeRTOS

void TimerCallback(TimerHandle_t xTimer) {
    // Your code here
}

// Create timer
TimerHandle_t xTimer = xTimerCreate(
    "TimerName",          // Timer name
    pdMS_TO_TICKS(1000),  // Timer period in ticks
    pdTRUE,               // Auto-reload (true for repeating)
    (void *)0,            // Timer ID
    TimerCallback         // Callback function
);

// Start timer
xTimerStart(xTimer, 0);

The callback function runs inside the timer service task context, not your main task.

Keep the callback code short and avoid blocking calls to not delay other timers.

Examples

This example toggles an LED every second using a repeating timer.

FreeRTOS

void MyTimerCallback(TimerHandle_t xTimer) {
    // Toggle LED
    ToggleLED();
}

TimerHandle_t xTimer = xTimerCreate("LEDTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, MyTimerCallback);
xTimerStart(xTimer, 0);

This example runs the callback once after 5 seconds using a one-shot timer.

FreeRTOS

void OneShotCallback(TimerHandle_t xTimer) {
    // Send a message once after delay
    SendMessage();
}

TimerHandle_t xTimer = xTimerCreate("OneShot", pdMS_TO_TICKS(5000), pdFALSE, NULL, OneShotCallback);
xTimerStart(xTimer, 0);

Sample Program

This program creates a repeating timer that calls the callback every second. The callback prints how many times it was called.

The timer runs in the timer service task context, so it does not block the main program.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"
#include <stdio.h>

void TimerCallback(TimerHandle_t xTimer) {
    static int count = 0;
    count++;
    printf("Timer callback called %d times\n", count);
}

int main(void) {
    TimerHandle_t xTimer = xTimerCreate("MyTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);
    if (xTimer == NULL) {
        printf("Timer creation failed\n");
        return 1;
    }

    if (xTimerStart(xTimer, 0) != pdPASS) {
        printf("Timer start failed\n");
        return 1;
    }

    // Start the scheduler
    vTaskStartScheduler();

    // Should never reach here
    return 0;
}

Important Notes

Timer callbacks run in the timer service task, so avoid long or blocking operations inside them.

Do not call FreeRTOS API functions that are not safe from timer callback context.

Use synchronization methods if sharing data between timer callbacks and other tasks.

Summary

Timer callbacks run when a timer expires, inside the timer service task context.

Keep callback code short and non-blocking to avoid delaying other timers.

Use timers to perform periodic or delayed actions without stopping main tasks.

Practice

1. In FreeRTOS, where does a timer callback function execute when a timer expires?

easy

A. Inside the main application task context

B. Inside an interrupt service routine (ISR)

C. Inside the timer service task context

D. Inside a hardware timer peripheral

Solution

1. Step 1: Understand timer callback execution

   Timer callbacks in FreeRTOS run when the timer expires, but not in the main task or ISR context.

2. Step 2: Identify the correct context

   The timer callback runs inside the timer service task context, which is a dedicated task for handling timers.

3. Final Answer:

   Inside the timer service task context -> Option C

4. Quick Check:

   Timer callback context = timer service task [OK]

Hint: Timer callbacks run in timer service task, not main or ISR [OK]

Common Mistakes:

• Thinking timer callbacks run in ISR context
• Assuming callbacks run in main application task
• Confusing hardware timer peripheral with callback context

2. Which of the following is the correct way to define a FreeRTOS timer callback function?

easy

A. void TimerCallback(TimerHandle_t xTimer)

B. void TimerCallback(void)

C. int TimerCallback(int timerId)

D. void TimerCallback(void *param)

Solution

1. Step 1: Recall FreeRTOS timer callback signature

   The callback function must accept a TimerHandle_t parameter representing the timer.

2. Step 2: Match the correct function signature

   Only the signature with void TimerCallback(TimerHandle_t xTimer) matches FreeRTOS requirements.

3. Final Answer:

   void TimerCallback(TimerHandle_t xTimer) -> Option A

4. Quick Check:

   Callback signature = void func(TimerHandle_t) [OK]

Hint: Timer callbacks take TimerHandle_t parameter only [OK]

Common Mistakes:

• Using no parameters in callback
• Using void* or int instead of TimerHandle_t
• Returning int instead of void

3. Given the following code snippet, what will be printed when the timer expires?

void vTimerCallback(TimerHandle_t xTimer) {
    printf("Timer expired\n");
}

int main() {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, vTimerCallback);
    xTimerStart(xTimer, 0);
    vTaskDelay(pdMS_TO_TICKS(1500));
    return 0;
}

medium

A. Timer expired

B. No output

C. Compilation error

D. Runtime error

Solution

1. Step 1: Analyze timer creation and start

   The timer is created with 1 second period, one-shot, and started, but vTaskStartScheduler() is missing.

2. Step 2: Check scheduler requirement

   FreeRTOS timers require the scheduler (and timer service task) to be running via vTaskStartScheduler(); without it, the callback never executes, producing no output.

3. Final Answer:

   No output -> Option B

4. Quick Check:

   Missing vTaskStartScheduler() = no callback execution [OK]

Hint: FreeRTOS timers require vTaskStartScheduler() [OK]

Common Mistakes:

• Assuming scheduler starts automatically
• Thinking timer expires without scheduler running
• Believing vTaskDelay alone allows callback execution

4. Identify the problem in this timer callback code snippet:

void vTimerCallback(TimerHandle_t xTimer) {
    vTaskDelay(pdMS_TO_TICKS(100));
    printf("Timer callback done\n");
}

medium

A. Using vTaskDelay inside timer callback blocks timer service task

B. Missing return statement in callback

C. Incorrect parameter type in callback

D. printf cannot be used in timer callbacks

Solution

1. Step 1: Understand timer callback context

   Timer callbacks run inside the timer service task, which handles all timers sequentially.

2. Step 2: Analyze use of vTaskDelay in callback

   Calling vTaskDelay blocks the timer service task, delaying other timers and causing timing issues.

3. Final Answer:

   Using vTaskDelay inside timer callback blocks timer service task -> Option A

4. Quick Check:

   Blocking calls in timer callback delay all timers [OK]

Hint: Never block or delay inside timer callbacks [OK]

Common Mistakes:

• Thinking callbacks can safely block or delay
• Assuming missing return causes error
• Believing printf is disallowed in callbacks

5. You want to toggle an LED every 500ms using a FreeRTOS timer. Which approach correctly uses the timer callback without blocking the timer service task?

void vTimerCallback(TimerHandle_t xTimer) {
    // Option 1
    // vTaskDelay(pdMS_TO_TICKS(100));
    // toggleLED();

    // Option 2
    toggleLED();
}

int main() {
    TimerHandle_t xTimer = xTimerCreate("LEDTimer", pdMS_TO_TICKS(500), pdTRUE, NULL, vTimerCallback);
    xTimerStart(xTimer, 0);
    vTaskStartScheduler();
    return 0;
}

hard

A. Use Option 1 with vTaskDelay inside callback

B. Call toggleLED from main task instead of timer

C. Use a blocking delay in main before starting timer

D. Use Option 2 with direct toggleLED call in callback

Solution

1. Step 1: Review timer callback best practices

   Timer callbacks should be short and non-blocking to avoid delaying other timers.

2. Step 2: Evaluate options for toggling LED

   Option 1 uses vTaskDelay inside callback, which blocks timer service task. Option 2 calls toggleLED directly without delay, which is correct.

3. Final Answer:

   Use Option 2 with direct toggleLED call in callback -> Option D

4. Quick Check:

   Non-blocking callback = direct toggleLED call [OK]

Hint: Keep timer callbacks short and non-blocking [OK]

Common Mistakes:

• Adding delays inside timer callbacks
• Trying to toggle LED outside timer context
• Blocking main task instead of using timer