Setting and clearing event bits in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set an event bit in FreeRTOS.

FreeRTOS

xEventGroupSetBits([1], BIT_0);

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

AeventGroup

BEventGroupHandle

CxEventGroup

DxHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using the event bit mask instead of the event group handle.

Using an undefined variable as the event group handle.

The function xEventGroupSetBits requires the event group handle as the first argument. Here, xEventGroup is the correct handle variable.

2fill in blank

medium

Complete the code to clear an event bit in FreeRTOS.

FreeRTOS

xEventGroupClearBits([1], BIT_1);

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

AxEventGroup

BeventGroup

CEventGroupHandle

DxHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the bit mask instead of the event group handle.

Using a variable that is not initialized as the event group handle.

The function xEventGroupClearBits requires the event group handle as the first argument. xEventGroup is the correct handle variable.

3fill in blank

hard

Fix the error in the code to set multiple event bits at once.

FreeRTOS

xEventGroupSetBits([1], BIT_0 | BIT_2);

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

AxEventGroup

BEventGroupHandle

CeventGroup

DxHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using the bit mask as the first argument.

Using an undefined or incorrect variable as the event group handle.

To set multiple bits, the event group handle must be passed as the first argument. xEventGroup is the correct handle variable.

4fill in blank

hard

Fill both blanks to create an event group and set BIT_3.

FreeRTOS

EventGroupHandle_t [1] = [2]();
xEventGroupSetBits([1], BIT_3);

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

AxEventGroupCreate

BxEventGroup

DEventGroupCreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names to create the event group.

Not matching the variable name in both places.

First, declare the event group handle variable xEventGroup. Then create the event group by calling xEventGroupCreate(). Finally, use the handle to set the bit.

5fill in blank

hard

Fill all three blanks to wait for BIT_0 and BIT_1 to be set, then clear them.

FreeRTOS

EventBits_t bits = xEventGroupWaitBits([1], [2], [3], pdTRUE, portMAX_DELAY);

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

AxEventGroup

BBIT_0 | BIT_1

CpdTRUE

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Passing incorrect bit masks or event group handles.

Using pdFALSE instead of pdTRUE when bits should be cleared.

Use the event group handle xEventGroup as the first argument. The bits to wait for are BIT_0 | BIT_1. The third argument pdTRUE clears the bits after they are received.

Practice

1. What is the primary purpose of xEventGroupSetBits() in FreeRTOS?

easy

A. To create a new event group

B. To clear (reset) specific bits in an event group

C. To set (signal) specific bits in an event group

D. To delete an existing event group

Solution

1. Step 1: Understand the function name

   xEventGroupSetBits() suggests setting bits, which means signaling events.

2. Step 2: Compare with other functions

   xEventGroupClearBits() is used to clear bits, so setting bits is done by xEventGroupSetBits().

3. Final Answer:

   To set (signal) specific bits in an event group -> Option C

4. Quick Check:

   Set bits = signal events [OK]

Hint: Set bits means signal events, clear bits means reset [OK]

Common Mistakes:

• Confusing setting bits with clearing bits
• Thinking it creates or deletes event groups
• Mixing up function names

2. Which of the following is the correct syntax to clear bits 0x03 in an event group named myEventGroup?

easy

A. xEventGroupClear(myEventGroup, 0x03);

B. xEventGroupSetBits(myEventGroup, 0x03);

C. xClearEventBits(myEventGroup, 0x03);

D. xEventGroupClearBits(myEventGroup, 0x03);

Solution

1. Step 1: Identify the correct function for clearing bits

   The function to clear bits is xEventGroupClearBits().

2. Step 2: Check the syntax

   The correct syntax is xEventGroupClearBits(eventGroupHandle, bitsToClear);. xEventGroupClearBits(myEventGroup, 0x03); matches this exactly.

3. Final Answer:

   xEventGroupClearBits(myEventGroup, 0x03); -> Option D

4. Quick Check:

   Clear bits syntax = xEventGroupClearBits(handle, bits) [OK]

Hint: Clear bits uses xEventGroupClearBits with group and bit mask [OK]

Common Mistakes:

• Using xEventGroupSetBits to clear bits
• Using incorrect function names
• Missing parameters or wrong order

3. Given the code snippet:

EventBits_t bits;
bits = xEventGroupSetBits(myEventGroup, 0x05);
bits = xEventGroupClearBits(myEventGroup, 0x01);

What is the value of bits after these calls if the initial bits were 0x00?

medium

A. 0x00

B. 0x05

C. 0x04

D. 0x01

Solution

1. Step 1: First call - xEventGroupSetBits(myEventGroup, 0x05)

   Initial event group: 0x00. Returns bits BEFORE setting: 0x00 (event group becomes 0x05). bits = 0x00.

2. Step 2: Second call - xEventGroupClearBits(myEventGroup, 0x01)

   Event group now: 0x05. Returns bits BEFORE clearing: 0x05 (event group becomes 0x04). bits = 0x05.

3. Final Answer:

   0x00 -> Option A

4. Quick Check:

   Return value = pre-operation bits [OK]

Hint: xEventGroupSetBits/ClearBits return bits BEFORE operation [OK]

Common Mistakes:

• Confusing return value (pre-op) with post-op event group state
• Miscalculating bitwise operations on event group
• Forgetting return value overwrites 'bits' each call

4. Identify the error in this code snippet:

EventGroupHandle_t myEventGroup;
myEventGroup = xEventGroupCreate();
xEventGroupClearBits(0x01, myEventGroup);

medium

A. Arguments to xEventGroupClearBits are reversed

B. xEventGroupCreate() returns void, not a handle

C. xEventGroupClearBits requires a pointer, not a value

D. Missing include for FreeRTOS headers

Solution

1. Step 1: Check function parameter order

   xEventGroupClearBits() expects (EventGroupHandle_t, EventBits_t). The code passes (0x01, myEventGroup), which is reversed.

2. Step 2: Confirm return type of xEventGroupCreate()

   xEventGroupCreate() returns an EventGroupHandle_t, so assignment is correct.

3. Final Answer:

   Arguments to xEventGroupClearBits are reversed -> Option A

4. Quick Check:

   Function parameters order matters [OK]

Hint: Check parameter order carefully for FreeRTOS API calls [OK]

Common Mistakes:

• Swapping parameters in API calls
• Assuming wrong return types
• Ignoring compiler warnings

5. You want to signal two events represented by bits 0x01 and 0x04, then immediately clear bit 0x01 without affecting others. Which sequence correctly achieves this?

hard

A. xEventGroupClearBits(myEventGroup, 0x05); xEventGroupSetBits(myEventGroup, 0x01);

B. xEventGroupSetBits(myEventGroup, 0x05); xEventGroupClearBits(myEventGroup, 0x01);

C. xEventGroupSetBits(myEventGroup, 0x01); xEventGroupClearBits(myEventGroup, 0x04);

D. xEventGroupClearBits(myEventGroup, 0x01); xEventGroupSetBits(myEventGroup, 0x05);

Solution

1. Step 1: Set bits 0x01 and 0x04 together

   Setting bits 0x05 (binary 0101) signals both events at once.

2. Step 2: Clear only bit 0x01

   Clearing bit 0x01 removes that event signal but leaves 0x04 set.

3. Final Answer:

   xEventGroupSetBits(myEventGroup, 0x05); xEventGroupClearBits(myEventGroup, 0x01); -> Option B

4. Quick Check:

   Set both bits then clear one bit [OK]

Hint: Set all bits first, then clear only the unwanted bit [OK]

Common Mistakes:

• Clearing bits before setting them
• Clearing wrong bits
• Not combining bits correctly