Deferred interrupt processing pattern in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interrupt handler function.

FreeRTOS

void [1](void) {
    // ISR code here
}

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

AvMyISR

Bmain

CvTaskDelay

DxQueueSend

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that conflicts with FreeRTOS API functions.

Adding parameters to the ISR function.

The interrupt handler function must have a proper name, here 'vMyISR' is used as an example.

2fill in blank

medium

Complete the code to send data to a queue from an ISR.

FreeRTOS

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR([1], &data, &xHigherPriorityTaskWoken);

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

AxTaskHandle

BxSemaphore

CxQueue

DxTimer

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a semaphore handle instead of a queue handle.

Using a task handle instead of a queue handle.

The function xQueueSendFromISR requires a queue handle as the first argument.

3fill in blank

hard

Fix the error in the ISR by correctly calling the yield function.

FreeRTOS

if (xHigherPriorityTaskWoken) {
    [1](xHigherPriorityTaskWoken);
}

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

AvTaskDelay

BvTaskStartScheduler

CxQueueReceive

DportYIELD_FROM_ISR

Attempts:

3 left

💡 Hint

Common Mistakes

Calling vTaskDelay inside ISR.

Calling scheduler start function inside ISR.

portYIELD_FROM_ISR(xHigherPriorityTaskWoken) is the correct macro to request a context switch from an ISR.

4fill in blank

hard

Fill both blanks to create a deferred processing task that waits on a queue.

FreeRTOS

void vDeferredTask(void *pvParameters) {
    uint32_t ulReceivedValue;
    for(;;) {
        if (xQueueReceive([1], &ulReceivedValue, [2]) == pdPASS) {
            // Process the received value
        }
    }
}

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

AxQueue

BportMAX_DELAY

CpdFALSE

DxSemaphore

Attempts:

3 left

💡 Hint

Common Mistakes

Using a semaphore handle instead of a queue handle.

Using pdFALSE as block time which means no wait.

The task waits indefinitely on the queue 'xQueue' using portMAX_DELAY to block.

5fill in blank

hard

Fill all three blanks to correctly create a queue, start the deferred task, and send data from ISR.

FreeRTOS

QueueHandle_t [1];

void vSetup() {
    [2] = xQueueCreate(10, sizeof(uint32_t));
    xTaskCreate(vDeferredTask, "Deferred", 1000, NULL, 2, NULL);
}

void vMyISR(void) {
    uint32_t ulData = 42;
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xQueueSendFromISR([3], &ulData, &xHigherPriorityTaskWoken);
    if (xHigherPriorityTaskWoken) {
        portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
    }
}

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

AxQueue

DxSemaphore

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the queue handle.

Using a semaphore handle instead of a queue handle.

The same queue handle 'xQueue' is used to create the queue, send data from ISR, and referenced in the task.

Practice

1. What is the main purpose of the deferred interrupt processing pattern in FreeRTOS?

easy

A. To move heavy processing from the ISR to a task for better system responsiveness

B. To execute all interrupt code inside the ISR for faster response

C. To disable all interrupts during task execution

D. To avoid using semaphores or queues in interrupt handling

Solution

1. Step 1: Understand ISR limitations

   Interrupt Service Routines (ISRs) should be short to keep system responsive.

2. Step 2: Role of deferred processing

   Heavy or time-consuming work is moved to a task, triggered by ISR using semaphores or queues.

3. Final Answer:

   To move heavy processing from the ISR to a task for better system responsiveness -> Option A

4. Quick Check:

   Deferred processing = move work out of ISR [OK]

Hint: Heavy work belongs in tasks, not ISRs [OK]

Common Mistakes:

• Thinking all work should be done inside ISR
• Believing interrupts should be disabled during tasks
• Ignoring use of semaphores or queues

2. Which of the following is the correct way to notify a task from an ISR in FreeRTOS?

easy

A. vTaskDelay()

B. xSemaphoreGive()

C. xSemaphoreGiveFromISR()

D. xQueueSend()

Solution

1. Step 1: Identify ISR-safe functions

   Functions called from ISRs must be ISR-safe variants, like xSemaphoreGiveFromISR().

2. Step 2: Compare options

   xSemaphoreGive() is for tasks, not ISRs; vTaskDelay() is a task delay; xQueueSend() is not ISR-safe.

3. Final Answer:

   xSemaphoreGiveFromISR() -> Option C

4. Quick Check:

   Use FromISR functions inside ISRs [OK]

Hint: Use FromISR suffix functions inside ISRs [OK]

Common Mistakes:

• Using task-only functions inside ISR
• Calling blocking functions in ISR
• Not using FromISR variants

3. Consider this snippet inside an ISR in FreeRTOS:

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

What is the purpose of portYIELD_FROM_ISR() here?

medium

A. To reset the semaphore count

B. To request a context switch if a higher priority task was woken

C. To delay the ISR until the task runs

D. To disable interrupts after giving the semaphore

Solution

1. Step 1: Understand xHigherPriorityTaskWoken

   This flag indicates if giving the semaphore woke a higher priority task.

2. Step 2: Role of portYIELD_FROM_ISR()

   If the flag is true, portYIELD_FROM_ISR() requests a context switch to run the higher priority task immediately.

3. Final Answer:

   To request a context switch if a higher priority task was woken -> Option B

4. Quick Check:

   portYIELD_FROM_ISR triggers context switch [OK]

Hint: portYIELD_FROM_ISR triggers immediate task switch [OK]

Common Mistakes:

• Thinking it disables interrupts
• Assuming it delays ISR execution
• Confusing it with semaphore reset

4. You wrote this ISR code to notify a task:

void ISR_Handler(void) {
    xSemaphoreGive(xSemaphore);
}

Why might this cause problems in FreeRTOS?

medium

A. xSemaphoreGive() is not safe to call from an ISR

B. The semaphore is given twice accidentally

C. The ISR does not clear the interrupt flag

D. The task will never unblock

Solution

1. Step 1: Check function usage in ISR

   xSemaphoreGive() is a task-level function and is not safe to call inside an ISR.

2. Step 2: Correct function to use

   Inside ISR, xSemaphoreGiveFromISR() must be used to safely notify the task.

3. Final Answer:

   xSemaphoreGive() is not safe to call from an ISR -> Option A

4. Quick Check:

   Use FromISR functions in ISRs [OK]

Hint: Never call task functions inside ISR, use FromISR versions [OK]

Common Mistakes:

• Calling task functions inside ISR
• Forgetting to clear interrupt flags
• Assuming semaphore gives always unblock tasks

5. You want to implement deferred interrupt processing where an ISR signals a task to process sensor data. Which sequence correctly follows FreeRTOS best practices?

hard

A. Task polls sensor data continuously, ISR disables interrupts

B. ISR processes sensor data directly, then calls vTaskDelay()

C. ISR sends data via xQueueSend(), task never blocks

D. ISR calls xSemaphoreGiveFromISR(), then task waits on semaphore and processes data

Solution

1. Step 1: ISR role in deferred processing

   ISR should be short and only notify the task using xSemaphoreGiveFromISR().

2. Step 2: Task role in processing

   The task waits on the semaphore and processes sensor data when notified.

3. Final Answer:

   ISR calls xSemaphoreGiveFromISR(), then task waits on semaphore and processes data -> Option D

4. Quick Check:

   ISR signals, task processes data [OK]

Hint: ISR signals; task does heavy work [OK]

Common Mistakes:

• Doing heavy work inside ISR
• Using blocking calls inside ISR
• Not synchronizing task and ISR properly