xTaskNotify() with value in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of xTaskNotify() in FreeRTOS?

xTaskNotify() is used to send a notification to a task, optionally with a 32-bit value, to signal events or pass data between tasks.

beginner

What does the ulValue parameter represent in xTaskNotify()?

ulValue is a 32-bit unsigned integer value sent with the notification. It can carry data or flags to the notified task.

intermediate

Explain the role of eAction parameter in xTaskNotify().

eAction defines how the notification value is updated in the receiving task. Actions include setting, incrementing, or bitwise operations on the task's notification value.

intermediate

What happens if xTaskNotify() is called with eSetValueWithOverwrite as eAction?

The notification value in the receiving task is overwritten with ulValue, even if the previous notification was not yet processed.

intermediate

How does xTaskNotify() differ from xTaskNotifyGive()?

xTaskNotify() can send a value with the notification and specify how to update it, while xTaskNotifyGive() simply increments the notification count without a value.

What type of value does xTaskNotify() send to a task?

A A string message

B A floating-point number

C A 32-bit unsigned integer value

D A pointer to a function

xTaskNotify() sends a 32-bit unsigned integer value as notification data.

Which eAction option overwrites the notification value regardless of previous notifications?

A eSetValueWithOverwrite

B eIncrement

C eSetBits

D eNoAction

eSetValueWithOverwrite overwrites the notification value even if it was not processed yet.

If you want to add bits to the current notification value, which eAction should you use?

A eSetBits

B eIncrement

C eSetValueWithOverwrite

D eNoAction

eSetBits performs a bitwise OR to add bits to the notification value.

What does xTaskNotify() return if the notification was successfully sent?

A pdFAIL

B pdPASS

C NULL

D 0

xTaskNotify() returns pdPASS on success.

Which function is simpler and only increments a task's notification count without sending a value?

A xTaskNotify()

B vTaskDelay()

C xTaskNotifyWait()

D xTaskNotifyGive()

xTaskNotifyGive() increments the notification count without a value.

Describe how xTaskNotify() can be used to send data to another task and how the eAction parameter affects the notification value.

Think about how you can pass numbers or flags to a friend and how you might update that information.

Explain the difference between xTaskNotify() and xTaskNotifyGive() in FreeRTOS task communication.

Consider when you want to send data versus just signal an event.

Practice

1.

What does the xTaskNotify() function primarily do in FreeRTOS?

easy

A. Creates a new task in the system

B. Deletes a task from the scheduler

C. Sends a notification value from one task to another

D. Suspends the current task indefinitely

Solution

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

   The function is designed to send a notification value or signal from one task to another in FreeRTOS.

2. Step 2: Compare with other task functions

   Creating, deleting, or suspending tasks are done by other FreeRTOS functions, not xTaskNotify().

3. Final Answer:

   Sends a notification value from one task to another -> Option C

4. Quick Check:

   xTaskNotify() sends notification value [OK]

Hint: Remember: xTaskNotify() sends signals between tasks [OK]

Common Mistakes:

• Confusing xTaskNotify() with task creation functions
• Thinking it suspends or deletes tasks
• Assuming it handles task priorities

2.

Which of the following is the correct syntax to send a notification with a value using xTaskNotify()?

BaseType_t xTaskNotify( TaskHandle_t xTaskToNotify, uint32_t ulValue, eNotifyAction eAction );

easy

A. xTaskNotify(taskHandle, 10, eSetBits);

B. xTaskNotify(10, taskHandle, eSetValueWithOverwrite);

C. xTaskNotify(taskHandle, eSetValueWithoutOverwrite, 10);

D. xTaskNotify(eSetValueWithOverwrite, taskHandle, 10);

Solution

1. Step 1: Identify parameter order in xTaskNotify()

   The correct order is: TaskHandle_t, uint32_t value, eNotifyAction.

2. Step 2: Match options to correct order

   xTaskNotify(taskHandle, 10, eSetBits); matches the correct order: taskHandle, 10, eSetBits. Others mix parameter order incorrectly.

3. Final Answer:

   xTaskNotify(taskHandle, 10, eSetBits); -> Option A

4. Quick Check:

   Correct parameter order: task, value, action [OK]

Hint: Remember parameter order: task, value, action [OK]

Common Mistakes:

• Swapping value and task handle parameters
• Using wrong enum for eNotifyAction
• Passing parameters in wrong order

3.

Consider the following code snippet:

uint32_t notifiedValue = 0;

// Task A sends notification
xTaskNotify(taskBHandle, 0x04, eSetBits);

// Task B waits for notification
xTaskNotifyWait(0x00, 0xFFFFFFFF, ¬ifiedValue, portMAX_DELAY);

// What is the value of notifiedValue after notification?

medium

A. 0x04

B. 0x00

C. 0xFFFFFFFF

D. Undefined

Solution

1. Step 1: Understand eSetBits action

   eSetBits sets bits in the notification value. Since initial is 0, setting bit 0x04 results in notifiedValue = 0x04.

2. Step 2: Check xTaskNotifyWait parameters

   It clears bits 0x00 (none) and waits indefinitely, storing the notified bits in notifiedValue.

3. Final Answer:

   0x04 -> Option A

4. Quick Check:

   eSetBits sets bits = 0x04 [OK]

Hint: eSetBits adds bits to notified value [OK]

Common Mistakes:

• Assuming notifiedValue remains zero
• Confusing eSetBits with eSetValueWithoutOverwrite
• Ignoring the effect of xTaskNotifyWait

4.

Identify the error in the following code snippet using xTaskNotify():

BaseType_t status;
status = xTaskNotify(NULL, 5, eSetValueWithOverwrite);
if(status == pdPASS) {
    // Notification sent
}

medium

A. Missing return statement in function

B. Using eSetValueWithOverwrite is invalid

C. Notification value must be zero

D. Passing NULL as task handle causes error

Solution

1. Step 1: Check task handle parameter

   xTaskNotify requires a valid task handle; NULL is invalid and causes failure.

2. Step 2: Validate other parameters

   eSetValueWithOverwrite and value 5 are valid; no return statement needed here.

3. Final Answer:

   Passing NULL as task handle causes error -> Option D

4. Quick Check:

   Task handle NULL invalid [OK]

Hint: Task handle must never be NULL [OK]

Common Mistakes:

• Passing NULL as task handle
• Thinking notification value must be zero
• Confusing eNotifyAction enums

5.

You want to send a notification value from Task A to Task B, but only if Task B's current notification value is zero. Which eNotifyAction should you use with xTaskNotify() to achieve this?

hard

A. eSetValueWithOverwrite

B. eSetValueWithoutOverwrite

C. eIncrement

D. eNoAction

Solution

1. Step 1: Understand eNotifyAction options

   eSetValueWithoutOverwrite sets the notification value only if it is currently zero.

2. Step 2: Compare with other actions

   eSetValueWithOverwrite always overwrites; eIncrement adds 1; eNoAction sends no value.

3. Final Answer:

   eSetValueWithoutOverwrite -> Option B

4. Quick Check:

   Set only if zero = eSetValueWithoutOverwrite [OK]

Hint: Use eSetValueWithoutOverwrite to avoid overwriting [OK]

Common Mistakes:

• Using eSetValueWithOverwrite and overwriting value
• Choosing eIncrement which changes value incorrectly
• Selecting eNoAction which sends no value