Why event groups coordinate tasks in FreeRTOS - Test Your Understanding

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'

AtaskHandle

BxSemaphore

CqueueHandle

DeventGroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that suggests a semaphore or queue instead of an event group.

The function xEventGroupCreate() creates an event group and returns its handle. Naming it eventGroup is clear and appropriate.

2fill in blank

medium

Complete the code to wait for event bits to be set.

FreeRTOS

xEventGroupWaitBits(eventGroup, [1], pdTRUE, pdFALSE, portMAX_DELAY);

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

A0x01

B0x10

C0xFF

D0x00

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x00 which waits for no bits.

Using 0xFF which waits for all bits.

The bit mask 0x01 waits for the first event bit to be set.

3fill in blank

hard

Fix the error in setting event bits.

FreeRTOS

xEventGroup[1]Bits(eventGroup, 0x01);

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AClear

BWait

CSet

DCreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Clear' instead of 'Set' causes the bits to be cleared, not set.

The function xEventGroupSetBits() sets the specified bits in the event group.

4fill in blank

hard

Fill both blanks to create and wait for event bits correctly.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();
uint32_t bits = xEventGroupWaitBits([2], 0x03, pdTRUE, pdFALSE, portMAX_DELAY);

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

AeventGroup

CxSemaphore

DtaskHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for creation and waiting causes errors.

We create an event group named eventGroup and then wait for bits on the same event group handle.

5fill in blank

hard

Fill all three blanks to set bits, wait for them, and clear bits properly.

FreeRTOS

xEventGroup[1]Bits(eventGroup, 0x01);
uint32_t bits = xEventGroupWaitBits(eventGroup, 0x01, [2], pdFALSE, portMAX_DELAY);
xEventGroup[3]Bits(eventGroup, 0x01);

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

ASet

BpdTRUE

CClear

DWait

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Wait' instead of 'Set' or 'Clear' in the function names.

Not clearing bits properly after waiting.

First, we set the bit with xEventGroupSetBits(). Then we wait for the bit with pdTRUE to clear it automatically after waiting. Finally, we can clear bits explicitly with xEventGroupClearBits().

Practice

1. What is the main purpose of event groups in FreeRTOS?

easy

A. To handle hardware interrupts directly

B. To create new tasks dynamically

C. To let tasks wait for or signal events using bits

D. To manage memory allocation

Solution

1. Step 1: Understand event groups role

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

2. Step 2: Compare with other options

   Creating tasks, managing memory, and handling interrupts are different FreeRTOS features.

3. Final Answer:

   To let tasks wait for or signal events using bits -> Option C

4. Quick Check:

   Event groups coordinate tasks by signaling bits [OK]

Hint: Event groups use bits to sync tasks quickly [OK]

Common Mistakes:

• Confusing event groups with task creation
• Thinking event groups manage memory
• Assuming event groups handle interrupts

2. Which of the following is the correct way to create an event group in FreeRTOS?

easy

A. vEventGroupCreate()

B. xEventGroupCreate()

C. xCreateEventGroup()

D. vCreateEventGroup()

Solution

1. Step 1: Recall FreeRTOS naming conventions

   Functions returning handles usually start with 'x' and create functions end with 'Create'.

2. Step 2: Identify correct function

   The correct function to create an event group is xEventGroupCreate().

3. Final Answer:

   xEventGroupCreate() -> Option B

4. Quick Check:

   Event group creation uses xEventGroupCreate() [OK]

Hint: Creation functions start with 'x' and end with 'Create' [OK]

Common Mistakes:

• Using 'v' prefix which is for void functions
• Swapping function name order
• Assuming event groups are created with task functions

3. Given the code snippet:

EventGroupHandle_t xEventGroup = xEventGroupCreate();
const EventBits_t BIT_0 = (1 << 0);
xEventGroupSetBits(xEventGroup, BIT_0);
EventBits_t uxBits = xEventGroupWaitBits(xEventGroup, BIT_0, pdTRUE, pdFALSE, portMAX_DELAY);

What will be the value of uxBits after xEventGroupWaitBits returns?

medium

A. BIT_0

B. portMAX_DELAY

C. 0

D. pdTRUE

Solution

1. Step 1: Understand bit setting and waiting

   BIT_0 is set before waiting, so the wait function will return with BIT_0 set.

2. Step 2: Analyze parameters

   pdTRUE clears the bits after return, but uxBits holds the bits that were set when the function returned.

3. Final Answer:

   BIT_0 -> Option A

4. Quick Check:

   Wait returns bits that were set, here BIT_0 [OK]

Hint: Wait returns bits set before clearing if pdTRUE used [OK]

Common Mistakes:

• Assuming bits clear before return
• Confusing return value with delay constant
• Ignoring bit mask value

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

EventGroupHandle_t xEventGroup;
xEventGroupSetBits(xEventGroup, (1 << 1));

medium

A. Event groups cannot set bits directly

B. Bit mask (1 << 1) is invalid

C. xEventGroupSetBits requires a pointer to the event group

D. xEventGroup is not initialized before use

Solution

1. Step 1: Check event group initialization

   xEventGroup is declared but never created or assigned a valid handle.

2. Step 2: Understand function requirements

   xEventGroupSetBits requires a valid event group handle, which is missing here.

3. Final Answer:

   xEventGroup is not initialized before use -> Option D

4. Quick Check:

   Event group must be created before setting bits [OK]

Hint: Always create event group before using it [OK]

Common Mistakes:

• Using uninitialized event group handle
• Misunderstanding bit mask validity
• Passing pointer instead of handle

5. You want two tasks to wait until both have signaled readiness using event groups. Which approach correctly coordinates this?

hard

A. Each task sets a unique bit; both wait for both bits set before proceeding

B. Both tasks set the same bit and wait for that bit to be set

C. Tasks use separate event groups and wait independently

D. Tasks use a global variable instead of event groups

Solution

1. Step 1: Understand event group bits for coordination

   Each task sets a unique bit representing its readiness.

2. Step 2: Wait for both bits set

   Tasks wait for the combination of both bits to be set before continuing.

3. Final Answer:

   Each task sets a unique bit; both wait for both bits set before proceeding -> Option A

4. Quick Check:

   Unique bits per task coordinate readiness [OK]

Hint: Use unique bits per task and wait for all bits [OK]

Common Mistakes:

• Using same bit for multiple tasks
• Using separate event groups losing coordination
• Ignoring event groups and using variables