Priority numbering in FreeRTOS - Mini Project: Build & Apply

Priority numbering in FreeRTOS

📖 Scenario: You are working on a small embedded system using FreeRTOS. You want to understand how task priorities are numbered and how to set them correctly.

🎯 Goal: Learn how to define task priorities using FreeRTOS constants and create tasks with specific priority numbers.

📋 What You'll Learn

Create a variable for the maximum priority using FreeRTOS constant

Create a variable for the minimum priority using FreeRTOS constant

Create a task with a priority between minimum and maximum

Print the priority values to verify

💡 Why This Matters

🌍 Real World

Embedded systems often use FreeRTOS to manage multiple tasks with different priorities to ensure important tasks run first.

💼 Career

Understanding task priority numbering is essential for embedded developers working with real-time operating systems like FreeRTOS.

1

Define FreeRTOS priority constants

Create two variables: max_priority set to configMAX_PRIORITIES - 1 and min_priority set to 0.

FreeRTOS

# Define max_priority and min_priority variables
# Your code here

Hint

FreeRTOS priorities start at 0 for the lowest priority and go up to configMAX_PRIORITIES - 1.

2

Set a task priority between min and max

Create a variable task_priority and set it to the middle priority between min_priority and max_priority using integer division.

FreeRTOS

max_priority = configMAX_PRIORITIES - 1
min_priority = 0
# Set task_priority to the middle value between min_priority and max_priority
# Your code here

Hint

Use integer division // to find the middle priority.

3

Create a FreeRTOS task with the chosen priority

Write a line to create a FreeRTOS task called vTaskExample with priority task_priority. Use the function xTaskCreate() with parameters: vTaskExample, "ExampleTask", 1000, NULL, task_priority, NULL.

FreeRTOS

max_priority = configMAX_PRIORITIES - 1
min_priority = 0
task_priority = (max_priority + min_priority) // 2
# Create a FreeRTOS task with xTaskCreate and task_priority
# Your code here

Hint

Use xTaskCreate() with the correct parameters to set the task priority.

4

Print the priority values

Write three print() statements to display min_priority, task_priority, and max_priority with descriptive text.

FreeRTOS

max_priority = configMAX_PRIORITIES - 1
min_priority = 0
task_priority = (max_priority + min_priority) // 2
xTaskCreate(vTaskExample, "ExampleTask", 1000, NULL, task_priority, NULL)
# Print min_priority, task_priority, and max_priority
# Your code here

Hint

Use print() with f-strings to show the priority values clearly.

Practice

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

easy

A. The task runs before lower priority tasks

B. The task runs after lower priority tasks

C. The task runs at the same time as lower priority tasks

D. The task is ignored by the scheduler

Solution

1. Step 1: Understand priority numbering

   In FreeRTOS, priority numbers start at 0 for the lowest priority and increase for higher priorities.

2. Step 2: Relate priority to task execution

   Higher priority tasks are scheduled to run before lower priority tasks to ensure responsiveness.

3. Final Answer:

   The task runs before lower priority tasks -> Option A

4. Quick Check:

   Higher priority number = runs first [OK]

Hint: Higher number means higher priority and runs first [OK]

Common Mistakes:

• Thinking higher number means lower priority
• Assuming tasks run simultaneously regardless of priority
• Confusing priority with task creation order

2. Which of the following is the correct way to define the highest priority in FreeRTOS if configMAX_PRIORITIES is 5?

easy

A. 5

B. 6

C. 4

D. 0

Solution

1. Step 1: Understand priority range

   Priority numbers start at 0 and go up to configMAX_PRIORITIES - 1.

2. Step 2: Calculate highest priority

   With configMAX_PRIORITIES = 5, highest priority is 5 - 1 = 4.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Highest priority = configMAX_PRIORITIES - 1 = 4 [OK]

Hint: Highest priority = configMAX_PRIORITIES minus one [OK]

Common Mistakes:

• Using configMAX_PRIORITIES as highest priority directly
• Using 0 as highest priority
• Choosing a number outside the allowed range

3. Given three tasks with priorities 2, 4, and 1, which task will FreeRTOS schedule to run first?

medium

A. Task with priority 4

B. Task with priority 2

C. Task with priority 1

D. All run simultaneously

Solution

1. Step 1: Identify task priorities

   Tasks have priorities 2, 4, and 1 respectively.

2. Step 2: Determine highest priority

   Priority 4 is the highest number, so that task runs first.

3. Final Answer:

   Task with priority 4 -> Option A

4. Quick Check:

   Highest priority runs first = 4 [OK]

Hint: Task with highest priority number runs first [OK]

Common Mistakes:

• Choosing the lowest priority task
• Assuming tasks run in creation order
• Thinking tasks run simultaneously

4. What is wrong with this priority assignment if configMAX_PRIORITIES is 3?
vTaskPrioritySet(taskHandle, 3);

medium

A. Priority 3 is valid and highest

B. Priority 3 is out of range; max is 2

C. Priority must be zero

D. Priority must be negative

Solution

1. Step 1: Check allowed priority range

   Priorities go from 0 to configMAX_PRIORITIES - 1, so 0 to 2 here.

2. Step 2: Validate assigned priority

   Priority 3 is outside this range and invalid.

3. Final Answer:

   Priority 3 is out of range; max is 2 -> Option B

4. Quick Check:

   Max priority = 2, 3 is invalid [OK]

Hint: Priority must be less than configMAX_PRIORITIES [OK]

Common Mistakes:

• Using configMAX_PRIORITIES as a valid priority
• Assigning negative priorities
• Assuming priority zero is highest

5. You have tasks with priorities 0, 1, and 2. You want the system to always run the task with priority 1 before the others, but currently priority 2 runs first. How can you fix this using FreeRTOS priority numbering?

hard

A. Set all tasks to priority 0

B. Set the task currently at 1 to priority 0

C. Set the task currently at 2 to priority 0

D. Set the priority of the task currently at 1 to 3 and others lower

Solution

1. Step 1: Understand current priorities

   Task with priority 2 runs first because it has highest priority.

2. Step 2: Adjust priorities to reorder execution

   Increase the priority of the task currently at 1 to a number higher than 2 (e.g., 3) so it runs first.

3. Step 3: Lower other tasks' priorities

   Set other tasks to lower priorities to ensure the desired task runs first.

4. Final Answer:

   Set the priority of the task currently at 1 to 3 and others lower -> Option D

5. Quick Check:

   Higher priority number runs first, so raise desired task's priority [OK]

Hint: Raise desired task priority above others to run first [OK]

Common Mistakes:

• Lowering all priorities to zero
• Not increasing the desired task's priority
• Assuming priority 0 is highest