ulTaskNotifyTake() for binary/counting notification in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to wait for a notification using ulTaskNotifyTake.

FreeRTOS

uint32_t ulNotificationValue = ulTaskNotifyTake(pdTRUE, [1]);

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

AportMAX_DELAY

B0

C1000

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as block time causes no waiting and immediate return.

Using pdFALSE instead of a block time is incorrect.

ulTaskNotifyTake waits indefinitely for a notification when the block time is set to portMAX_DELAY.

2fill in blank

medium

Complete the code to clear the notification value on exit.

FreeRTOS

uint32_t ulNotificationValue = ulTaskNotifyTake([1], portMAX_DELAY);

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

A0

BpdFALSE

C1

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE keeps the notification count unchanged.

Using 0 or 1 instead of pdTRUE or pdFALSE can cause confusion.

Setting ulClearCountOnExit to pdTRUE clears the notification value when the function returns.

3fill in blank

hard

Fix the error in the code to correctly receive a counting notification.

FreeRTOS

uint32_t ulNotificationValue = ulTaskNotifyTake(pdTRUE, [1]);

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

A0

B-1

CportMAX_DELAY

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes no wait and immediate return.

Using negative values is invalid.

The block time must be a valid tick count or portMAX_DELAY to wait properly. Using 0 causes immediate return.

4fill in blank

hard

Fill both blanks to create a counting notification wait that clears on exit and waits forever.

FreeRTOS

uint32_t ulNotificationValue = ulTaskNotifyTake([1], [2]);

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

ApdTRUE

BpdFALSE

CportMAX_DELAY

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping parameters causes wrong behavior.

Using 0 as block time causes no wait.

Use pdTRUE to clear the count on exit and portMAX_DELAY to wait indefinitely.

5fill in blank

hard

Fill all three blanks to check if a notification was received and print its value.

FreeRTOS

uint32_t ulNotificationValue = ulTaskNotifyTake([1], [2]);
if (ulNotificationValue [3] 0) {
    printf("Notification count: %u\n", ulNotificationValue);
}

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

ApdTRUE

BportMAX_DELAY

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == 0 checks for no notification, not received.

Not clearing count can cause repeated notifications.

Clear count on exit with pdTRUE, wait forever with portMAX_DELAY, and check if notification count is greater than 0.

Practice

1. What does the ulTaskNotifyTake() function do in FreeRTOS?

easy

A. It deletes a task from the scheduler.

B. It creates a new task in the system.

C. It sends a notification to another task.

D. It blocks a task until a notification is received.

Solution

1. Step 1: Understand the purpose of ulTaskNotifyTake()

   This function is designed to make a task wait (block) until it receives a notification from another task or interrupt.

2. Step 2: Differentiate from other task functions

   Creating, sending, or deleting tasks are done by other FreeRTOS functions, not ulTaskNotifyTake().

3. Final Answer:

   It blocks a task until a notification is received. -> Option D

4. Quick Check:

   ulTaskNotifyTake() waits for notification [OK]

Hint: ulTaskNotifyTake() makes task wait for notification [OK]

Common Mistakes:

• Confusing ulTaskNotifyTake() with task creation functions
• Thinking it sends notifications instead of receiving
• Assuming it deletes tasks

2. Which parameter of ulTaskNotifyTake() controls whether the notification count resets on exit?

easy

A. xClearCountOnExit

B. xTicksToWait

C. ulNotificationValue

D. xNotifyOption

Solution

1. Step 1: Identify parameters of ulTaskNotifyTake()

   ulTaskNotifyTake() takes two parameters: xClearCountOnExit and xTicksToWait.

2. Step 2: Understand xClearCountOnExit role

   This parameter decides if the notification count resets to zero when the function exits (pdTRUE) or not (pdFALSE).

3. Final Answer:

   xClearCountOnExit -> Option A

4. Quick Check:

   xClearCountOnExit resets count [OK]

Hint: xClearCountOnExit resets notification count [OK]

Common Mistakes:

• Mixing xClearCountOnExit with xTicksToWait
• Assuming ulNotificationValue is a parameter
• Confusing with notification sending options

3. Consider this code snippet:

uint32_t count = ulTaskNotifyTake(pdFALSE, portMAX_DELAY);
// Assume 3 notifications were sent before this call

What will be the value of count after this call?

medium

A. 1

B. 0

C. 3

D. portMAX_DELAY

Solution

1. Step 1: Analyze xClearCountOnExit parameter

   pdFALSE means the notification count is not cleared on exit, so it counts all notifications received.

2. Step 2: Understand the count returned

   Since 3 notifications were sent before, ulTaskNotifyTake() returns the total count, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   pdFALSE keeps count, so count = 3 [OK]

Hint: pdFALSE keeps count, so returns total notifications [OK]

Common Mistakes:

• Assuming count resets to 1 regardless
• Confusing portMAX_DELAY as return value
• Thinking count is zero if notifications sent before

4. What is wrong with this code snippet?

uint32_t count = ulTaskNotifyTake(pdTRUE, 100);
if(count == 0) {
    // No notification received
}
// Task expects to count multiple notifications

medium

A. Using pdTRUE clears count, so multiple notifications can't be counted.

B. Timeout value 100 is invalid and causes error.

C. ulTaskNotifyTake() cannot be used with if statements.

D. count variable should be int, not uint32_t.

Solution

1. Step 1: Check xClearCountOnExit usage

   pdTRUE clears the notification count on exit, so only a binary notification is counted, not multiple.

2. Step 2: Understand task expectation

   The task expects to count multiple notifications, but pdTRUE resets count, so it cannot accumulate multiple counts.

3. Final Answer:

   Using pdTRUE clears count, so multiple notifications can't be counted. -> Option A

4. Quick Check:

   pdTRUE clears count, no multiple counting [OK]

Hint: pdTRUE clears count; use pdFALSE for counting multiple [OK]

Common Mistakes:

• Thinking timeout 100 is invalid
• Believing ulTaskNotifyTake() can't be in if
• Confusing variable type for count

5. You want a task to sleep until it receives exactly 2 notifications, then proceed. Which ulTaskNotifyTake() usage is correct?

hard

A. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) twice and sum results.

B. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2.

C. Call ulTaskNotifyTake(pdTRUE, 0) in a loop until count is 2.

D. Call ulTaskNotifyTake(pdTRUE, portMAX_DELAY) once and check if return is 2.

Solution

1. Step 1: Understand counting notifications

   Using pdFALSE keeps the count of notifications, so the function returns the total number received.

2. Step 2: Use single call to wait indefinitely

   Calling ulTaskNotifyTake(pdFALSE, portMAX_DELAY) waits until at least one notification arrives and returns the count accumulated.

3. Step 3: Check if count equals 2

   After the call returns, check if the count is exactly 2 to proceed.

4. Final Answer:

   Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2. -> Option B

5. Quick Check:

   pdFALSE counts notifications; check return for 2 [OK]

Hint: Use pdFALSE and check return count for exact notifications [OK]

Common Mistakes:

• Calling ulTaskNotifyTake multiple times and summing counts
• Using pdTRUE which resets count
• Using zero timeout causing immediate return