Multiple tasks running concurrently in FreeRTOS

Introduction

Running multiple tasks at the same time lets your program do many things together. This makes your device more efficient and responsive.

When you want to read sensors and control motors at the same time.

When you need to handle user input while processing data in the background.

When you want to blink an LED while also communicating over Wi-Fi.

When your program must respond quickly to different events without waiting.

Syntax

FreeRTOS

void Task1(void *pvParameters) {
    for(;;) {
        // Task1 code here
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void Task2(void *pvParameters) {
    for(;;) {
        // Task2 code here
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

int main(void) {
    xTaskCreate(Task1, "Task 1", 1000, NULL, 1, NULL);
    xTaskCreate(Task2, "Task 2", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    for(;;);
}

Each task is a function that runs in an infinite loop.

vTaskDelay pauses the task for a set time, letting other tasks run.

Examples

Two tasks: one blinks an LED every 500ms, the other reads a sensor every 1000ms.

FreeRTOS

void BlinkLED(void *pvParameters) {
    for(;;) {
        // Toggle LED
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

void ReadSensor(void *pvParameters) {
    for(;;) {
        // Read sensor data
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

Creating tasks with different priorities: LED task has higher priority (2) than sensor task (1).

FreeRTOS

xTaskCreate(BlinkLED, "LED Task", 1000, NULL, 2, NULL);
xTaskCreate(ReadSensor, "Sensor Task", 1000, NULL, 1, NULL);

Sample Program

This program creates two tasks that print messages at different intervals. Task 1 prints every 1 second, Task 2 every 0.5 seconds. They run together, showing multitasking.

FreeRTOS

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

void Task1(void *pvParameters) {
    for(;;) {
        printf("Task 1 is running\n");
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void Task2(void *pvParameters) {
    for(;;) {
        printf("Task 2 is running\n");
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

int main(void) {
    xTaskCreate(Task1, "Task 1", 1000, NULL, 1, NULL);
    xTaskCreate(Task2, "Task 2", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    for(;;);
}

Important Notes

FreeRTOS switches between tasks quickly to make them run together.

Use vTaskDelay to let other tasks run and avoid blocking.

Task priorities help decide which task runs first if both are ready.

Summary

Multiple tasks let your program do many things at once.

Each task runs in a loop and uses vTaskDelay to pause.

FreeRTOS manages task switching automatically.

Practice

1. What is the main purpose of running multiple tasks concurrently in FreeRTOS?

easy

A. To allow the program to perform several operations at the same time

B. To make the program run slower

C. To use more memory without benefit

D. To avoid using any delays in the code

Solution

1. Step 1: Understand task concurrency

   Multiple tasks let the program handle different jobs at once, improving efficiency.

2. Step 2: Identify the purpose in FreeRTOS

   FreeRTOS switches between tasks automatically, enabling multitasking.

3. Final Answer:

   To allow the program to perform several operations at the same time -> Option A

4. Quick Check:

   Multiple tasks = multitasking = To allow the program to perform several operations at the same time [OK]

Hint: Multiple tasks mean doing many things at once [OK]

Common Mistakes:

• Thinking tasks slow down the program
• Believing tasks waste memory
• Ignoring the role of delays

2. Which of the following is the correct way to create a task in FreeRTOS?

easy

A. TaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

B. createTask(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

C. xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

D. xCreateTask(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

Solution

1. Step 1: Recall FreeRTOS task creation function

   The correct function to create a task is xTaskCreate.

2. Step 2: Match the exact syntax

   xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); matches the correct function name and parameters.

3. Final Answer:

   xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); -> Option C

4. Quick Check:

   Task creation function = xTaskCreate = xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); [OK]

Hint: Remember the function starts with 'xTaskCreate' [OK]

Common Mistakes:

• Using incorrect function names like createTask
• Mixing up parameter order
• Missing the 'x' prefix in function name

3. Consider this FreeRTOS code snippet:

void Task1(void *pvParameters) {
  for (;;) {
    printf("Task1 running\n");
    vTaskDelay(pdMS_TO_TICKS(1000));
  }
}

void Task2(void *pvParameters) {
  for (;;) {
    printf("Task2 running\n");
    vTaskDelay(pdMS_TO_TICKS(500));
  }
}

What will be the output pattern when both tasks run concurrently?

medium

A. Task1 prints every 1 second; Task2 prints every 0.5 seconds

B. Task1 and Task2 print at the same time every 1 second

C. Task1 prints every 0.5 seconds; Task2 prints every 1 second

D. Only Task1 prints because Task2 is blocked

Solution

1. Step 1: Analyze vTaskDelay timing for Task1

   Task1 delays 1000 ms (1 second) between prints.

2. Step 2: Analyze vTaskDelay timing for Task2

   Task2 delays 500 ms (0.5 seconds) between prints.

3. Step 3: Understand concurrent execution

   Both tasks run independently, so Task2 prints twice as often as Task1.

4. Final Answer:

   Task1 prints every 1 second; Task2 prints every 0.5 seconds -> Option A

5. Quick Check:

   Delays control print rate = Task1 prints every 1 second; Task2 prints every 0.5 seconds [OK]

Hint: Check each task's delay time to predict print frequency [OK]

Common Mistakes:

• Assuming tasks print simultaneously
• Mixing up delay times
• Thinking one task blocks the other

4. Given this code snippet, what is the error?

void TaskA(void *pvParameters) {
  while(1) {
    // Do something
    vTaskDelay(1000);
  }
}

xTaskCreate(TaskA, "TaskA", 1000, NULL, 1, NULL);

medium

A. Task function must return int, not void

B. vTaskDelay requires delay in ticks, not milliseconds

C. Stack size 1000 is too small

D. Task name must be longer than 5 characters

Solution

1. Step 1: Check vTaskDelay parameter

   vTaskDelay expects delay in ticks, not milliseconds.

2. Step 2: Identify correct delay usage

   Use pdMS_TO_TICKS(1000) to convert 1000 ms to ticks.

3. Final Answer:

   vTaskDelay requires delay in ticks, not milliseconds -> Option B

4. Quick Check:

   Delay unit must be ticks = vTaskDelay requires delay in ticks, not milliseconds [OK]

Hint: Use pdMS_TO_TICKS() to convert ms to ticks for vTaskDelay [OK]

Common Mistakes:

• Passing milliseconds directly to vTaskDelay
• Wrong return type for task function
• Incorrect stack size assumptions

5. You want two tasks to print messages alternately every 500 ms without overlapping prints. Which approach ensures this behavior in FreeRTOS?

hard

A. Use global variables without any synchronization

B. Use vTaskDelay with different delay times for each task

C. Run both tasks at the same priority without synchronization

D. Use a binary semaphore to synchronize tasks before printing

Solution

1. Step 1: Understand the need for synchronization

   To avoid overlapping prints, tasks must coordinate access to the printing resource.

2. Step 2: Identify synchronization method

   A binary semaphore allows one task to print while the other waits, alternating access.

3. Step 3: Evaluate other options

   Different delays or same priority without sync can cause overlapping prints; global variables alone don't prevent conflicts.

4. Final Answer:

   Use a binary semaphore to synchronize tasks before printing -> Option D

5. Quick Check:

   Synchronization via semaphore = Use a binary semaphore to synchronize tasks before printing [OK]

Hint: Use semaphores to control shared resource access [OK]

Common Mistakes:

• Relying only on delays for synchronization
• Ignoring task priority effects
• Not using any synchronization mechanism