vTaskDelay() for periodic tasks in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delay the task for 100 ticks.

FreeRTOS

vTaskDelay([1]);

Drag options to blanks, or click blank then click option'

A100

B1000

C10

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using milliseconds instead of ticks.

Forgetting that the argument is in ticks, not time units.

The function vTaskDelay() pauses the task for the specified number of ticks. Here, 100 ticks is the correct delay.

2fill in blank

medium

Complete the code to create a periodic task that delays for 200 ticks.

FreeRTOS

for (;;) {
    // Task code here
    vTaskDelay([1]);
}

Drag options to blanks, or click blank then click option'

A50

B20

C200

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using too short or too long delay values.

Not placing the delay inside the infinite loop.

The task delays for 200 ticks each cycle to run periodically every 200 ticks.

3fill in blank

hard

Fix the error in the code to correctly delay the task for 500 ticks.

FreeRTOS

vTaskDelay([1]); // Delay for 500 ticks

Drag options to blanks, or click blank then click option'

A5

B500

C50

D5000

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing milliseconds with ticks.

Using a value ten times too large or too small.

The correct delay is 500 ticks, so the argument must be 500.

4fill in blank

hard

Fill both blanks to create a periodic task that delays for 100 ticks and prints "Tick" each cycle.

FreeRTOS

for (;;) {
    printf([1]);
    vTaskDelay([2]);
}

Drag options to blanks, or click blank then click option'

A"Tick\n"

B100

C"Hello"

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Printing the wrong string.

Using the wrong delay value.

The task prints "Tick" and then delays for 100 ticks to run periodically.

5fill in blank

hard

Fill all three blanks to create a periodic task that prints the count, delays for 50 ticks, and increments the count.

FreeRTOS

int count = 0;
for (;;) {
    printf([1], [2]);
    vTaskDelay([3]);
    count++;
}

Drag options to blanks, or click blank then click option'

A"Count: %d\n"

Bcount

C50

D"Value: %d\n"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong format string.

Printing a wrong variable.

Using incorrect delay value.

The task prints the current count, delays for 50 ticks, then increments the count for the next cycle.

Practice

1. What is the primary purpose of vTaskDelay() in FreeRTOS?

easy

A. To pause a task for a specific number of ticks

B. To create a new task

C. To delete a task

D. To send data between tasks

Solution

1. Step 1: Understand the function role

   vTaskDelay() is designed to pause or block a task for a set number of tick counts, allowing other tasks to run.

2. Step 2: Compare with other options

   Creating or deleting tasks and sending data are done by other FreeRTOS functions, not vTaskDelay().

3. Final Answer:

   To pause a task for a specific number of ticks -> Option A

4. Quick Check:

   vTaskDelay() pauses task = A [OK]

Hint: vTaskDelay() always pauses task execution for ticks [OK]

Common Mistakes:

• Confusing vTaskDelay() with task creation functions
• Thinking vTaskDelay() sends data
• Assuming vTaskDelay() deletes tasks

2. Which of the following is the correct way to delay a task for 100 milliseconds using vTaskDelay()?

easy

A. vTaskDelay(100);

B. vTaskDelayMs(100);

C. vTaskDelay(pdMS_TO_TICKS(100));

D. vTaskDelayTicks(100);

Solution

1. Step 1: Understand tick conversion

   FreeRTOS uses ticks, not milliseconds directly. To convert milliseconds to ticks, use pdMS_TO_TICKS().

2. Step 2: Check syntax correctness

   Only vTaskDelay(pdMS_TO_TICKS(100)); correctly converts 100 ms to ticks and delays the task.

3. Final Answer:

   vTaskDelay(pdMS_TO_TICKS(100)); -> Option C

4. Quick Check:

   Use pdMS_TO_TICKS() inside vTaskDelay() [OK]

Hint: Always convert ms to ticks with pdMS_TO_TICKS() [OK]

Common Mistakes:

• Passing milliseconds directly without conversion
• Using non-existent functions like vTaskDelayMs()
• Confusing ticks and milliseconds

3. Given the following code snippet, what will be the approximate delay between each print statement?

void TaskFunction(void *pvParameters) {
    while(1) {
        printf("Tick\n");
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

medium

A. Approximately 1 second

B. Approximately 500 milliseconds

C. No delay, prints continuously

D. Approximately 50 milliseconds

Solution

1. Step 1: Analyze vTaskDelay parameter

   The delay is set to pdMS_TO_TICKS(500), which converts 500 milliseconds to ticks.

2. Step 2: Understand loop timing

   Each loop iteration prints "Tick" then delays for about 500 ms, so prints occur roughly every 500 ms.

3. Final Answer:

   Approximately 500 milliseconds -> Option B

4. Quick Check:

   vTaskDelay(pdMS_TO_TICKS(500)) = 500 ms delay [OK]

Hint: vTaskDelay(pdMS_TO_TICKS(x)) delays about x milliseconds [OK]

Common Mistakes:

• Ignoring pdMS_TO_TICKS conversion
• Assuming no delay in loop
• Confusing milliseconds with ticks

4. Identify the error in this code snippet intended to delay a task for 200 milliseconds:

void Task(void *params) {
    while(1) {
        doWork();
        vTaskDelay(200);
    }
}

medium

A. vTaskDelay cannot be used inside a while loop

B. doWork() must be called after vTaskDelay()

C. The task function must return a value

D. vTaskDelay should use pdMS_TO_TICKS(200) to convert milliseconds

Solution

1. Step 1: Check vTaskDelay argument

   The argument 200 is treated as ticks, not milliseconds. To delay 200 ms, convert using pdMS_TO_TICKS(200).

2. Step 2: Verify other code parts

   Calling doWork() before delay is valid. The task function returns void, which is correct.

3. Final Answer:

   vTaskDelay should use pdMS_TO_TICKS(200) to convert milliseconds -> Option D

4. Quick Check:

   Convert ms to ticks with pdMS_TO_TICKS() [OK]

Hint: Always convert ms to ticks before passing to vTaskDelay() [OK]

Common Mistakes:

• Passing milliseconds directly to vTaskDelay()
• Thinking vTaskDelay can't be in loops
• Expecting task functions to return values

5. You want a task to run exactly every 1 second, but your system tick rate is 1000 Hz (1 tick = 1 ms). Which is the best way to implement this periodic delay using vTaskDelay()?

hard

A. Use vTaskDelayUntil() instead of vTaskDelay() for precise periodic timing

B. Call vTaskDelay(pdMS_TO_TICKS(1000)); inside the task loop

C. Call vTaskDelay(pdMS_TO_TICKS(1000)); and then call vTaskDelay(0);

D. Call vTaskDelay(1000); inside the task loop

Solution

1. Step 1: Understand vTaskDelay limitations

   vTaskDelay() delays relative to current time, so small variations accumulate causing drift in periodic tasks.

2. Step 2: Identify precise periodic method

   vTaskDelayUntil() uses an absolute wake time, preventing drift and ensuring exact periodic execution.

3. Final Answer:

   Use vTaskDelayUntil() instead of vTaskDelay() for precise periodic timing -> Option A

4. Quick Check:

   vTaskDelayUntil() prevents drift in periodic tasks [OK]

Hint: Use vTaskDelayUntil() for exact periodic task timing [OK]

Common Mistakes:

• Using vTaskDelay() causes timing drift
• Assuming tick count matches ms without conversion
• Adding extra delays unnecessarily