Choosing priorities for real applications in FreeRTOS - Mini Project: Build & Apply

Choosing Priorities for Real Applications in FreeRTOS

📖 Scenario: You are developing a simple embedded system using FreeRTOS. The system has three tasks: reading sensor data, controlling an actuator, and logging data. Each task needs a priority to decide which runs first when multiple tasks are ready.Choosing the right priorities helps the system work smoothly and respond quickly to important events.

🎯 Goal: You will create three FreeRTOS tasks with specific priorities to manage sensor reading, actuator control, and data logging. You will assign priorities so that the actuator control runs first, sensor reading second, and logging last.

📋 What You'll Learn

Create three tasks named SensorTask, ActuatorTask, and LoggerTask

Assign priority 3 to ActuatorTask

Assign priority 2 to SensorTask

Assign priority 1 to LoggerTask

Use xTaskCreate to create each task with the correct priority

💡 Why This Matters

🌍 Real World

Embedded systems like robots, home automation, and industrial machines use FreeRTOS tasks with priorities to manage multiple jobs smoothly.

💼 Career

Understanding task priorities is essential for embedded software engineers working on real-time operating systems to ensure timely and reliable system behavior.

1

Create three empty FreeRTOS tasks

Write three empty task functions named SensorTask, ActuatorTask, and LoggerTask. Each should take a void *pvParameters argument and have an infinite loop with vTaskDelay(1000 / portTICK_PERIOD_MS); inside.

FreeRTOS

# Write the three task functions below

# Your code here

Hint

Define each task as a function with void *pvParameters parameter and an infinite loop using for (;;).

2

Declare task handles for each task

Declare three variables of type TaskHandle_t named SensorTaskHandle, ActuatorTaskHandle, and LoggerTaskHandle to hold the task handles.

FreeRTOS

void SensorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void ActuatorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void LoggerTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

// Declare task handles below
# Your code here

Hint

Use TaskHandle_t type to declare variables for each task handle.

3

Create the three tasks with correct priorities

Use xTaskCreate to create ActuatorTask with priority 3, SensorTask with priority 2, and LoggerTask with priority 1. Pass the address of each task handle variable to store the created task handle.

FreeRTOS

void SensorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void ActuatorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void LoggerTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

TaskHandle_t SensorTaskHandle;
TaskHandle_t ActuatorTaskHandle;
TaskHandle_t LoggerTaskHandle;

// Create the tasks below
# Your code here

Hint

Use xTaskCreate with parameters: task function, name, stack size (1000), NULL for parameters, priority, and address of task handle.

4

Print the task priorities to verify

Use printf to print the priorities of ActuatorTaskHandle, SensorTaskHandle, and LoggerTaskHandle using uxTaskPriorityGet. Print in this exact format: "ActuatorTask priority: 3\nSensorTask priority: 2\nLoggerTask priority: 1\n".

FreeRTOS

void SensorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void ActuatorTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

void LoggerTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(1000 / portTICK_PERIOD_MS);
    }
}

TaskHandle_t SensorTaskHandle;
TaskHandle_t ActuatorTaskHandle;
TaskHandle_t LoggerTaskHandle;

xTaskCreate(ActuatorTask, "ActuatorTask", 1000, NULL, 3, &ActuatorTaskHandle);
xTaskCreate(SensorTask, "SensorTask", 1000, NULL, 2, &SensorTaskHandle);
xTaskCreate(LoggerTask, "LoggerTask", 1000, NULL, 1, &LoggerTaskHandle);

// Print the priorities below
# Your code here

Hint

Use printf and uxTaskPriorityGet to print each task's priority exactly as shown.

Practice

1. In FreeRTOS, what does a higher task priority number mean?

easy

A. The task runs before lower priority tasks

B. The task uses less memory

C. The task runs slower

D. The task is paused

Solution

1. Step 1: Understand task priority meaning

   In FreeRTOS, task priority determines which task the scheduler runs first.

2. Step 2: Higher number means higher priority

   A higher priority number means the task is more important and runs before lower priority tasks.

3. Final Answer:

   The task runs before lower priority tasks -> Option A

4. Quick Check:

   Higher priority = runs first [OK]

Hint: Higher number means task runs first [OK]

Common Mistakes:

• Thinking higher priority means slower task
• Confusing priority with memory usage
• Assuming priority pauses the task

2. Which of the following is the correct way to set a task priority in FreeRTOS?

easy

A. xTaskCreate(taskFunction, "Task", stackSize, NULL, -1, NULL);

B. xTaskCreate(taskFunction, "Task", stackSize, NULL, "high", NULL);

C. xTaskCreate(taskFunction, "Task", stackSize, NULL, 5, NULL);

D. xTaskCreate(taskFunction, "Task", stackSize, NULL, 0.5, NULL);

Solution

1. Step 1: Check parameter for priority in xTaskCreate

   The fifth parameter in xTaskCreate is the task priority and must be an integer.

2. Step 2: Validate options

   xTaskCreate(taskFunction, "Task", stackSize, NULL, 5, NULL); uses integer 5, which is valid. xTaskCreate(taskFunction, "Task", stackSize, NULL, "high", NULL); uses a string, invalid. xTaskCreate(taskFunction, "Task", stackSize, NULL, -1, NULL); uses negative priority, invalid. xTaskCreate(taskFunction, "Task", stackSize, NULL, 0.5, NULL); uses float, invalid.

3. Final Answer:

   xTaskCreate(taskFunction, "Task", stackSize, NULL, 5, NULL); -> Option C

4. Quick Check:

   Priority must be integer >= 0 [OK]

Hint: Priority is an integer in xTaskCreate [OK]

Common Mistakes:

• Using string or float for priority
• Using negative numbers for priority
• Confusing parameter order

3. Given three tasks with priorities: TaskA=3, TaskB=5, TaskC=2. Which task will run first when all are ready?

medium

A. TaskA

B. They run simultaneously

C. TaskC

D. TaskB

Solution

1. Step 1: Compare task priorities

   TaskB has priority 5, TaskA has 3, TaskC has 2.

2. Step 2: Highest priority runs first

   Since 5 is highest, TaskB runs first when all are ready.

3. Final Answer:

   TaskB -> Option D

4. Quick Check:

   Highest priority = 5 = TaskB [OK]

Hint: Highest priority number runs first [OK]

Common Mistakes:

• Choosing lowest priority task
• Thinking tasks run simultaneously
• Ignoring priority values

4. You set two tasks with the same priority in FreeRTOS. What is a likely issue?

medium

A. Tasks will share CPU time but may cause unpredictable timing

B. Tasks may starve lower priority tasks

C. One task will never run

D. System will crash immediately

Solution

1. Step 1: Understand equal priority behavior

   Tasks with same priority share CPU time via time slicing.

2. Step 2: Effects of equal priority

   This can cause unpredictable timing because tasks switch frequently, but no starvation occurs.

3. Final Answer:

   Tasks will share CPU time but may cause unpredictable timing -> Option A

4. Quick Check:

   Same priority = time slicing, possible timing issues [OK]

Hint: Same priority tasks share CPU, timing may vary [OK]

Common Mistakes:

• Assuming one task never runs
• Expecting system crash
• Ignoring time slicing behavior

5. You have three tasks: SensorRead (must respond fast), DataLogging (less urgent), and UIUpdate (medium urgency). How should you assign priorities?

hard

A. DataLogging=3, UIUpdate=2, SensorRead=1

B. SensorRead=3, UIUpdate=2, DataLogging=1

C. UIUpdate=3, SensorRead=2, DataLogging=1

D. All tasks priority 1

Solution

1. Step 1: Identify task urgency

   SensorRead must respond fastest, so highest priority. UIUpdate is medium, DataLogging less urgent.

2. Step 2: Assign priorities accordingly

   Assign SensorRead=3 (highest), UIUpdate=2, DataLogging=1 (lowest).

3. Final Answer:

   SensorRead=3, UIUpdate=2, DataLogging=1 -> Option B

4. Quick Check:

   Fastest task gets highest priority [OK]

Hint: Fastest task gets highest priority [OK]

Common Mistakes:

• Giving low priority to fastest task
• Assigning same priority to all
• Reversing urgency order