Event-driven architecture in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an event group in FreeRTOS.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();

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

AeventGroup

BxQueue

CtaskHandle

DtimerID

Attempts:

3 left

💡 Hint

Common Mistakes

Using a queue or task handle variable name instead of an event group handle.

Not assigning the return value of xEventGroupCreate() to a variable.

The function xEventGroupCreate() returns a handle to an event group, which we store in eventGroup.

2fill in blank

medium

Complete the code to wait for an event bit to be set in the event group.

FreeRTOS

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

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

A0xFF

B0x00

C0x10

D0x01

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x00 which waits for no bits.

Using 0xFF which waits for all bits set.

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

3fill in blank

hard

Fix the error in the code to set an event bit in the event group.

FreeRTOS

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

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

ACreate

BClear

CSet

DWait

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ClearBits' which clears bits instead of setting.

Using 'WaitBits' which waits for bits instead of setting.

The function to set bits in an event group is xEventGroupSetBits().

4fill in blank

hard

Fill both blanks to create an event group and check if a specific bit is set.

FreeRTOS

EventGroupHandle_t [1] = xEventGroupCreate();
if (xEventGroupGetBits([2]) & 0x02) {
    // Event bit 2 is set
}

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

AmyEventGroup

BeventQueue

DtaskHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for creation and checking.

Using a queue or task handle instead of an event group handle.

We create the event group with myEventGroup and check its bits using the same handle.

5fill in blank

hard

Fill all three blanks to set multiple event bits and then clear one bit.

FreeRTOS

xEventGroupSetBits([1], 0x03);
xEventGroupClearBits([2], [3]);

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

AeventGroupHandle

C0x01

D0x02

Attempts:

3 left

💡 Hint

Common Mistakes

Using different handles for set and clear functions.

Clearing the wrong bit mask.

We set bits 0 and 1 with 0x03, then clear bit 0 with 0x01 on the same event group handle.

Practice

1. What is the main benefit of using event-driven architecture in FreeRTOS?

easy

A. It makes the program run all tasks continuously without waiting.

B. It requires more CPU power to check all events constantly.

C. The program reacts only when an event occurs, saving CPU time.

D. It disables multitasking to focus on one task at a time.

Solution

1. Step 1: Understand event-driven architecture basics

   Event-driven architecture means the program waits and reacts only when events happen, not running tasks all the time.

2. Step 2: Connect to FreeRTOS event groups

   FreeRTOS event groups let tasks wait efficiently for events, saving CPU time by not running unnecessarily.

3. Final Answer:

   The program reacts only when an event occurs, saving CPU time. -> Option C

4. Quick Check:

   Event-driven = React on event, save CPU [OK]

Hint: Event-driven means react only on events, not always running [OK]

Common Mistakes:

• Thinking event-driven runs all tasks continuously
• Believing it uses more CPU by checking constantly
• Confusing event-driven with disabling multitasking

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

easy

A. EventGroup xEventGroup = xCreateEventGroup();

B. EventGroupHandle_t xEventGroup = xEventGroupCreate();

C. xEventGroup = EventGroupCreate();

D. EventGroupHandle_t xEventGroup = createEventGroup();

Solution

1. Step 1: Recall FreeRTOS event group creation syntax

   The official FreeRTOS function to create an event group is xEventGroupCreate(), returning EventGroupHandle_t.

2. Step 2: Match the correct syntax

   EventGroupHandle_t xEventGroup = xEventGroupCreate(); uses the correct function name and type: EventGroupHandle_t xEventGroup = xEventGroupCreate();

3. Final Answer:

   EventGroupHandle_t xEventGroup = xEventGroupCreate(); -> Option B

4. Quick Check:

   Correct function and type = EventGroupHandle_t xEventGroup = xEventGroupCreate(); [OK]

Hint: FreeRTOS event groups use xEventGroupCreate() function [OK]

Common Mistakes:

• Using wrong function names like createEventGroup()
• Missing the correct handle type EventGroupHandle_t
• Incorrect capitalization or function syntax

3. Given the code snippet:
<pre>EventGroupHandle_t xEventGroup = xEventGroupCreate(); // Task A waits for bit 0 and bit 1 EventBits_t uxBits = xEventGroupWaitBits(xEventGroup, (1 << 0) | (1 << 1), pdTRUE, pdTRUE, portMAX_DELAY); // Task B sets bit 0 xEventGroupSetBits(xEventGroup, (1 << 0)); // Task C sets bit 1 xEventGroupSetBits(xEventGroup, (1 << 1)); // What is the value of uxBits after Task A unblocks?

medium

A. 0x03 (both bits 0 and 1 set)

B. 0x01 (only bit 0 set)

C. 0x02 (only bit 1 set)

D. 0x00 (no bits set)

Solution

1. Step 1: Understand xEventGroupWaitBits parameters

   Task A waits for bits 0 and 1 together (bitmask 0x03), clears bits on exit (pdTRUE), waits for all bits (pdTRUE), and waits indefinitely.

2. Step 2: Analyze bit setting by Task B and Task C

   Task B sets bit 0 (0x01), Task C sets bit 1 (0x02). When both bits are set, Task A unblocks and uxBits contains both bits set (0x03).

3. Final Answer:

   0x03 (both bits 0 and 1 set) -> Option A

4. Quick Check:

   Wait for bits 0 & 1 set = 0x03 [OK]

Hint: WaitBits returns all requested bits set when unblocked [OK]

Common Mistakes:

• Assuming only one bit is set when unblocked
• Ignoring that wait is for all bits (pdTRUE)
• Confusing bit masks with decimal values

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

EventGroupHandle_t xEventGroup;
xEventGroupSetBits(xEventGroup, (1 << 0));

medium

A. xEventGroupSetBits requires a pointer, not a handle.

B. Bit mask (1 << 0) is invalid.

C. Event groups cannot set bits directly.

D. xEventGroup is not created before use.

Solution

1. Step 1: Check event group initialization

   xEventGroup is declared but never created with xEventGroupCreate(), so it is uninitialized (likely NULL).

2. Step 2: Understand consequences of uninitialized handle

   Calling xEventGroupSetBits with an uninitialized handle causes undefined behavior or crash.

3. Final Answer:

   xEventGroup is not created before use. -> Option D

4. Quick Check:

   Create event group before use [OK]

Hint: Always create event group before setting bits [OK]

Common Mistakes:

• Forgetting to call xEventGroupCreate()
• Thinking bit mask is invalid
• Misunderstanding handle vs pointer usage

5. You want a FreeRTOS task to wait for either bit 2 or bit 3 to be set in an event group, but not both. Which call correctly implements this behavior?

hard

A. xEventGroupWaitBits(xEventGroup, (1 << 2) | (1 << 3), pdTRUE, pdFALSE, portMAX_DELAY);

B. xEventGroupWaitBits(xEventGroup, (1 << 2) | (1 << 3), pdTRUE, pdTRUE, portMAX_DELAY);

C. xEventGroupWaitBits(xEventGroup, (1 << 2) & (1 << 3), pdTRUE, pdFALSE, portMAX_DELAY);

D. xEventGroupWaitBits(xEventGroup, (1 << 2) | (1 << 3), pdFALSE, pdTRUE, portMAX_DELAY);

Solution

1. Step 1: Understand the parameters for xEventGroupWaitBits

   The third parameter clears bits on exit (pdTRUE), the fourth parameter controls if waiting for all bits (pdTRUE) or any bit (pdFALSE).

2. Step 2: Match requirement to wait for either bit 2 or bit 3, but not both

   Waiting for any bit means pdFALSE for the fourth parameter. Using bitmask (1 << 2) | (1 << 3) waits for either bit 2 or bit 3.

3. Final Answer:

   xEventGroupWaitBits(xEventGroup, (1 << 2) | (1 << 3), pdTRUE, pdFALSE, portMAX_DELAY); -> Option A

4. Quick Check:

   Wait for any bit = pdFALSE, bitmask ORed = xEventGroupWaitBits(xEventGroup, (1 << 2) | (1 << 3), pdTRUE, pdFALSE, portMAX_DELAY); [OK]

Hint: Use pdFALSE to wait for any bit, not all bits [OK]

Common Mistakes:

• Using pdTRUE to wait for all bits instead of any
• Using bitwise AND instead of OR for bitmask
• Not clearing bits on exit when needed