Choosing priorities for real applications in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task priority example?

Consider two tasks created with different priorities. What will be the output order of the print statements?

FreeRTOS

void Task1(void *pvParameters) {
    printf("Task 1 running\n");
    printf("Task 1 finished\n");
    vTaskDelete(NULL);
}

void Task2(void *pvParameters) {
    printf("Task 2 running\n");
    printf("Task 2 finished\n");
    vTaskDelete(NULL);
}

int main() {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    return 0;
}

A Task 1 running\nTask 2 running\nTask 2 finished\nTask 1 finished\n

B Task 1 running\nTask 1 finished\nTask 2 running\nTask 2 finished\n

C Task 2 running\nTask 2 finished\nTask 1 running\nTask 1 finished\n

D Task 2 running\nTask 1 running\nTask 1 finished\nTask 2 finished\n

💡 Hint

Remember, higher priority tasks preempt lower priority ones immediately.

Task2 has priority 3, higher than Task1's priority 2. So Task2 runs first, prints its messages, then Task1 runs.

🧠 Conceptual

intermediate

Which priority assignment best avoids priority inversion?

You have three tasks: A (high priority), B (medium priority), and C (low priority). Task C holds a resource needed by Task A. Which priority assignment helps avoid priority inversion?

A Assign all tasks the same priority.

B Keep Task C at the lowest priority always.

C Assign Task B the highest priority to preempt Task C.

D Assign Task C the highest priority temporarily while it holds the resource.

💡 Hint

Think about priority inheritance to prevent blocking.

Temporarily boosting Task C's priority prevents it from being preempted by Task B, avoiding priority inversion.

🔧 Debug

advanced

Why does this FreeRTOS system freeze with these priorities?

Given three tasks with priorities 1, 2, and 3, the system freezes after startup. What is the likely cause?

FreeRTOS

void TaskLow(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

void TaskMedium(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

void TaskHigh(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

int main() {
    xTaskCreate(TaskLow, "Low", 1000, NULL, 1, NULL);
    xTaskCreate(TaskMedium, "Medium", 1000, NULL, 2, NULL);
    xTaskCreate(TaskHigh, "High", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    return 0;
}

A TaskHigh is starving lower priority tasks causing system freeze.

B TaskLow has a blocking call causing deadlock.

C TaskMedium has a syntax error preventing scheduler start.

D Stack sizes are too small causing overflow.

💡 Hint

Consider how higher priority tasks affect lower priority ones.

TaskHigh runs continuously without delay, starving lower priority tasks and freezing system progress.

📝 Syntax

advanced

Which option correctly sets a task priority in FreeRTOS?

Choose the correct way to set a task's priority to 5 after creation.

FreeRTOS

TaskHandle_t xHandle = NULL;
xTaskCreate(TaskFunction, "Task", 1000, NULL, 3, &xHandle);
// Set priority here

A xTaskSetPriority(xHandle, 5);

B vTaskPrioritySet(xHandle, 5);

C vTaskSetPriority(xHandle, 5);

D xTaskPriorityChange(xHandle, 5);

💡 Hint

Check the FreeRTOS API naming conventions.

The correct API to set a task priority is vTaskPrioritySet().

🚀 Application

expert

How many tasks will run concurrently with these priorities and delays?

Three tasks are created with priorities 4, 3, and 2. Task with priority 4 calls vTaskDelay(100) in its loop. How many tasks run concurrently during the delay?

FreeRTOS

void TaskHigh(void *pvParameters) {
    while(1) {
        // Do work
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void TaskMid(void *pvParameters) {
    while(1) {
        // Do work
    }
}

void TaskLow(void *pvParameters) {
    while(1) {
        // Do work
    }
}

int main() {
    xTaskCreate(TaskHigh, "High", 1000, NULL, 4, NULL);
    xTaskCreate(TaskMid, "Mid", 1000, NULL, 3, NULL);
    xTaskCreate(TaskLow, "Low", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    return 0;
}

A Only one task runs during the delay: Mid.

B No tasks run during the delay.

C All three tasks run concurrently during the delay.

D Two tasks run concurrently during the delay: Mid and Low.

💡 Hint

Remember that vTaskDelay makes the task blocked, allowing lower priority tasks to run.

When TaskHigh delays, it is blocked. TaskMid (priority 3) runs next and never blocks or yields, starving TaskLow (priority 2).

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