Waiting for event bits (AND/OR) in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to wait for event bits using FreeRTOS API.

FreeRTOS

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

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

A0x01

B0x00

C0x03

D0xFF

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x00 means no bits are waited on.

Using 0xFF waits for all bits, which may be unnecessary.

The mask 0x03 waits for bits 0 and 1 to be set.

2fill in blank

medium

Complete the code to clear bits automatically after waiting.

FreeRTOS

EventBits_t bits = xEventGroupWaitBits(eventGroup, 0x0F, [1], pdFALSE, 1000);

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

ApdFALSE

BpdPASS

CportMAX_DELAY

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE means bits remain set after waiting.

Using portMAX_DELAY here is incorrect as it is a timeout value.

Setting clearOnExit to pdTRUE clears the bits after they are received.

3fill in blank

hard

Fix the error in the code to wait for any bit in the mask.

FreeRTOS

EventBits_t bits = xEventGroupWaitBits(eventGroup, 0x0F, pdTRUE, [1], portMAX_DELAY);

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

ApdFAIL

BpdFALSE

CpdPASS

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdTRUE waits for all bits, not any bit.

Using pdPASS or pdFAIL are invalid here.

Setting waitForAllBits to pdFALSE waits for any bit in the mask.

4fill in blank

hard

Fill both blanks to wait for bits 0x05 and clear them after waiting.

FreeRTOS

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

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

A0x05

BpdTRUE

CpdFALSE

D0x0A

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x0A waits for wrong bits.

Setting clearOnExit to pdFALSE does not clear bits.

Mask 0x05 waits for bits 0 and 2. Setting clearOnExit to pdTRUE clears them after waiting.

5fill in blank

hard

Fill all three blanks to wait for bits 0x03, do not clear bits, and wait for all bits.

FreeRTOS

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

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

ApdTRUE

BpdFALSE

D0x03

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping clearOnExit and waitForAllBits values.

Using wrong mask value.

Mask 0x03 waits for bits 0 and 1. clearOnExit pdFALSE means bits are not cleared. waitForAllBits pdTRUE waits for all bits.

Practice

1. What does waiting for event bits in FreeRTOS allow a task to do?

easy

A. Pause execution until specific event bits are set

B. Run continuously without interruption

C. Automatically clear all event bits on start

D. Create new event bits dynamically

Solution

1. Step 1: Understand event bits purpose

   Event bits are signals that tasks can wait for to synchronize actions.

2. Step 2: Identify what waiting means

   Waiting means the task pauses until the required bits are set.

3. Final Answer:

   Pause execution until specific event bits are set -> Option A

4. Quick Check:

   Waiting for event bits = pause task until bits set [OK]

Hint: Waiting means pausing until bits signal event [OK]

Common Mistakes:

• Thinking waiting means running continuously
• Confusing event bits with task creation
• Assuming bits clear automatically on start

2. Which of the following is the correct syntax to wait for event bits using AND mode in FreeRTOS?

easy

A. xEventGroupClearBits(eventGroup, bitsToWaitFor);

B. xEventGroupWaitBits(eventGroup, bitsToWaitFor, clearOnExit, pdFALSE, timeout);

C. xEventGroupWaitBits(eventGroup, bitsToWaitFor, clearOnExit, pdTRUE, timeout);

D. xEventGroupSetBits(eventGroup, bitsToWaitFor);

Solution

1. Step 1: Identify the AND mode parameter

   The fourth parameter is pdTRUE to wait for all bits (AND mode).

2. Step 2: Confirm function usage

   xEventGroupWaitBits waits for bits with parameters: event group, bits, clear flag, wait mode, timeout.

3. Final Answer:

   xEventGroupWaitBits(eventGroup, bitsToWaitFor, clearOnExit, pdTRUE, timeout); -> Option C

4. Quick Check:

   pdTRUE means AND mode in wait bits [OK]

Hint: pdTRUE means wait for all bits (AND) [OK]

Common Mistakes:

• Using pdFALSE for AND mode
• Confusing SetBits with WaitBits
• Mixing up clearOnExit parameter

3. Given the code snippet:

EventBits_t bits = xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdFALSE, 1000);

What does bits contain if only BIT_1 is set in the event group before the call?

medium

A. Only BIT_1 set in bits

B. Both BIT_1 and BIT_2 set in bits

C. Zero (no bits set)

D. Function blocks indefinitely

Solution

1. Step 1: Understand wait mode pdFALSE

   pdFALSE means OR mode: wait for any bit set.

2. Step 2: Check which bits are set before call

   Only BIT_1 is set, but the third parameter is pdTRUE (clearOnExit), and the fourth parameter is pdFALSE (waitForAll = false), so the function waits for any bit set.

3. Step 3: Analyze the code

   However, the third parameter is pdTRUE (clearOnExit), so bits are cleared on exit. But since waitForAll is pdFALSE, the function returns when any bit is set.

4. Step 4: But the question is what bits contains if only BIT_1 is set before call

   Since waitForAll is false, the function returns immediately with bits containing BIT_1 set.

5. Final Answer:

   Only BIT_1 set in bits -> Option A

6. Quick Check:

   OR mode returns bits set among requested [OK]

Hint: pdFALSE means OR mode, returns any set bits [OK]

Common Mistakes:

• Assuming both bits must be set for return
• Thinking function blocks if not all bits set
• Confusing pdTRUE and pdFALSE meanings

4. Identify the error in this code snippet:

EventBits_t bits = xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdTRUE, 500);

What is wrong?

medium

A. pdTRUE for waitForAll means wait for all bits, but clearOnExit should be false to avoid clearing

B. pdTRUE for clearOnExit clears bits on exit, which is valid; no error

C. Using pdTRUE for both clearOnExit and waitForAll bits is correct

D. pdTRUE for waitForAll is invalid; must be pdFALSE

Solution

1. Step 1: Understand clearOnExit parameter

   pdTRUE means bits are cleared when function returns.

2. Step 2: Understand waitForAll parameter

   pdTRUE means wait for all bits (AND mode), which is valid.

3. Final Answer:

   pdTRUE for clearOnExit clears bits on exit, which is valid; no error -> Option B

4. Quick Check:

   clearOnExit pdTRUE is allowed and common [OK]

Hint: clearOnExit pdTRUE clears bits on return, allowed [OK]

Common Mistakes:

• Thinking clearOnExit pdTRUE is always wrong
• Confusing waitForAll with clearOnExit
• Assuming pdTRUE for waitForAll is invalid

5. You want a task to wait until both BIT_1 and BIT_2 are set, then clear those bits automatically on exit. Which call achieves this correctly?

hard

A. xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdFALSE, pdTRUE, portMAX_DELAY);

B. xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdFALSE, pdFALSE, portMAX_DELAY);

C. xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdFALSE, portMAX_DELAY);

D. xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdTRUE, portMAX_DELAY);

Solution

1. Step 1: Identify wait for all bits

   Fourth parameter pdTRUE means wait for all bits (AND mode).

2. Step 2: Identify clear bits on exit

   Third parameter pdTRUE means clear bits on exit.

3. Step 3: Match parameters to options

   xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdTRUE, portMAX_DELAY); uses clearOnExit = pdTRUE and waitForAll = pdTRUE, correct for requirement.

4. Final Answer:

   xEventGroupWaitBits(eventGroup, BIT_1 | BIT_2, pdTRUE, pdTRUE, portMAX_DELAY); -> Option D

5. Quick Check:

   Clear bits and wait all bits = clearOnExit pdTRUE, waitForAll pdTRUE [OK]

Hint: clearOnExit and waitForAll both pdTRUE for AND + clear [OK]

Common Mistakes:

• Mixing up clearOnExit and waitForAll parameters
• Using pdFALSE for waitForAll when AND needed
• Not setting timeout to wait indefinitely