vTaskDelayUntil() for precise timing in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the variable that stores the last wake time for precise delay.

FreeRTOS

TickType_t [1] = xTaskGetTickCount();

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

AxLastWakeTime

BdelayTime

CtickCount

DlastTick

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not match the expected naming convention.

Not initializing the variable with the current tick count.

The variable xLastWakeTime stores the last wake time and is used with vTaskDelayUntil() for precise timing.

2fill in blank

medium

Complete the code to delay the task precisely for 100 ticks using vTaskDelayUntil.

FreeRTOS

vTaskDelayUntil(&[1], pdMS_TO_TICKS(100));

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

AdelayTime

BlastTick

CtickCount

DxLastWakeTime

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the wrong variable or a non-pointer argument.

Using vTaskDelay() instead of vTaskDelayUntil().

The function vTaskDelayUntil() requires a pointer to xLastWakeTime to maintain precise periodic timing.

3fill in blank

hard

Fix the error in the code to correctly use vTaskDelayUntil for a 500 ms delay.

FreeRTOS

TickType_t xLastWakeTime = xTaskGetTickCount();
vTaskDelayUntil([1], pdMS_TO_TICKS(500));

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

A&xLastWakeTime

BxLastWakeTime

C*xLastWakeTime

DxLastWakeTime&

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable without the address operator.

Using incorrect pointer syntax.

vTaskDelayUntil() requires a pointer to the last wake time variable, so &xLastWakeTime is correct.

4fill in blank

hard

Fill both blanks to create a periodic task that delays precisely for 200 ms.

FreeRTOS

TickType_t [1] = xTaskGetTickCount();
vTaskDelayUntil(&[2], pdMS_TO_TICKS(200));

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

AxLastWakeTime

BdelayTime

DtickStart

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for declaration and usage.

Not passing the address of the variable in the function call.

Both blanks should be xLastWakeTime to correctly track and update the last wake time for precise delays.

5fill in blank

hard

Fill all three blanks to implement a task that toggles an LED every 250 ms using vTaskDelayUntil.

FreeRTOS

void vTaskToggleLED(void *pvParameters) {
    TickType_t [1] = xTaskGetTickCount();
    for(;;) {
        toggleLED();
        vTaskDelayUntil(&[2], pdMS_TO_TICKS([3]));
    }
}

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

AxLastWakeTime

C250

D200

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in declaration and function call.

Setting the wrong delay time.

Not passing the address of the variable.

The variable xLastWakeTime is used to track the last wake time, and the delay is set to 250 ms for toggling the LED periodically.

Practice

1. What is the main purpose of vTaskDelayUntil() in FreeRTOS?

easy

A. To delay a task until a fixed periodic time, keeping timing precise

B. To delay a task for a random amount of time

C. To suspend a task indefinitely

D. To immediately resume a suspended task

Solution

1. Step 1: Understand the function's purpose

   vTaskDelayUntil() is designed to delay a task until a specific time, maintaining a fixed periodic interval.

2. Step 2: Compare options with function behavior

   Only To delay a task until a fixed periodic time, keeping timing precise describes this precise periodic delay. Other options describe unrelated behaviors.

3. Final Answer:

   To delay a task until a fixed periodic time, keeping timing precise -> Option A

4. Quick Check:

   Precise periodic delay = A [OK]

Hint: vTaskDelayUntil keeps timing steady, not random delays [OK]

Common Mistakes:

• Confusing vTaskDelayUntil with vTaskDelay
• Thinking it suspends tasks indefinitely
• Assuming it resumes tasks immediately

2. Which of the following is the correct way to declare the variable used with vTaskDelayUntil() to track the last wake time?

easy

A. TickType_t xLastWakeTime = 0;

B. int xLastWakeTime = 0;

C. TickType_t xLastWakeTime;

D. void *xLastWakeTime = NULL;

Solution

1. Step 1: Identify the correct type for last wake time

   The FreeRTOS API specifies TickType_t as the type for tick counts and time tracking.

2. Step 2: Initialization rules for vTaskDelayUntil

   The variable must be declared but not necessarily initialized to zero; it is initialized by calling xLastWakeTime = xTaskGetTickCount(); before the loop.

3. Final Answer:

   TickType_t xLastWakeTime; -> Option C

4. Quick Check:

   Use TickType_t without initial zero for last wake time [OK]

Hint: Declare with TickType_t, initialize with xTaskGetTickCount() [OK]

Common Mistakes:

• Using int instead of TickType_t
• Initializing to zero instead of using xTaskGetTickCount()
• Using void pointer type

3. Given the code snippet below, what will be the approximate delay between each "Task running" print?

TickType_t xLastWakeTime;
xLastWakeTime = xTaskGetTickCount();

for (;;) {
    vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(1000));
    printf("Task running\n");
}

medium

A. Approximately 1000 milliseconds, steady without drift

B. Exactly 1000 milliseconds between prints

C. Variable delay, sometimes less than 1000 milliseconds

D. No delay, prints continuously

Solution

1. Step 1: Understand vTaskDelayUntil behavior

   It delays the task until the next fixed interval based on the last wake time, keeping timing steady.

2. Step 2: Analyze the delay duration

   The delay is set to 1000 ms converted to ticks, so the task runs approximately every 1000 ms without accumulating drift.

3. Final Answer:

   Approximately 1000 milliseconds, steady without drift -> Option A

4. Quick Check:

   vTaskDelayUntil = steady periodic delay [OK]

Hint: vTaskDelayUntil keeps intervals steady, not exact but no drift [OK]

Common Mistakes:

• Assuming exact millisecond precision
• Confusing with vTaskDelay which causes drift
• Thinking delay varies randomly

4. Identify the error in the following FreeRTOS task code using vTaskDelayUntil():

void vTaskFunction(void *pvParameters) {
    TickType_t xLastWakeTime = 0;
    for (;;) {
        vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(500));
        // Task code here
    }
}

medium

A. vTaskDelayUntil() should not be used inside a loop

B. TickType_t cannot be initialized to zero

C. pdMS_TO_TICKS() cannot be used as a delay parameter

D. xLastWakeTime should be initialized with xTaskGetTickCount() before the loop

Solution

1. Step 1: Check initialization of xLastWakeTime

   It must be initialized to the current tick count using xTaskGetTickCount() before the loop to set the reference time.

2. Step 2: Verify usage of vTaskDelayUntil

   The function is correctly used inside the loop, and pdMS_TO_TICKS is valid for converting milliseconds to ticks.

3. Final Answer:

   xLastWakeTime should be initialized with xTaskGetTickCount() before the loop -> Option D

4. Quick Check:

   Initialize xLastWakeTime with xTaskGetTickCount() [OK]

Hint: Always init last wake time with xTaskGetTickCount() before loop [OK]

Common Mistakes:

• Not initializing xLastWakeTime before loop
• Misusing vTaskDelayUntil outside loops
• Incorrect delay parameter types

5. You want a FreeRTOS task to run exactly every 200 ms without drift, but your task sometimes takes 50 ms to execute. How should you use vTaskDelayUntil() to achieve this?

hard

A. Use vTaskDelayUntil() with 50 ms delay to match task execution time

B. Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(200)) inside the loop after task code

C. Initialize xLastWakeTime to zero and call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(250)) before task code

D. Call vTaskDelay(pdMS_TO_TICKS(200)) after task code to delay fixed 200 ms

Solution

1. Step 1: Understand timing requirements

   The task must run every 200 ms total, including 50 ms execution time, so delay should be 200 ms fixed interval.

2. Step 2: Correct usage of vTaskDelayUntil

   Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil() with 200 ms delay after task code to maintain steady timing without drift.

3. Final Answer:

   Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(200)) inside the loop after task code -> Option B

4. Quick Check:

   Init last wake time + vTaskDelayUntil with full period = D [OK]

Hint: Init last wake time, delay full period after task code [OK]

Common Mistakes:

• Using vTaskDelay instead of vTaskDelayUntil causing drift
• Not accounting for task execution time in delay
• Initializing xLastWakeTime incorrectly