Hard real-time vs soft real-time in FreeRTOS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a task that must meet a strict deadline (hard real-time).

FreeRTOS

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

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

AconfigMAX_PRIORITIES - 1

BtskIDLE_PRIORITY

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using low priority for hard real-time tasks.

Confusing priority values.

Hard real-time tasks require the highest priority to meet strict deadlines, so we use configMAX_PRIORITIES - 1.

2fill in blank

medium

Complete the code to delay a soft real-time task without blocking other tasks.

FreeRTOS

vTaskDelay([1]);

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

A0

BpdMS_TO_TICKS(100)

C100

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Passing raw milliseconds instead of ticks.

Using portMAX_DELAY which blocks indefinitely.

Use pdMS_TO_TICKS(100) to convert milliseconds to ticks for a non-blocking delay.

3fill in blank

hard

Fix the error in the code to ensure a hard real-time task does not miss its deadline.

FreeRTOS

if (xTaskGetTickCount() > [1]) {
    // handle deadline miss
}

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

AxTaskGetTickCount()

BportMAX_DELAY

CpdMS_TO_TICKS(1000)

Ddeadline

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing to portMAX_DELAY which is a constant for max delay.

Comparing to current tick count which is always true.

The code should compare the current tick count to a predefined deadline to detect deadline misses.

4fill in blank

hard

Fill both blanks to create a soft real-time task that waits for a notification with a timeout.

FreeRTOS

if (xTaskNotifyWait(0, 0, NULL, [1]) == [2]) {
    // notification received
}

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

ApdMS_TO_TICKS(5000)

BpdPASS

CportMAX_DELAY

DpdFAIL

Attempts:

3 left

💡 Hint

Common Mistakes

Using portMAX_DELAY which waits indefinitely.

Checking for pdFAIL instead of pdPASS.

The task waits up to 5000 ms (pdMS_TO_TICKS(5000)) and checks if the notification was successful (pdPASS).

5fill in blank

hard

Fill all three blanks to define a hard real-time task that suspends itself and resumes after an interrupt.

FreeRTOS

void [1](void *pvParameters) {
    for (;;) {
        vTaskSuspend(NULL);
        // task resumed by [2]
        [3]();
    }
}

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

AHardRealTimeTask

Ban ISR

CprocessCriticalData

DSoftRealTimeTask

Attempts:

3 left

💡 Hint

Common Mistakes

Using a soft real-time task name for a hard real-time task.

Not calling the processing function after resume.

The task named HardRealTimeTask suspends itself and is resumed by an interrupt service routine (an ISR), then calls processCriticalData().

Practice

1. Which statement best describes a hard real-time system in FreeRTOS?

easy

A. Tasks can miss deadlines occasionally without major issues.

B. Tasks run only when the CPU is idle.

C. Tasks must meet deadlines strictly without any misses.

D. Tasks have no timing constraints.

Solution

1. Step 1: Understand hard real-time definition

   Hard real-time means deadlines are strict and must never be missed.

2. Step 2: Compare options with definition

   The statement 'Tasks must meet deadlines strictly without any misses' matches the definition of hard real-time.

3. Final Answer:

   Tasks must meet deadlines strictly without any misses. -> Option C

4. Quick Check:

   Hard real-time = strict deadlines [OK]

Hint: Hard real-time means no deadline misses allowed [OK]

Common Mistakes:

• Confusing hard real-time with soft real-time
• Thinking tasks run only when CPU is idle
• Ignoring timing constraints

2. Which FreeRTOS API function is used to delay a task for a specific time period?

easy

A. vTaskDelay()

B. xTaskCreate()

C. vTaskStartScheduler()

D. xQueueSend()

Solution

1. Step 1: Identify delay function in FreeRTOS

   vTaskDelay() pauses a task for a given tick count, controlling timing.

2. Step 2: Check other options

   xTaskCreate() creates tasks, vTaskStartScheduler() starts the OS, xQueueSend() sends to queues.

3. Final Answer:

   vTaskDelay() -> Option A

4. Quick Check:

   Delay task = vTaskDelay() [OK]

Hint: Delay tasks with vTaskDelay() in FreeRTOS [OK]

Common Mistakes:

• Confusing task creation with delay
• Using scheduler start as delay
• Mixing queue functions with delays

3. Consider this FreeRTOS task priority setup:

Task A priority = 3
Task B priority = 2
Task C priority = 1

If Task A blocks for 100ms using vTaskDelay(), which task runs next?

medium

A. Task A again immediately

B. Task B

C. Task C

D. No task runs until Task A finishes delay

Solution

1. Step 1: Understand task priorities and blocking

   Task A has highest priority (3) but blocks for 100ms, so it is not ready.

2. Step 2: Identify next highest priority ready task

   Task B has priority 2 and is ready, so it runs next.

3. Final Answer:

   Task B -> Option B

4. Quick Check:

   Next task after blocking = highest ready priority [OK]

Hint: Next runs highest priority ready task after delay [OK]

Common Mistakes:

• Assuming blocked task runs immediately
• Ignoring task priorities
• Thinking no task runs during delay

4. Identify the error in this FreeRTOS code snippet for a hard real-time task:

void vTaskFunction(void *pvParameters) {
  while(1) {
    // Critical processing
    vTaskDelay(1000);
  }
}

What is the problem?

medium

A. vTaskDelay should be replaced with vTaskDelete.

B. The task function lacks a return statement.

C. The infinite loop will crash FreeRTOS.

D. Using vTaskDelay causes deadline misses in hard real-time tasks.

Solution

1. Step 1: Analyze vTaskDelay in hard real-time context

   vTaskDelay pauses the task, which can cause missing strict deadlines in hard real-time.

2. Step 2: Check other options for correctness

   Return statement is not needed in infinite loop, infinite loops are normal, vTaskDelete deletes task, not delay.

3. Final Answer:

   Using vTaskDelay causes deadline misses in hard real-time tasks. -> Option D

4. Quick Check:

   Hard real-time tasks avoid vTaskDelay [OK]

Hint: Hard real-time tasks avoid delays that cause misses [OK]

Common Mistakes:

• Thinking infinite loops crash FreeRTOS
• Confusing vTaskDelay with task deletion
• Expecting return in infinite loops

5. You need to design a FreeRTOS system where some tasks must never miss deadlines (hard real-time), and others can miss occasionally (soft real-time). Which approach best achieves this?

hard

A. Assign highest priorities to hard real-time tasks and use vTaskDelay carefully for soft real-time tasks.

B. Use vTaskDelay for all tasks to simplify timing.

C. Assign equal priorities to all tasks and rely on round-robin scheduling.

D. Run hard real-time tasks only when soft real-time tasks are idle.

Solution

1. Step 1: Understand priority and delay roles in FreeRTOS

   Hard real-time tasks need highest priority to avoid preemption and deadline misses.

2. Step 2: Use vTaskDelay carefully for soft real-time tasks

   Soft real-time tasks can tolerate delays, so vTaskDelay helps manage their timing without affecting hard real-time tasks.

3. Step 3: Evaluate other options

   Equal priorities or running hard real-time tasks only when soft are idle do not guarantee deadlines.

4. Final Answer:

   Assign highest priorities to hard real-time tasks and use vTaskDelay carefully for soft real-time tasks. -> Option A

5. Quick Check:

   Priority + careful delay = mixed real-time system [OK]

Hint: Prioritize hard real-time tasks highest, delay soft tasks [OK]

Common Mistakes:

• Giving equal priority to all tasks
• Using delays in hard real-time tasks
• Running hard real-time tasks only after soft tasks