xTaskNotify() with value in FreeRTOS - Mini Project: Build & Apply

Using xTaskNotify() with Value in FreeRTOS

📖 Scenario: You are working on a FreeRTOS-based embedded system where two tasks need to communicate. One task will send a notification with a value to another task to trigger an action.

🎯 Goal: Build a simple FreeRTOS program where TaskSender sends a notification with a value to TaskReceiver using xTaskNotify(). The receiver task will print the received value.

📋 What You'll Learn

Create two tasks: TaskSender and TaskReceiver

Use xTaskNotify() to send a value from TaskSender to TaskReceiver

Use xTaskNotifyWait() in TaskReceiver to receive the notification and value

Print the received notification value in TaskReceiver

💡 Why This Matters

🌍 Real World

Task notifications with values are used in embedded systems to signal events or pass small data between tasks efficiently.

💼 Career

Understanding task notifications is essential for embedded software developers working with FreeRTOS to build responsive and efficient multitasking applications.

1

Create Task Handles

Declare two task handles called TaskSenderHandle and TaskReceiverHandle initialized to NULL.

FreeRTOS

// Declare task handles
// Your code here

Hint

Task handles are pointers used to identify tasks in FreeRTOS. Initialize them to NULL.

2

Create TaskReceiver Task Function

Write a task function called TaskReceiver that waits for a notification using xTaskNotifyWait() and stores the received value in a variable called receivedValue.

FreeRTOS

TaskHandle_t TaskSenderHandle = NULL;
TaskHandle_t TaskReceiverHandle = NULL;

// Define TaskReceiver function
// Your code here

Hint

Use xTaskNotifyWait() with clear bits 0x00, clear all bits on exit 0xffffffff, and wait forever portMAX_DELAY.

3

Create TaskSender Task Function and Send Notification

Write a task function called TaskSender that sends a notification with the value 123 to TaskReceiverHandle using xTaskNotify() with the action eSetValueWithOverwrite.

FreeRTOS

TaskHandle_t TaskSenderHandle = NULL;
TaskHandle_t TaskReceiverHandle = NULL;

void TaskReceiver(void *pvParameters) {
    uint32_t receivedValue = 0;
    for (;;) {
        xTaskNotifyWait(0x00, 0xffffffff, &receivedValue, portMAX_DELAY);
        // Process receivedValue here
    }
}

// Define TaskSender function
// Your code here

Hint

Use vTaskDelay() to wait 1000 milliseconds between notifications.

4

Print Received Value in TaskReceiver

Add a printf statement inside TaskReceiver after xTaskNotifyWait() to print the received value in the format: "Received notification value: %lu\n".

FreeRTOS

TaskHandle_t TaskSenderHandle = NULL;
TaskHandle_t TaskReceiverHandle = NULL;

void TaskReceiver(void *pvParameters) {
    uint32_t receivedValue = 0;
    for (;;) {
        xTaskNotifyWait(0x00, 0xffffffff, &receivedValue, portMAX_DELAY);
        // Print received value here
    }
}

void TaskSender(void *pvParameters) {
    for (;;) {
        xTaskNotify(TaskReceiverHandle, 123, eSetValueWithOverwrite);
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

Hint

Use printf to show the value received from the notification.

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