vTaskPrioritySet() dynamic priority in FreeRTOS - Mini Project: Build & Apply

Using vTaskPrioritySet() to Change Task Priority Dynamically

📖 Scenario: You are working on a FreeRTOS-based embedded system with two tasks: a SensorTask and a LoggerTask. Initially, the SensorTask has a higher priority to read sensor data frequently. However, when the LoggerTask needs to write data to storage, it should temporarily get a higher priority to avoid data loss.

🎯 Goal: Build a FreeRTOS program that creates two tasks with initial priorities, then dynamically changes the LoggerTask's priority using vTaskPrioritySet() to a higher priority, and finally prints the current priorities of both tasks.

📋 What You'll Learn

Create two tasks named SensorTask and LoggerTask with initial priorities 2 and 1 respectively.

Create a variable loggerNewPriority and set it to 3.

Use vTaskPrioritySet() to change LoggerTask priority to loggerNewPriority.

Print the priorities of both tasks after the change.

💡 Why This Matters

🌍 Real World

In embedded systems, dynamically changing task priorities helps manage critical operations that need immediate attention, like logging important data or handling interrupts.

💼 Career

Understanding task priority management is essential for embedded software engineers working with real-time operating systems like FreeRTOS to ensure system responsiveness and reliability.

1

Create two tasks with initial priorities

Create two FreeRTOS tasks named SensorTask and LoggerTask with initial priorities 2 and 1 respectively. Use xTaskCreate() to create both tasks and store their handles in variables sensorTaskHandle and loggerTaskHandle.

FreeRTOS

// Create SensorTask and LoggerTask with priorities 2 and 1
// Your code here

Hint

Use xTaskCreate() with the correct task function, name, stack size, parameters, priority, and handle pointer.

2

Create a variable for the new LoggerTask priority

Create an integer variable called loggerNewPriority and set it to 3.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include 

TaskHandle_t sensorTaskHandle = NULL;
TaskHandle_t loggerTaskHandle = NULL;

void SensorTask(void *pvParameters) {
    while(1) {
        // Simulate sensor reading
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void LoggerTask(void *pvParameters) {
    while(1) {
        // Simulate logging
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

int main(void) {
    xTaskCreate(SensorTask, "SensorTask", 1000, NULL, 2, &sensorTaskHandle);
    xTaskCreate(LoggerTask, "LoggerTask", 1000, NULL, 1, &loggerTaskHandle);

    // Create loggerNewPriority variable and set to 3
    // Your code here

    vTaskStartScheduler();
    return 0;
}

Hint

Declare int loggerNewPriority = 3; inside main() before starting the scheduler.

3

Change LoggerTask priority dynamically

Use vTaskPrioritySet() to change the priority of LoggerTask to the value stored in loggerNewPriority. Place this call inside main() after creating the tasks and before starting the scheduler.

FreeRTOS

    vTaskPrioritySet(loggerTaskHandle, loggerNewPriority);
    // Print priorities of SensorTask and LoggerTask

Hint

Call vTaskPrioritySet(loggerTaskHandle, loggerNewPriority); inside main() after task creation.

4

Print the priorities of both tasks

Use uxTaskPriorityGet() to get the priorities of SensorTask and LoggerTask. Then print their priorities using printf(). Place this code inside main() after changing the LoggerTask priority and before starting the scheduler.

FreeRTOS

Hint

Use uxTaskPriorityGet() to get priorities and printf() to display them.

Practice

1.

What does the FreeRTOS function vTaskPrioritySet() do?

easy

A. It changes the priority of a task while the program is running.

B. It creates a new task with a fixed priority.

C. It deletes a task permanently.

D. It suspends a task temporarily.

Solution

1. Step 1: Understand the function purpose

   vTaskPrioritySet() is designed to change the priority of an existing task dynamically during runtime.

2. Step 2: Compare with other task functions

   Creating, deleting, or suspending tasks are done by different functions, not vTaskPrioritySet().

3. Final Answer:

   It changes the priority of a task while the program is running. -> Option A

4. Quick Check:

   vTaskPrioritySet() changes priority [OK]

Hint: Remember: vTaskPrioritySet changes priority dynamically [OK]

Common Mistakes:

• Confusing vTaskPrioritySet with task creation functions
• Thinking it deletes or suspends tasks
• Assuming priority is fixed at task creation

2.

Which of the following is the correct syntax to change a task's priority using vTaskPrioritySet()?

TaskHandle_t xTaskHandle;
// Change priority to 3
_____;

easy

A. vTaskPrioritySetPriority(xTaskHandle, 3);

B. vTaskPrioritySet(xTaskHandle, 3);

C. vTaskPrioritySet(3, xTaskHandle);

D. vTaskSetPriority(xTaskHandle, 3);

Solution

1. Step 1: Recall the function signature

   The correct function is vTaskPrioritySet(TaskHandle_t xTask, UBaseType_t uxNewPriority), so the task handle comes first, then the new priority.

2. Step 2: Check the options

   vTaskPrioritySet(xTaskHandle, 3); matches the correct syntax exactly. Options A, B, and D either swap parameters or use incorrect function names.

3. Final Answer:

   vTaskPrioritySet(xTaskHandle, 3); -> Option B

4. Quick Check:

   Correct syntax = vTaskPrioritySet(task, priority) [OK]

Hint: Function name and parameter order matter: task handle first [OK]

Common Mistakes:

• Swapping the order of parameters
• Using incorrect function names
• Passing priority before task handle

3.

Consider the following code snippet:

TaskHandle_t xTask1;
// Assume xTask1 is created with priority 2
vTaskPrioritySet(xTask1, 5);
UBaseType_t newPriority = uxTaskPriorityGet(xTask1);
printf("%u", newPriority);

What will be printed?

medium

A. 0

B. 2

C. 5

D. Compilation error

Solution

1. Step 1: Understand priority change

   The call vTaskPrioritySet(xTask1, 5) changes the task's priority from 2 to 5.

2. Step 2: Check the priority retrieval

   uxTaskPriorityGet(xTask1) returns the current priority, which is now 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Priority updated to 5, so output is 5 [OK]

Hint: Priority changes immediately, so get returns new value [OK]

Common Mistakes:

• Assuming priority stays the same after set
• Confusing priority with task handle
• Thinking printf prints task handle

4.

What is wrong with this code snippet?

TaskHandle_t xTask2 = NULL;
vTaskPrioritySet(xTask2, 4);

medium

A. The function vTaskPrioritySet returns a value that is ignored.

B. Priority 4 is invalid; priorities must be 0 or 1 only.

C. vTaskPrioritySet requires a pointer to the task, not the handle.

D. xTask2 is NULL, so setting priority causes undefined behavior.

Solution

1. Step 1: Check the task handle value

   The variable xTask2 is set to NULL, meaning it does not point to a valid task.

2. Step 2: Understand effect of NULL handle

   Calling vTaskPrioritySet() with a NULL handle leads to undefined behavior or runtime error because no valid task exists to change priority.

3. Final Answer:

   xTask2 is NULL, so setting priority causes undefined behavior. -> Option D

4. Quick Check:

   NULL task handle causes error [OK]

Hint: Never pass NULL as task handle to vTaskPrioritySet [OK]

Common Mistakes:

• Ignoring NULL task handle
• Assuming priority range is only 0 or 1
• Confusing handle with pointer to handle

5.

You have two tasks: TaskA with priority 3 and TaskB with priority 5. You want TaskA to temporarily run with higher priority than TaskB during a critical section. Which code snippet correctly achieves this?

TaskHandle_t xTaskA, xTaskB;
// Initially priorities: TaskA=3, TaskB=5
// Critical section starts
_____;
// Critical section ends
vTaskPrioritySet(xTaskA, 3);

hard

A. vTaskPrioritySet(xTaskA, 6);

B. vTaskPrioritySet(xTaskB, 2);

C. vTaskPrioritySet(xTaskA, 5);

D. vTaskPrioritySet(xTaskB, 3);

Solution

1. Step 1: Identify priority requirements

   TaskA must have higher priority than TaskB (which is 5) during the critical section.

2. Step 2: Choose correct priority change

   Setting TaskA to 6 makes it higher than TaskB's 5. Options A and D lower TaskB's priority instead. vTaskPrioritySet(xTaskA, 5); sets TaskA to 5, equal to TaskB, not higher.

3. Step 3: Restore original priority after critical section

   After the critical section, TaskA's priority is reset to 3 as shown.

4. Final Answer:

   vTaskPrioritySet(xTaskA, 6); -> Option A

5. Quick Check:

   TaskA priority > TaskB priority during critical section [OK]

Hint: Raise TaskA priority above TaskB, then restore after [OK]

Common Mistakes:

• Setting TaskA priority equal to TaskB
• Lowering TaskB priority instead of raising TaskA
• Not restoring original priority after critical section