Tick timer and scheduler in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the FreeRTOS scheduler.

FreeRTOS

vTaskStartScheduler[1];

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

A[]

B;

C{}

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the parentheses after the function name.

Adding a semicolon inside the parentheses.

The function vTaskStartScheduler() starts the FreeRTOS scheduler and takes no parameters, so it requires parentheses.

2fill in blank

medium

Complete the code to create a FreeRTOS task with the correct priority.

FreeRTOS

xTaskCreate(taskFunction, "Task1", configMINIMAL_STACK_SIZE, NULL, [1], NULL);

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A1

BtskIDLE_PRIORITY

C0

DconfigMAX_PRIORITIES

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero as priority which is idle priority.

Using configMAX_PRIORITIES which is out of range.

The priority parameter is usually set to a value between tskIDLE_PRIORITY (0) and configMAX_PRIORITIES - 1. Here, 1 is a valid priority above idle.

3fill in blank

hard

Fix the error in the tick hook function declaration.

FreeRTOS

void vApplicationTickHook[1]

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A()

B(void)

C(int)

D(void*)

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty parentheses which is not valid in C for no-parameter functions.

Adding parameters to the tick hook function.

The tick hook function must be declared with void parameter list: void vApplicationTickHook(void).

4fill in blank

hard

Fill both blanks to create a dictionary of task names and their states.

FreeRTOS

const char* taskStates[] = {"Running", "Ready", "Blocked", "Suspended"};
for(int i = 0; i < [1]; i++) {
    printf("Task %s is %s\n", taskNames[i], taskStates[[2]]);
}

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AconfigMAX_PRIORITIES

BuxTaskGetNumberOfTasks()

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_PRIORITIES which is unrelated to task count.

Using a fixed zero index for all task states.

The loop should run for the number of tasks returned by uxTaskGetNumberOfTasks(). The index i is used to access the current task state.

5fill in blank

hard

Fill all three blanks to correctly implement a tick hook that increments a counter and yields if needed.

FreeRTOS

static BaseType_t xHigherPriorityTaskWoken = pdFALSE;
void vApplicationTickHook(void) {
    [1]++;
    if (xSemaphoreGiveFromISR(xSemaphore, &[2]) == pdTRUE) {
        port[3]();
    }
}

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

AulTickCount

BxHigherPriorityTaskWoken

CYIELD_FROM_ISR

DvTaskDelay

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong variable name for the tick count.

Calling vTaskDelay inside an ISR which is invalid.

Not passing the address of the flag variable.

The tick count variable ulTickCount is incremented. The xHigherPriorityTaskWoken flag is passed to the semaphore function. If a higher priority task was woken, portYIELD_FROM_ISR() is called to request a context switch.

Practice

1. What is the main role of the tick timer in FreeRTOS?

easy

A. To handle input/output operations

B. To store task priorities

C. To allocate memory for tasks

D. To count time in small steps called ticks

Solution

1. Step 1: Understand tick timer function

   The tick timer counts time in small units called ticks, which are the basic time measurement in FreeRTOS.

2. Step 2: Differentiate from other functions

   Other options like storing priorities or handling I/O are not related to the tick timer's role.

3. Final Answer:

   To count time in small steps called ticks -> Option D

4. Quick Check:

   Tick timer = counts ticks [OK]

Hint: Tick timer counts time steps, not priorities or memory [OK]

Common Mistakes:

• Confusing tick timer with task priority management
• Thinking tick timer handles memory or I/O
• Mixing tick timer with scheduler directly

2. Which of the following is the correct way to delay a task for 100 ticks in FreeRTOS?

easy

A. xTaskDelay(100);

B. vTaskDelay(1000);

C. vTaskDelay(100);

D. delayTask(100);

Solution

1. Step 1: Identify correct function name

   The correct FreeRTOS function to delay a task is vTaskDelay().

2. Step 2: Check parameter correctness

   The parameter is the number of ticks to delay. For 100 ticks, vTaskDelay(100); is correct.

3. Final Answer:

   vTaskDelay(100); -> Option C

4. Quick Check:

   Delay task = vTaskDelay(ticks) [OK]

Hint: Use vTaskDelay with tick count to pause tasks [OK]

Common Mistakes:

• Using wrong function name like xTaskDelay or delayTask
• Passing milliseconds instead of ticks
• Using too large or incorrect delay values

3. Consider this code snippet in FreeRTOS:

TickType_t start = xTaskGetTickCount();
vTaskDelay(50);
TickType_t end = xTaskGetTickCount();
TickType_t diff = end - start;

What is the expected value of diff?

medium

A. 0

B. 50

C. 100

D. Depends on task priority

Solution

1. Step 1: Understand xTaskGetTickCount()

   This function returns the current tick count since the scheduler started.

2. Step 2: Analyze vTaskDelay(50)

   The task delays itself for 50 ticks, so the tick count advances by 50 during the delay.

3. Step 3: Calculate difference

   Subtracting start from end gives the delay duration, which should be 50 ticks.

4. Final Answer:

   50 -> Option B

5. Quick Check:

   Delay ticks = 50 [OK]

Hint: Delay of 50 ticks means tick count difference is 50 [OK]

Common Mistakes:

• Assuming delay is zero or doubled
• Thinking task priority affects tick count difference
• Confusing ticks with milliseconds

4. What is wrong with this FreeRTOS code snippet?

vTaskDelay(0);
TickType_t ticks = xTaskGetTickCount();

medium

A. vTaskDelay(0) does not delay the task

B. xTaskGetTickCount() returns an error

C. vTaskDelay requires a pointer argument

D. TickType_t is not a valid type

Solution

1. Step 1: Check vTaskDelay parameter

   Passing 0 to vTaskDelay means no delay; the task yields but continues immediately.

2. Step 2: Verify xTaskGetTickCount()

   This function correctly returns the current tick count; no error here.

3. Final Answer:

   vTaskDelay(0) does not delay the task -> Option A

4. Quick Check:

   Delay(0) means no wait [OK]

Hint: vTaskDelay(0) yields but does not pause task [OK]

Common Mistakes:

• Expecting delay with zero ticks
• Thinking xTaskGetTickCount returns error
• Misunderstanding TickType_t type

5. You want a task to run every 200 ticks precisely using FreeRTOS. Which approach ensures this timing despite task execution time?

hard

A. Use vTaskDelayUntil() with a previous wake time variable

B. Use vTaskDelay(200) at the end of the task loop

C. Use xTaskGetTickCount() inside the task and delay manually

D. Use vTaskDelay(100) twice in the task loop

Solution

1. Step 1: Understand vTaskDelay() limitation

   Using vTaskDelay(200) delays from current time, so task execution time adds up causing drift.

2. Step 2: Use vTaskDelayUntil() for fixed periodic timing

   vTaskDelayUntil() uses a previous wake time to maintain exact periodic intervals regardless of execution time.

3. Step 3: Evaluate other options

   Manual delay or multiple delays do not guarantee precise timing; only vTaskDelayUntil() does.

4. Final Answer:

   Use vTaskDelayUntil() with a previous wake time variable -> Option A

5. Quick Check:

   Precise periodic task = vTaskDelayUntil() [OK]

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

Common Mistakes:

• Using vTaskDelay causes timing drift
• Trying manual delay calculations
• Ignoring task execution time in timing