ISR-safe API functions (FromISR suffix) in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to send a character to a queue safely from an ISR.

FreeRTOS

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

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

Aitem

Bdata

Cvalue

Dcharacter

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable not declared or unrelated to the data to send.

Forgetting to pass the address of the data.

The variable data holds the character to send to the queue from the ISR.

2fill in blank

medium

Complete the code to yield from an ISR if a higher priority task was woken.

FreeRTOS

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

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

A<

B!=

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality operator which causes incorrect behavior.

Using comparison operators like < or > which are not appropriate here.

The condition checks if xHigherPriorityTaskWoken is equal to pdTRUE to decide if a context switch is needed.

3fill in blank

hard

Fix the error in the ISR-safe queue receive function call.

FreeRTOS

BaseType_t result = xQueueReceiveFromISR(queue, [1], NULL);

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

A&buffer

Bbuffer

C*buffer

Dbuffer[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the buffer variable directly instead of its address.

Dereferencing the buffer which causes errors.

The function expects a pointer to the buffer, so &buffer is correct.

4fill in blank

hard

Fill both blanks to correctly send data from an ISR and handle task wakeup.

FreeRTOS

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
if (xQueueSendFromISR(queue, &[1], &[2]) != pdPASS) {
    // Handle error
}

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

Adata

BxHigherPriorityTaskWoken

CxTaskWoken

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names for the task wakeup flag.

Not passing the address of the data variable.

data is the variable sent to the queue, and xHigherPriorityTaskWoken tracks if a higher priority task was woken.

5fill in blank

hard

Fill all three blanks to create an ISR-safe queue send and yield if needed.

FreeRTOS

BaseType_t [1] = pdFALSE;
if (xQueueSendFromISR(queue, &[2], &[3]) == pdPASS) {
    portYIELD_FROM_ISR([3]);
}

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

AxHigherPriorityTaskWoken

Bdata

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not passing the address of the data variable.

xHigherPriorityTaskWoken is declared and used to track if a higher priority task was woken. data is the variable sent to the queue.

Practice

1.

What is the main reason to use FreeRTOS API functions with the FromISR suffix inside an interrupt service routine (ISR)?

easy

A. They disable all interrupts during execution.

B. They are safe to call from ISRs and handle interrupt context correctly.

C. They run tasks faster than normal API functions.

D. They automatically create new tasks inside the ISR.

Solution

1. Step 1: Understand ISR context constraints

   Inside an ISR, normal FreeRTOS API functions are unsafe because they may block or cause race conditions.

2. Step 2: Role of FromISR functions

   Functions with the FromISR suffix are designed to be safe in interrupt context by avoiding blocking and managing critical sections properly.

3. Final Answer:

   They are safe to call from ISRs and handle interrupt context correctly. -> Option B

4. Quick Check:

   FromISR functions = ISR safe calls [OK]

Hint: Use FromISR functions only inside interrupts for safety [OK]

Common Mistakes:

• Calling normal API functions inside ISR causing crashes
• Assuming FromISR functions disable interrupts
• Thinking FromISR creates tasks automatically

2.

Which of the following is the correct way to declare a variable to track if a higher priority task was woken inside an ISR?

?

easy

A. int higherPriorityTaskWoken = 0;

B. bool higherPriorityTaskWoken = false;

C. BaseType_t higherPriorityTaskWoken = pdFALSE;

D. volatile int higherPriorityTaskWoken = 1;

Solution

1. Step 1: Identify FreeRTOS type for ISR task wake flag

   FreeRTOS uses BaseType_t for portability and defines pdFALSE and pdTRUE for boolean values.

2. Step 2: Correct initialization

   The variable must start as pdFALSE to indicate no task was woken yet.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; -> Option C

4. Quick Check:

   Use BaseType_t and pdFALSE for ISR flags [OK]

Hint: Use BaseType_t and pdFALSE to track task wake in ISR [OK]

Common Mistakes:

• Using plain int instead of BaseType_t
• Initializing flag to 1 instead of pdFALSE
• Using bool which may not be portable

3.

Consider this ISR code snippet:

BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) {
    portYIELD_FROM_ISR();
}

What happens if higherPriorityTaskWoken is set to pdTRUE after xQueueSendFromISR?

medium

A. The ISR requests a context switch to the higher priority task immediately after ISR.

B. The ISR disables all interrupts until the higher priority task runs.

C. The queue send operation fails silently.

D. The higher priority task is blocked until the ISR finishes.

Solution

1. Step 1: Understand higherPriorityTaskWoken flag

   This flag indicates if a task with higher priority than the current one was made ready by the ISR.

2. Step 2: Role of portYIELD_FROM_ISR()

   If the flag is true, calling portYIELD_FROM_ISR() requests a context switch to that higher priority task immediately after ISR exit.

3. Final Answer:

   The ISR requests a context switch to the higher priority task immediately after ISR. -> Option A

4. Quick Check:

   higherPriorityTaskWoken true triggers context switch [OK]

Hint: If higherPriorityTaskWoken true, call portYIELD_FROM_ISR() [OK]

Common Mistakes:

• Thinking interrupts are disabled by portYIELD_FROM_ISR()
• Assuming queue send fails if flag is true
• Believing higher priority task waits for ISR to finish

4.

Identify the error in this ISR code snippet:

void ISR_Handler(void) {
    BaseType_t higherPriorityTaskWoken = pdFALSE;
    xSemaphoreGive(semaphore);
    if (higherPriorityTaskWoken) {
        portYIELD_FROM_ISR();
    }
}

What is the problem here?

medium

A. The variable higherPriorityTaskWoken should be global.

B. The semaphore is given twice causing overflow.

C. The portYIELD_FROM_ISR() call is outside the ISR.

D. The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR.

Solution

1. Step 1: Check function used for semaphore give

   The code uses xSemaphoreGive, which is not safe inside ISR context.

2. Step 2: Correct ISR-safe function usage

   Inside ISR, xSemaphoreGiveFromISR must be used with higherPriorityTaskWoken passed by reference to track task wake.

3. Final Answer:

   The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR. -> Option D

4. Quick Check:

   Use FromISR functions with higherPriorityTaskWoken [OK]

Hint: Always pass higherPriorityTaskWoken to FromISR functions [OK]

Common Mistakes:

• Using normal API functions inside ISR
• Not passing higherPriorityTaskWoken pointer
• Declaring higherPriorityTaskWoken as global unnecessarily

5.

You want to send data from an ISR to a task using a queue. Which sequence correctly uses FreeRTOS ISR-safe API functions to do this and ensure the higher priority task runs immediately if woken?

?

hard

A. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

B. xQueueSend(queue, &data, 0);
BaseType_t higherPriorityTaskWoken = pdFALSE;
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

C. BaseType_t higherPriorityTaskWoken = pdTRUE;
xQueueSendFromISR(queue, &data, NULL);
portYIELD_FROM_ISR();

D. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSend(queue, &data, 0);
portYIELD_FROM_ISR();

Solution

1. Step 1: Use ISR-safe queue send function

   Inside ISR, xQueueSendFromISR must be used with a pointer to higherPriorityTaskWoken.

2. Step 2: Check higherPriorityTaskWoken and yield

   If higherPriorityTaskWoken is set to pdTRUE, call portYIELD_FROM_ISR() to request immediate context switch.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken); if (higherPriorityTaskWoken) portYIELD_FROM_ISR(); -> Option A

4. Quick Check:

   Use FromISR + check flag + yield [OK]

Hint: Use xQueueSendFromISR with flag and yield if true [OK]

Common Mistakes:

• Using normal xQueueSend inside ISR
• Not passing higherPriorityTaskWoken pointer
• Calling portYIELD_FROM_ISR unconditionally