Priority numbering in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a task priority to the highest level in FreeRTOS.

FreeRTOS

vTaskPrioritySet(taskHandle, [1]);

Drag options to blanks, or click blank then click option'

AconfigMAX_PRIORITIES - 1

BconfigMAX_PRIORITIES

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES as the highest priority causes errors.

Setting priority to 0 is the lowest priority, not highest.

In FreeRTOS, priorities start at 0 (lowest) and go up to configMAX_PRIORITIES - 1 (highest). So the highest priority is configMAX_PRIORITIES - 1.

2fill in blank

medium

Complete the code to create a task with priority 2 in FreeRTOS.

FreeRTOS

xTaskCreate(taskFunction, "Task", 1000, NULL, [1], NULL);

Drag options to blanks, or click blank then click option'

AconfigMAX_PRIORITIES

BconfigMAX_PRIORITIES - 1

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES as priority causes errors.

Setting priority to 0 creates the lowest priority task.

The priority parameter sets the task priority. Here, setting it to 2 means the task has priority level 2.

3fill in blank

hard

Fix the error in setting a task priority to the highest level.

FreeRTOS

vTaskPrioritySet(taskHandle, [1]);

Drag options to blanks, or click blank then click option'

A1

BconfigMAX_PRIORITIES

C0

DconfigMAX_PRIORITIES - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES instead of configMAX_PRIORITIES - 1.

Confusing priority numbering starting at zero.

The highest priority is configMAX_PRIORITIES - 1. Using configMAX_PRIORITIES is invalid and causes errors.

4fill in blank

hard

Fill both blanks to create a task with the lowest priority and then increase its priority by 1.

FreeRTOS

xTaskCreate(taskFunc, "Task", 500, NULL, [1], &handle);
vTaskPrioritySet(handle, handle->uxPriority [2] 1);

Drag options to blanks, or click blank then click option'

A0

B+

C-

DconfigMAX_PRIORITIES - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES as initial priority causes errors.

Using '-' instead of '+' decreases priority.

The task is created with priority 0 (lowest). Then its priority is increased by 1 using the '+' operator.

5fill in blank

hard

Fill all three blanks to create a task with priority 3, then set its priority to the highest level.

FreeRTOS

xTaskCreate(func, "Tsk", 256, NULL, [1], &h);
vTaskPrioritySet(h, [2]);
configASSERT([3] <= configMAX_PRIORITIES - 1);

Drag options to blanks, or click blank then click option'

A3

BconfigMAX_PRIORITIES - 1

Ch->uxPriority

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or configMAX_PRIORITIES as priority values incorrectly.

Not using the task handle's priority for assertion.

The task is created with priority 3. Then its priority is set to the highest level configMAX_PRIORITIES - 1. The assertion checks that the priority is valid by comparing the task's current priority h->uxPriority.

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