Event group for task synchronization in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an event group.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();

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

AmyEventGroup

BeventGroup

CxEventGroup

Devent_handle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that conflicts with FreeRTOS internal names.

Using an invalid variable name.

The event group handle is usually declared with a meaningful name like myEventGroup.

2fill in blank

medium

Complete the code to wait for bits 0x01 and 0x02 to be set in the event group.

FreeRTOS

xEventGroupWaitBits(myEventGroup, [1], pdTRUE, pdTRUE, portMAX_DELAY);

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

A0x03

B0x01

C0x02

D0x04

Attempts:

3 left

💡 Hint

Common Mistakes

Waiting for only one bit instead of both.

Using incorrect bit values.

Bits 0x01 and 0x02 combined is 0x03 (bitwise OR).

3fill in blank

hard

Fix the error in the code to set bit 0x04 in the event group.

FreeRTOS

xEventGroupSetBits([1], 0x04);

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

AeventGroup

Bevent_handle

CmyEventGroup

DxEventGroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using an undefined or incorrect variable name.

Confusing event group handle with other variables.

The event group handle used must be the one created and named myEventGroup.

4fill in blank

hard

Fill both blanks to create an event group and wait for bit 0x08 without clearing it.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();
xEventGroupWaitBits([2], 0x08, pdFALSE, pdTRUE, portMAX_DELAY);

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

AsyncEventGroup

BmyEventGroup

CeventGroup

DxEventGroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for creation and waiting.

Forgetting to create the event group before waiting.

Use the same event group handle syncEventGroup for creation and waiting.

5fill in blank

hard

Fill all three blanks to set bits 0x01 and 0x02, then wait for bit 0x01 and clear it after waiting.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();
xEventGroupSetBits([2], 0x03);
xEventGroupWaitBits([3], 0x01, pdTRUE, pdFALSE, portMAX_DELAY);

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

AtaskEventGroup

BmyEventGroup

CeventGroup

DxEventGroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the same event group.

Not clearing the bit after waiting when required.

Use the same event group handle myEventGroup for creating the event group, setting bits, and waiting.

Practice

1. What is the primary purpose of an event group in FreeRTOS for task synchronization?

easy

A. To allocate memory dynamically for tasks

B. To allow tasks to wait for multiple events using bits

C. To create queues for message passing

D. To manage task priorities automatically

Solution

1. Step 1: Understand event groups in FreeRTOS

   Event groups use bits to represent events that tasks can wait for or signal.

2. Step 2: Identify the main use of event groups

   They help tasks synchronize by waiting on or setting bits representing events.

3. Final Answer:

   To allow tasks to wait for multiple events using bits -> Option B

4. Quick Check:

   Event groups = wait for multiple events [OK]

Hint: Event groups use bits to sync tasks, not memory or priorities [OK]

Common Mistakes:

• Confusing event groups with queues
• Thinking event groups manage memory
• Assuming event groups change task priorities

2. Which of the following is the correct function to wait for bits in an event group in FreeRTOS?

easy

A. xEventGroupClearBits()

B. xEventGroupSetBits()

C. xEventGroupWaitBits()

D. xEventGroupCreate()

Solution

1. Step 1: Recall FreeRTOS event group functions

   xEventGroupWaitBits() is used to wait for specific bits to be set.

2. Step 2: Differentiate from other functions

   xEventGroupSetBits() sets bits, ClearBits clears bits, Create creates the group.

3. Final Answer:

   xEventGroupWaitBits() -> Option C

4. Quick Check:

   Wait for bits = xEventGroupWaitBits() [OK]

Hint: Wait uses xEventGroupWaitBits(), set uses xEventGroupSetBits() [OK]

Common Mistakes:

• Using xEventGroupSetBits() to wait
• Confusing create with wait functions
• Mixing clear bits with wait bits

3. Given the following code snippet, what will be the value of bits after xEventGroupWaitBits() returns?

EventBits_t bits;
bits = xEventGroupWaitBits(eventGroup, BIT_0 | BIT_1, pdTRUE, pdFALSE, portMAX_DELAY);

Assume BIT_0 and BIT_1 are both set in eventGroup before the call.

medium

A. Only BIT_0 set

B. Only BIT_1 set

C. No bits set

D. Both BIT_0 and BIT_1 set

Solution

1. Step 1: Understand xEventGroupWaitBits parameters

   The function waits for BIT_0 or BIT_1 to be set, clears bits on exit (pdTRUE), and does not wait for all bits (pdFALSE).

2. Step 2: Analyze the bits set before call

   Both BIT_0 and BIT_1 are set, so the function returns with both bits set in the return value.

3. Final Answer:

   Both BIT_0 and BIT_1 set -> Option D

4. Quick Check:

   WaitBits returns bits set before clearing [OK]

Hint: WaitBits returns bits set before clearing them [OK]

Common Mistakes:

• Assuming only one bit is returned
• Ignoring bits cleared on exit
• Confusing wait for all bits with wait for any

4. Identify the error in the following FreeRTOS event group usage:

EventGroupHandle_t eventGroup = xEventGroupCreate();

void Task1(void *pvParameters) {
    xEventGroupWaitBits(eventGroup, BIT_0, pdFALSE, pdTRUE, portMAX_DELAY);
    // Task code
}

void Task2(void *pvParameters) {
    xEventGroupSetBits(eventGroup, BIT_0);
}

medium

A. Not checking if eventGroup creation succeeded

B. Using portMAX_DELAY as wait time

C. Using pdFALSE for clearOnExit in xEventGroupWaitBits

D. Setting bits before event group creation

Solution

1. Step 1: Check event group creation

   xEventGroupCreate() can return NULL if creation fails; code does not check this.

2. Step 2: Verify other parameters

   Using pdFALSE for clearOnExit is valid; portMAX_DELAY is valid; bits are set after creation.

3. Final Answer:

   Not checking if eventGroup creation succeeded -> Option A

4. Quick Check:

   Always check event group creation success [OK]

Hint: Always verify event group creation returned non-NULL [OK]

Common Mistakes:

• Ignoring NULL return from xEventGroupCreate
• Misusing clearOnExit parameter
• Setting bits before creating event group

5. You want two tasks to synchronize so that Task A waits until Task B signals both BIT_0 and BIT_1. Which xEventGroupWaitBits() call correctly waits for both bits before continuing?

hard

A. xEventGroupWaitBits(eventGroup, BIT_0 | BIT_1, pdTRUE, pdTRUE, portMAX_DELAY);

B. xEventGroupWaitBits(eventGroup, BIT_0 & BIT_1, pdTRUE, pdTRUE, portMAX_DELAY);

C. xEventGroupWaitBits(eventGroup, BIT_0 | BIT_1, pdTRUE, pdFALSE, portMAX_DELAY);

D. xEventGroupWaitBits(eventGroup, BIT_0 & BIT_1, pdFALSE, pdFALSE, portMAX_DELAY);

Solution

1. Step 1: Understand wait for all bits parameter

   The fourth parameter (xWaitForAllBits) must be pdTRUE to wait for both bits together.

2. Step 2: Check bit mask and parameters

   Use BIT_0 | BIT_1 to specify both bits; pdTRUE for clearOnExit clears bits after wait.

3. Final Answer:

   xEventGroupWaitBits(eventGroup, BIT_0 | BIT_1, pdTRUE, pdTRUE, portMAX_DELAY); -> Option A

4. Quick Check:

   Wait for all bits = xEventGroupWaitBits with xWaitForAllBits = pdTRUE [OK]

Hint: Set xWaitForAllBits to pdTRUE to wait for all bits [OK]

Common Mistakes:

• Using bitwise AND (&) instead of OR (|) for bit mask
• Setting xWaitForAllBits to pdFALSE when waiting for all bits
• Confusing clearOnExit with wait for all bits