Queue from ISR (xQueueSendFromISR) in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

FreeRTOS

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

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

AxHigherPriorityTaskWoken

BNULL

CxTaskHandle

DxQueueHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Passing NULL instead of the address of xHigherPriorityTaskWoken.

Using a variable with wrong type or name.

The third parameter of xQueueSendFromISR is a pointer to a variable that will be set to pdTRUE if a higher priority task was woken by sending to the queue. This variable is usually named xHigherPriorityTaskWoken.

2fill in blank

medium

Complete the code to perform a context switch if needed after sending to the queue from ISR.

FreeRTOS

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

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

AvTaskDelay

BportYIELD_FROM_ISR

CxQueueReceive

DvTaskStartScheduler

Attempts:

3 left

💡 Hint

Common Mistakes

Calling vTaskDelay inside ISR.

Calling scheduler start function inside ISR.

portYIELD_FROM_ISR(xHigherPriorityTaskWoken) is called to request a context switch from the ISR if a higher priority task was woken.

3fill in blank

hard

Fix the error in the ISR queue send call by completing the blank.

FreeRTOS

BaseType_t result = xQueueSendFromISR(queueHandle, &data, [1]);
if (result != pdPASS) {
    // Handle error
}

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

ANULL

B&xHigherPriorityTaskWoken

CxHigherPriorityTaskWoken

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable without address operator.

Passing pdTRUE instead of a pointer.

Passing NULL as the third parameter means the ISR does not care about context switching. This is valid but no task switch will be requested.

4fill in blank

hard

Fill both blanks to create a queue and send data from ISR correctly.

FreeRTOS

QueueHandle_t queueHandle = xQueueCreate([1], sizeof(int));
BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queueHandle, &data, &[2]);

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

A10

BxHigherPriorityTaskWoken

CxTaskHandle

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable name for the context switch flag.

Creating queue with zero or negative size.

The queue is created with 10 slots for int data. The address of xHigherPriorityTaskWoken is passed to xQueueSendFromISR to detect if a context switch is needed.

5fill in blank

hard

Fill all three blanks to send data from ISR and perform a context switch if needed.

FreeRTOS

BaseType_t [1] = pdFALSE;
BaseType_t result = xQueueSendFromISR(queueHandle, &data, &[2]);
if ([3]) {
    portYIELD_FROM_ISR([1]);
}

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

AxHigherPriorityTaskWoken

Cresult

DxQueueHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not passing the address of the context switch flag.

xHigherPriorityTaskWoken is declared and passed by address to xQueueSendFromISR. The xHigherPriorityTaskWoken variable is checked to decide if portYIELD_FROM_ISR(xHigherPriorityTaskWoken) should be called.

Practice

1.

What is the primary purpose of xQueueSendFromISR() in FreeRTOS?

easy

A. To send data to a queue safely from an interrupt service routine (ISR)

B. To create a new queue in the system

C. To delete a queue from an ISR

D. To block a task until data is received

Solution

1. Step 1: Understand the function context

   xQueueSendFromISR() is designed to be called inside an ISR, where normal queue functions that can block are unsafe.

2. Step 2: Identify the function's role

   It safely sends data to a queue without blocking, allowing communication from ISR to tasks.

3. Final Answer:

   To send data to a queue safely from an interrupt service routine (ISR) -> Option A

4. Quick Check:

   Purpose of xQueueSendFromISR = safe queue send from ISR [OK]

Hint: Remember: xQueueSendFromISR is for sending data inside interrupts [OK]

Common Mistakes:

• Confusing it with queue creation or deletion
• Thinking it blocks the ISR
• Using normal queue send functions inside ISR

2.

Which of the following is the correct way to call xQueueSendFromISR()?

BaseType_t xQueueSendFromISR(QueueHandle_t xQueue, const void * pvItemToQueue, BaseType_t *pxHigherPriorityTaskWoken);

easy

A. xQueueSendFromISR(queue, data, NULL);

B. xQueueSendFromISR(queue, &data, NULL);

C. xQueueSendFromISR(&queue, &data, NULL);

D. xQueueSendFromISR(queue, &data);

Solution

1. Step 1: Check parameter types

   The function expects a queue handle, a pointer to the data, and a pointer to a BaseType_t variable for task wake flag.

2. Step 2: Match correct call

   xQueueSendFromISR(queue, &data, NULL); passes the queue handle, address of data, and NULL for the last parameter, which is valid if you don't care about task wake.

3. Final Answer:

   xQueueSendFromISR(queue, &data, NULL); -> Option B

4. Quick Check:

   Correct parameters and pointers used = xQueueSendFromISR(queue, &data, NULL); [OK]

Hint: Always pass data address and pxHigherPriorityTaskWoken pointer [OK]

Common Mistakes:

• Passing data instead of its address
• Passing queue address instead of handle
• Omitting the third parameter

3.

Given the following code snippet inside an ISR, what will be the value of result after execution?

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
int data = 10;
BaseType_t result = xQueueSendFromISR(myQueue, &data, &xHigherPriorityTaskWoken);

Assume myQueue has space available.

medium

A. pdFAIL (0)

B. pdFALSE (0)

C. pdTRUE (1)

D. pdPASS (1)

Solution

1. Step 1: Understand return values of xQueueSendFromISR

   The function returns pdPASS (1) if the item was successfully sent to the queue, pdFAIL (0) otherwise.

2. Step 2: Analyze queue state

   Since myQueue has space, the send will succeed, so result will be pdPASS (1).

3. Final Answer:

   pdPASS (1) -> Option D

4. Quick Check:

   Queue has space, send success = pdPASS (1) [OK]

Hint: If queue has space, xQueueSendFromISR returns pdPASS (1) [OK]

Common Mistakes:

• Confusing pdPASS with pdTRUE
• Assuming it returns pdFALSE on success
• Ignoring queue capacity

4.

Identify the error in this ISR code snippet using xQueueSendFromISR():

void ISR_Handler(void) {
    int value = 5;
    BaseType_t xHigherPriorityTaskWoken;
    xQueueSendFromISR(myQueue, &value, &xHigherPriorityTaskWoken);
    if(xHigherPriorityTaskWoken) {
        portYIELD_FROM_ISR();
    }
}

medium

A. value should be a pointer, not an int

B. portYIELD_FROM_ISR() is called incorrectly

C. xHigherPriorityTaskWoken is not initialized before use

D. xQueueSendFromISR() cannot be called inside ISR

Solution

1. Step 1: Check variable initialization

   xHigherPriorityTaskWoken must be initialized to pdFALSE before calling xQueueSendFromISR() to avoid unpredictable behavior.

2. Step 2: Verify other parts

   Calling portYIELD_FROM_ISR() conditionally is correct; value is passed by address; calling xQueueSendFromISR() inside ISR is valid.

3. Final Answer:

   xHigherPriorityTaskWoken is not initialized before use -> Option C

4. Quick Check:

   Initialize xHigherPriorityTaskWoken = pdFALSE before use [OK]

Hint: Always init xHigherPriorityTaskWoken to pdFALSE before sending [OK]

Common Mistakes:

• Not initializing xHigherPriorityTaskWoken
• Misusing portYIELD_FROM_ISR
• Passing data incorrectly

5.

You want to send multiple sensor readings from an ISR to a task using xQueueSendFromISR(). Which approach correctly ensures the task runs immediately if it has higher priority?

void SensorISR(void) {
    static int sensorData = 0;
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    sensorData++;
    xQueueSendFromISR(sensorQueue, &sensorData, &xHigherPriorityTaskWoken);
    // What should be done here?
}

hard

A. Call portYIELD_FROM_ISR(xHigherPriorityTaskWoken); after sending

B. Call vTaskDelay(1); inside the ISR

C. Call taskYIELD(); inside the ISR unconditionally

D. Do nothing; the task will run eventually

Solution

1. Step 1: Understand task wake mechanism

   After sending data from ISR, if a higher priority task was woken, you must request a context switch immediately.

2. Step 2: Use correct yield function

   portYIELD_FROM_ISR(xHigherPriorityTaskWoken); triggers a context switch only if needed, which is the correct approach.

3. Final Answer:

   Call portYIELD_FROM_ISR(xHigherPriorityTaskWoken); after sending -> Option A

4. Quick Check:

   Use portYIELD_FROM_ISR with xHigherPriorityTaskWoken to yield [OK]

Hint: Use portYIELD_FROM_ISR with flag to yield only if needed [OK]

Common Mistakes:

• Calling blocking functions like vTaskDelay in ISR
• Calling taskYIELD unconditionally inside ISR
• Ignoring portYIELD_FROM_ISR call