Multiple tasks running concurrently in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a task in FreeRTOS?

A task in FreeRTOS is like a small program or job that runs independently and can be managed by the FreeRTOS scheduler to run alongside other tasks.

beginner

How does FreeRTOS run multiple tasks at the same time?

FreeRTOS uses a scheduler to switch between tasks quickly, giving the illusion that tasks run at the same time by sharing CPU time.

intermediate

What is the purpose of task priorities in FreeRTOS?

Task priorities tell FreeRTOS which task is more important, so the scheduler runs higher priority tasks before lower priority ones.

beginner

What function is used to create a task in FreeRTOS?

The function xTaskCreate() is used to create a new task in FreeRTOS.

intermediate

Why should tasks avoid blocking for too long in FreeRTOS?

If a task blocks for too long, other tasks may not get CPU time, causing delays or missed deadlines in the system.

Which FreeRTOS function creates a new task?

A xTaskCreate()

B vTaskStart()

C xTaskRun()

D vTaskInit()

xTaskCreate() is the correct function to create a new task in FreeRTOS.

What does the FreeRTOS scheduler do?

A Runs tasks one after another without switching

B Stops all tasks except the highest priority

C Switches between tasks to share CPU time

D Runs only one task forever

The scheduler switches between tasks quickly to give the appearance of running multiple tasks concurrently.

What happens if two tasks have the same priority in FreeRTOS?

A The scheduler runs the first created task only

B The higher memory task runs first

C The system crashes

D The scheduler runs them in a round-robin fashion

Tasks with the same priority share CPU time in a round-robin way.

Why is it important to keep tasks short and non-blocking?

A To save memory

B To allow other tasks to run smoothly

C To increase task priority automatically

D To avoid using the scheduler

Short, non-blocking tasks let the scheduler run other tasks without delay.

Which of these is NOT a benefit of running multiple tasks concurrently?

A Tasks run slower individually

B Improved responsiveness

C Better CPU utilization

D Simpler program structure

Running multiple tasks concurrently does not necessarily make individual tasks slower; it improves overall system efficiency.

Explain how FreeRTOS manages multiple tasks running at the same time.

Think about how the CPU switches between tasks quickly.

Describe why task priorities are important in FreeRTOS and how they affect task execution.

Consider which task the system chooses to run first.

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