Timer callback execution context in FreeRTOS - Mini Project: Build & Apply

Timer Callback Execution Context in FreeRTOS

📖 Scenario: You are working on a simple embedded system using FreeRTOS. You want to create a timer that triggers a callback function every second to toggle an LED. Understanding the execution context of the timer callback is important to write safe and correct code.

🎯 Goal: Build a FreeRTOS program that creates a software timer with a callback function. The callback toggles a variable representing an LED state. You will learn how to set up the timer, configure its callback, and understand the context in which the callback runs.

📋 What You'll Learn

Create a FreeRTOS software timer named xTimer with a 1000 ms period

Define a callback function named vTimerCallback that toggles an ledState variable

Start the timer using xTimerStart

Print the ledState value inside the callback to observe changes

💡 Why This Matters

🌍 Real World

Timers are used in embedded systems to perform actions periodically, such as blinking LEDs, reading sensors, or sending data.

💼 Career

Understanding timer callbacks and their execution context is essential for embedded software developers working with real-time operating systems like FreeRTOS.

1

Create the LED state variable and timer handle

Create a global variable called ledState initialized to 0. Also declare a variable called xTimer of type TimerHandle_t.

FreeRTOS

int ledState = 0;
TimerHandle_t xTimer;
// Your code here

Hint

Use int ledState = 0; to represent the LED off state. Declare xTimer as TimerHandle_t to hold the timer reference.

2

Create the timer with a callback function

Define a function called vTimerCallback that takes a TimerHandle_t xTimer parameter. Inside it, toggle ledState between 0 and 1. Then create the timer xTimer using xTimerCreate with name "LEDTimer", period 1000 ms, auto-reload enabled, id 0, and callback vTimerCallback.

FreeRTOS

int ledState = 0;
TimerHandle_t xTimer;

// Define the timer callback function
// Create the timer with xTimerCreate
// Your code here

Hint

The callback toggles ledState using ledState = !ledState;. Use pdMS_TO_TICKS(1000) to convert 1000 ms to ticks. Set auto-reload to pdTRUE.

3

Start the timer

Start the timer xTimer by calling xTimerStart with block time 0.

FreeRTOS

int ledState = 0;
TimerHandle_t xTimer;

void vTimerCallback(TimerHandle_t xTimer) {
    ledState = !ledState;
}

xTimer = xTimerCreate("LEDTimer", pdMS_TO_TICKS(1000), pdTRUE, (void *)0, vTimerCallback);

// Start the timer here
// Your code here

Hint

Use xTimerStart(xTimer, 0); to start the timer immediately without blocking.

4

Print the LED state inside the callback

Modify the vTimerCallback function to print the current ledState value using printf with the format "LED State: %d\n".

FreeRTOS

int ledState = 0;
TimerHandle_t xTimer;

void vTimerCallback(TimerHandle_t xTimer) {
    ledState = !ledState;
    // Print ledState here
    // Your code here
}

xTimer = xTimerCreate("LEDTimer", pdMS_TO_TICKS(1000), pdTRUE, (void *)0, vTimerCallback);
xTimerStart(xTimer, 0);

Hint

Use printf("LED State: %d\n", ledState); inside the callback to show the LED state changes.

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