Priority numbering in FreeRTOS

Introduction

Priority numbering helps FreeRTOS decide which task should run first. It keeps your program organized and responsive.

When you want certain tasks to run before others, like handling urgent sensor data.

When you have background tasks that can wait, like logging information.

When you want to avoid tasks blocking each other by setting clear priorities.

When you need smooth multitasking in embedded systems.

When you want to control how your system reacts to different events.

Syntax

FreeRTOS

UBaseType_t uxPriority = 0; // Lowest priority
uxPriority = configMAX_PRIORITIES - 1; // Highest priority

Priority numbers start at 0, which is the lowest priority.

The highest priority is one less than configMAX_PRIORITIES.

Examples

Defines the lowest and highest priority values based on FreeRTOS settings.

FreeRTOS

const UBaseType_t lowPriority = 0;
const UBaseType_t highPriority = configMAX_PRIORITIES - 1;

Creates a task with priority 2, which is higher than 0 but lower than the max.

FreeRTOS

xTaskCreate(taskFunction, "Task1", 1000, NULL, 2, NULL);

Creates a task with the highest priority allowed.

FreeRTOS

xTaskCreate(taskFunction, "Task2", 1000, NULL, configMAX_PRIORITIES - 1, NULL);

Sample Program

This program creates two tasks: one with low priority and one with the highest priority. The high priority task runs more often because FreeRTOS always runs the highest priority task ready to run.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void vTaskLowPriority(void *pvParameters) {
    for (;;) {
        printf("Low priority task running\n");
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void vTaskHighPriority(void *pvParameters) {
    for (;;) {
        printf("High priority task running\n");
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

int main(void) {
    xTaskCreate(vTaskLowPriority, "LowTask", 1000, NULL, 1, NULL);
    xTaskCreate(vTaskHighPriority, "HighTask", 1000, NULL, configMAX_PRIORITIES - 1, NULL);
    vTaskStartScheduler();
    for (;;) {}
    return 0;
}

Important Notes

Tasks with higher priority always run before lower priority tasks.

If two tasks have the same priority, FreeRTOS switches between them to share CPU time.

Setting priorities carefully helps your system stay fast and responsive.

Summary

Priority numbers start at 0 (lowest) and go up to configMAX_PRIORITIES - 1 (highest).

Higher priority tasks run before lower priority ones.

Use priority numbering to control task order and system responsiveness.

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