Setting and clearing event bits in FreeRTOS - Mini Project: Build & Apply

Setting and Clearing Event Bits in FreeRTOS

📖 Scenario: You are working on a FreeRTOS-based embedded system where tasks communicate using event groups. You want to control task synchronization by setting and clearing event bits.

🎯 Goal: Build a simple FreeRTOS program that creates an event group, sets specific event bits, clears some bits, and then prints the final event bits value.

📋 What You'll Learn

Create an event group variable called eventGroup

Create a configuration variable BIT_0 with value (1 << 0)

Set the event bit BIT_0 in eventGroup

Clear the event bit BIT_0 in eventGroup

Print the final value of the event bits

💡 Why This Matters

🌍 Real World

Event groups are used in embedded systems to signal and synchronize tasks efficiently without busy waiting.

💼 Career

Understanding event bits is important for embedded software engineers working with real-time operating systems like FreeRTOS.

1

Create an event group

Create an event group variable called eventGroup using xEventGroupCreate().

FreeRTOS

# Create the event group
# Your code here

Hint

Use xEventGroupCreate() to create the event group and assign it to eventGroup.

2

Define the event bit mask

Create a macro called BIT_0 with the value (1 << 0) to represent the first event bit.

FreeRTOS

#include "FreeRTOS.h"
#include "event_groups.h"

EventGroupHandle_t eventGroup = NULL;

#define BIT_0 (1 << 0)  // Define event bit mask

int main(void) {
    eventGroup = xEventGroupCreate();
    return 0;
}

// Your code here

Hint

Use #define BIT_0 (1 << 0) to create the bit mask.

3

Set and clear the event bit

Use xEventGroupSetBits(eventGroup, BIT_0) to set the event bit BIT_0. Then use xEventGroupClearBits(eventGroup, BIT_0) to clear the same bit.

FreeRTOS

#include "FreeRTOS.h"
#include "event_groups.h"

EventGroupHandle_t eventGroup = NULL;

#define BIT_0 (1 << 0)

int main(void) {
    eventGroup = xEventGroupCreate();
    // Set the event bit BIT_0
    // Your code here
    // Clear the event bit BIT_0
    // Your code here
    return 0;
}

Hint

Call xEventGroupSetBits() and xEventGroupClearBits() with eventGroup and BIT_0.

4

Print the final event bits value

Use uxEventGroupGetBits(eventGroup) to get the current event bits and print the value using printf.

FreeRTOS

#include "FreeRTOS.h"
#include "event_groups.h"
#include 

EventGroupHandle_t eventGroup = NULL;

#define BIT_0 (1 << 0)

int main(void) {
    eventGroup = xEventGroupCreate();
    xEventGroupSetBits(eventGroup, BIT_0);
    xEventGroupClearBits(eventGroup, BIT_0);
    // Get the current event bits
    EventBits_t bits = uxEventGroupGetBits(eventGroup);
    // Print the bits value
    // Your code here
    return 0;
}

Hint

Use printf("Final event bits value: %u\n", (unsigned int)bits); to print the bits.

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