ISR-to-task notification pattern in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to notify a task from an ISR using FreeRTOS.

FreeRTOS

BaseType_t xHigherPriorityTaskWoken = pdFALSE;

/* Notify the task */
xTaskNotifyGiveFromISR(taskHandle, [1]);

portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

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ANULL

BpdFALSE

CpdTRUE

D&xHigherPriorityTaskWoken

Attempts:

3 left

💡 Hint

Common Mistakes

Passing NULL instead of the pointer to xHigherPriorityTaskWoken

Passing pdTRUE or pdFALSE directly instead of the pointer

Not passing any argument

The second argument to xTaskNotifyGiveFromISR must be a pointer to xHigherPriorityTaskWoken so the ISR can inform the scheduler if a context switch is needed.

2fill in blank

medium

Complete the code to wait for a notification in a task.

FreeRTOS

void vTaskFunction(void *pvParameters) {
    for(;;) {
        ulTaskNotifyTake(pdTRUE, [1]);
        /* Process notification */
    }
}

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AportMAX_DELAY

B0

CpdFALSE

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 which causes no wait

Using pdFALSE which is not a valid block time

Using a numeric value without understanding its meaning

Using portMAX_DELAY makes the task wait indefinitely for a notification.

3fill in blank

hard

Fix the error in the ISR notification code to correctly notify the task and request a context switch if needed.

FreeRTOS

void vISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;

    /* Notify the task */
    xTaskNotifyGiveFromISR(taskHandle, [1]);

    if(xHigherPriorityTaskWoken) {
        [2];
    }
}

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A&xHigherPriorityTaskWoken

BxHigherPriorityTaskWoken

CportYIELD_FROM_ISR(xHigherPriorityTaskWoken)

DportYIELD_FROM_ISR(0)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable instead of its address

Calling portYIELD_FROM_ISR with 0 or no argument

Not calling portYIELD_FROM_ISR at all

The notification function requires the address of xHigherPriorityTaskWoken. Then, if it is set, portYIELD_FROM_ISR() is called with that variable to request a context switch.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps task names to their notification counts only if the count is greater than zero.

FreeRTOS

notification_counts = {taskName: [1] for taskName, count in taskNotifications.items() if count [2] 0}

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Acount

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality instead of greater than

Using less than instead of greater than

Using taskName instead of count as value

The comprehension maps each taskName to its count only if the count is greater than zero.

5fill in blank

hard

Fill all three blanks to create a task notification handler that clears the notification, waits for a notification, and processes it if received.

FreeRTOS

void vTaskNotificationHandler(void *pvParameters) {
    for(;;) {
        ulTaskNotifyTake([1], [2]);
        if([3]) {
            /* Process notification */
        }
    }
}

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ApdTRUE

BportMAX_DELAY

CulTaskNotifyTake(pdTRUE, portMAX_DELAY)

DulTaskNotifyTake(pdFALSE, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Not clearing the notification on entry

Using zero block time causing no wait

Not checking the return value before processing

The function ulTaskNotifyTake is called with pdTRUE to clear the notification on entry and portMAX_DELAY to wait indefinitely. The returned value is checked to see if a notification was received.