Semaphore for ISR-to-task signaling in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to give a semaphore from an ISR.

FreeRTOS

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

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

AxTimer

BxSemaphore

CxTaskHandle

DxQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a task handle instead of a semaphore handle.

Using the normal xSemaphoreGive() function inside an ISR.

The function xSemaphoreGiveFromISR() requires the semaphore handle as the first argument.

2fill in blank

medium

Complete the code to check if a higher priority task was woken after giving the semaphore.

FreeRTOS

if( [1] == pdTRUE ) {
    portYIELD_FROM_ISR();
}

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

AxQueue

BxTaskHandle

CxSemaphore

DxHigherPriorityTaskWoken

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the semaphore handle instead of the woken flag.

Not calling portYIELD_FROM_ISR() when needed.

The variable xHigherPriorityTaskWoken is set by xSemaphoreGiveFromISR() to indicate if a context switch is needed.

3fill in blank

hard

Fix the error in the ISR code to correctly give the semaphore and yield if needed.

FreeRTOS

void vISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR( [1], &xHigherPriorityTaskWoken );
    if( xHigherPriorityTaskWoken == pdTRUE ) {
        portYIELD_FROM_ISR();
    }
}

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

AxSemaphore

BxSemaphoreFromISR

DxSemaphoreHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreGive() inside an ISR.

Not passing the address of xHigherPriorityTaskWoken.

The ISR must use xSemaphoreGiveFromISR() instead of xSemaphoreGive(). The blank should be the semaphore handle passed to xSemaphoreGiveFromISR().

4fill in blank

hard

Fill both blanks to correctly give a semaphore from ISR and check if a context switch is needed.

FreeRTOS

BaseType_t [1] = pdFALSE;
xSemaphoreGiveFromISR( [2], &[1] );
if( [1] == pdTRUE ) {
    portYIELD_FROM_ISR();
}

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

AxHigherPriorityTaskWoken

BxSemaphore

CxTaskHandle

DxQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the semaphore handle and the woken flag variable.

Not passing the address of the woken flag.

The variable xHigherPriorityTaskWoken tracks if a higher priority task was woken. The semaphore handle xSemaphore is given from ISR.

5fill in blank

hard

Fill all three blanks to create a task that waits for a semaphore given from an ISR.

FreeRTOS

void vTaskFunction(void *pvParameters) {
    for( ;; ) {
        if( xSemaphoreTake( [1], portMAX_DELAY ) == [2] ) {
            // Process event
            [3]();
        }
    }
}

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

AxSemaphore

BpdTRUE

ChandleEvent

DxQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong return value to check semaphore take success.

Not calling the event handler function after taking the semaphore.

The task waits indefinitely for xSemaphore. When xSemaphoreTake() returns pdTRUE, it calls handleEvent() to process the event.

Practice

1. What is the main purpose of using a semaphore in FreeRTOS for ISR-to-task signaling?

easy

A. To disable interrupts during task execution

B. To create a new task inside an ISR

C. To safely notify a task from an interrupt service routine (ISR)

D. To allocate memory dynamically in an ISR

Solution

1. Step 1: Understand semaphore role in FreeRTOS

   Semaphores are synchronization tools used to signal between tasks or between ISRs and tasks.

2. Step 2: Identify ISR-to-task signaling purpose

   In FreeRTOS, semaphores let an ISR notify a task safely without blocking or complex synchronization.

3. Final Answer:

   To safely notify a task from an interrupt service routine (ISR) -> Option C

4. Quick Check:

   Semaphore signals task from ISR = A [OK]

Hint: Semaphores signal tasks from ISRs safely [OK]

Common Mistakes:

• Thinking semaphores create tasks
• Confusing semaphores with disabling interrupts
• Using semaphores for memory allocation

2. Which FreeRTOS function is used inside an ISR to give a semaphore to a task?

easy

A. xSemaphoreGiveFromISR()

B. xSemaphoreTakeFromISR()

C. xSemaphoreCreateFromISR()

D. xSemaphoreGive()

Solution

1. Step 1: Recall ISR-safe semaphore functions

   FreeRTOS provides special functions for ISR context to avoid blocking and ensure safety.

2. Step 2: Identify the correct ISR semaphore give function

   The function to give a semaphore from an ISR is xSemaphoreGiveFromISR().

3. Final Answer:

   xSemaphoreGiveFromISR() -> Option A

4. Quick Check:

   Give semaphore from ISR = xSemaphoreGiveFromISR() [OK]

Hint: ISR uses xSemaphoreGiveFromISR() to signal tasks [OK]

Common Mistakes:

• Using xSemaphoreGive() inside ISR (not safe)
• Confusing take and give functions
• Assuming xSemaphoreCreateFromISR() exists

3. Given the code snippet below, what will be the output printed by the task?

SemaphoreHandle_t xSemaphore;

void ISR_Handler() {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
}

void TaskFunction(void *pvParameters) {
    if (xSemaphoreTake(xSemaphore, portMAX_DELAY) == pdTRUE) {
        printf("Semaphore received\n");
    }
}

medium

A. Semaphore received twice

B. No output

C. Runtime error

D. Semaphore received

Solution

1. Step 1: Analyze ISR giving semaphore

   The ISR calls xSemaphoreGiveFromISR() which releases the semaphore.

2. Step 2: Analyze task waiting on semaphore

   The task waits indefinitely with xSemaphoreTake() and prints "Semaphore received" when it gets the semaphore.

3. Final Answer:

   Semaphore received -> Option D

4. Quick Check:

   Task prints after semaphore take = C [OK]

Hint: Task prints after successful semaphore take [OK]

Common Mistakes:

• Thinking semaphore is given twice
• Assuming no output if ISR runs
• Expecting runtime error without initialization

4. Identify the error in the following ISR code snippet for giving a semaphore:

void ISR_Handler() {
    xSemaphoreGive(xSemaphore);
}

medium

A. xSemaphoreGive() cannot be called from an ISR

B. Semaphore must be taken before giving

C. ISR must not use semaphores

D. xSemaphoreGive() requires a pointer argument

Solution

1. Step 1: Check function suitability for ISR

   xSemaphoreGive() is not safe to call inside an ISR because it may block or cause race conditions.

2. Step 2: Identify correct ISR function

   Use xSemaphoreGiveFromISR() inside ISRs to safely give semaphores.

3. Final Answer:

   xSemaphoreGive() cannot be called from an ISR -> Option A

4. Quick Check:

   Use xSemaphoreGiveFromISR() in ISR, not xSemaphoreGive() [OK]

Hint: Use xSemaphoreGiveFromISR() inside ISR, not xSemaphoreGive() [OK]

Common Mistakes:

• Calling xSemaphoreGive() inside ISR
• Ignoring ISR context restrictions
• Assuming semaphore must be taken before giving

5. You want a task to respond immediately when a hardware interrupt occurs. Which sequence correctly uses a semaphore for ISR-to-task signaling in FreeRTOS?

hard

A. Create counting semaphore; task calls xSemaphoreGive(); ISR calls xSemaphoreTakeFromISR()

B. Create binary semaphore; ISR calls xSemaphoreGiveFromISR(); task calls xSemaphoreTake() with portMAX_DELAY

C. Create mutex; ISR calls xSemaphoreGive(); task calls xSemaphoreTake() with zero timeout

D. Create binary semaphore; task calls xSemaphoreGiveFromISR(); ISR calls xSemaphoreTake()

Solution

1. Step 1: Identify correct semaphore type and usage

   Binary semaphore is suitable for signaling events from ISR to task.

2. Step 2: Confirm correct function calls and order

   ISR must call xSemaphoreGiveFromISR() to signal; task waits with xSemaphoreTake() blocking indefinitely (portMAX_DELAY) until signaled.

3. Final Answer:

   Create binary semaphore; ISR calls xSemaphoreGiveFromISR(); task calls xSemaphoreTake() with portMAX_DELAY -> Option B

4. Quick Check:

   Binary semaphore + ISR give + task take = A [OK]

Hint: ISR gives semaphore, task takes with infinite wait [OK]

Common Mistakes:

• Using mutex instead of semaphore
• Calling give in task and take in ISR
• Using zero timeout causing missed signals