Choosing priorities for real applications in FreeRTOS - Step-by-Step Execution

Concept Flow - Choosing priorities for real applications

Identify tasks

↓

Assign priorities based on importance

↓

Check for resource conflicts

↓

Adjust priorities to avoid starvation

↓

Implement and test

↓

Monitor and fine-tune

This flow shows how to pick task priorities step-by-step in a real FreeRTOS application.

Execution Sample

FreeRTOS

void TaskA(void *pvParameters) {
  for(;;) {
    // High priority task work
    vTaskDelay(100 / portTICK_PERIOD_MS);
  }
}

void TaskB(void *pvParameters) {
  for(;;) {
    // Low priority task work
    vTaskDelay(200 / portTICK_PERIOD_MS);
  }
}

Two tasks run with different priorities and delays to show priority effects.

Execution Table

Step	Running Task	Priority	Action	Result
1	TaskA	High	Starts running	TaskA runs first
2	TaskA	High	Calls vTaskDelay(100 / portTICK_PERIOD_MS)	TaskA blocks, scheduler runs TaskB
3	TaskB	Low	Starts running	TaskB runs while TaskA is blocked
4	TaskB	Low	Calls vTaskDelay(200 / portTICK_PERIOD_MS)	TaskB blocks, no ready tasks
5	Idle Task	Lowest	Runs	CPU idle until TaskA delay ends
6	TaskA	High	Delay ends, becomes ready	TaskA runs again
7	TaskA	High	Repeats cycle	TaskA keeps priority advantage

💡 Simulation ends after one full cycle of TaskA and TaskB delays.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 6	Final
TaskA State	Ready	Blocked (delay)	Blocked	Blocked	Running	Running
TaskB State	Ready	Running	Running	Blocked (delay)	Blocked	Blocked
CPU Running Task	None	TaskB	TaskB	Idle	TaskA	TaskA

Key Moments - 3 Insights

Why does TaskA run before TaskB even though both are ready at the start?

Because TaskA has a higher priority (see execution_table step 1), FreeRTOS always runs the highest priority ready task.

What happens when TaskA calls vTaskDelay?

TaskA blocks and stops running (step 2), so the scheduler runs the next highest priority ready task, TaskB (step 3).

Why does the Idle Task run at step 5?

Both TaskA and TaskB are blocked on delay, so no user tasks are ready; the Idle Task runs to keep CPU busy.

Visual Quiz - 3 Questions

Look at the execution_table, which task is running at step 3?

A TaskB

B Idle Task

C TaskA

D No task running

💡 Hint

Check the 'Running Task' column at step 3 in the execution_table.

At which step does TaskA become ready again after delay?

A Step 4

B Step 6

C Step 2

D Step 7

💡 Hint

Look for when TaskA state changes from blocked to ready in variable_tracker and execution_table.

If TaskB had higher priority than TaskA, what would happen at step 1?

A TaskA runs first

B Idle Task runs first

C TaskB runs first

D Both run simultaneously

💡 Hint

Recall FreeRTOS runs the highest priority ready task (see concept_flow and execution_table step 1).

Concept Snapshot

In FreeRTOS, assign task priorities based on importance.
Higher priority tasks preempt lower ones.
Use delays or blocking to allow lower priority tasks to run.
Avoid priority inversion and starvation by adjusting priorities.
Test and monitor to fine-tune priorities for smooth operation.

Full Transcript

Choosing priorities in FreeRTOS means deciding which tasks are more important so they run first. We start by listing tasks, then assign priorities based on how critical they are. The scheduler always runs the highest priority ready task. If a high priority task blocks (like calling vTaskDelay), lower priority tasks get CPU time. The Idle Task runs only when no other tasks are ready. This example shows TaskA with high priority running first, then blocking, letting TaskB run. When both block, the Idle Task runs. Understanding this helps avoid problems like starvation and ensures smooth multitasking.

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