Choosing priorities for real applications in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the highest priority for a critical task.

FreeRTOS

xTaskCreate(taskFunction, "Task1", 1000, NULL, [1], NULL);

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

AconfigMAX_PRIORITIES - 1

B0

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as the highest priority, which is actually the lowest.

Using a priority higher than configMAX_PRIORITIES - 1, which is invalid.

The highest priority in FreeRTOS is configMAX_PRIORITIES - 1 because priorities start at 0.

2fill in blank

medium

Complete the code to create a low priority background task.

FreeRTOS

xTaskCreate(backgroundTask, "BGTask", 500, NULL, [1], NULL);

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

A1

B0

CconfigMAX_PRIORITIES

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a high priority to background tasks, which can block important tasks.

Using a priority value outside the allowed range.

Priority 0 is the lowest priority in FreeRTOS, suitable for background tasks.

3fill in blank

hard

Fix the error in setting task priority to avoid invalid priority assignment.

FreeRTOS

xTaskCreate(myTask, "MyTask", 256, NULL, [1], NULL);

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

AconfigMAX_PRIORITIES

B-1

CconfigMAX_PRIORITIES - 1

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES as a priority, which is out of range.

Using negative priority values.

Task priority must be less than configMAX_PRIORITIES. Using configMAX_PRIORITIES - 1 is valid.

4fill in blank

hard

Fill both blanks to create a medium priority task that runs periodically.

FreeRTOS

xTaskCreate([1], "PeriodicTask", 512, NULL, [2], NULL);

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

AperiodicTaskFunction

BconfigMAX_PRIORITIES - 1

C5

DidleTaskFunction

Attempts:

3 left

💡 Hint

Common Mistakes

Using the idle task function for a periodic task.

Assigning the highest priority to a periodic task unnecessarily.

The task function is periodicTaskFunction and priority 5 is a medium priority.

5fill in blank

hard

Fill all three blanks to create a task with a custom stack size, priority, and name.

FreeRTOS

xTaskCreate([1], [2], [3], NULL, 3, NULL);

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

AcustomTaskFunction

B"CustomTask"

C1024

D"DefaultTask"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number instead of a string for the task name.

Mixing up the order of arguments.

The task uses customTaskFunction, named "CustomTask", with a stack size of 1024.

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