Event-driven architecture in FreeRTOS

Introduction

Event-driven architecture helps your program react to things when they happen, like a button press or a sensor alert. It makes your program efficient and easy to manage.

When you want your program to respond immediately to user actions like button presses.

When sensors send data at unpredictable times and you need to handle it quickly.

When you want to avoid wasting CPU time by waiting and instead react only when needed.

When managing multiple tasks that depend on different events happening.

When building embedded systems that must be responsive and efficient.

Syntax

FreeRTOS

void vTaskFunction(void *pvParameters) {
    for(;;) {
        if(xEventGroupWaitBits(xEventGroup, EVENT_BIT, pdTRUE, pdFALSE, portMAX_DELAY) != 0) {
            // Handle event here
        }
    }
}

xEventGroupWaitBits waits for specific event bits to be set.

Use pdTRUE to clear bits after handling the event.

Examples

This waits forever until BIT_0 event happens, then clears it.

FreeRTOS

EventGroupHandle_t xEventGroup = xEventGroupCreate();

// In task:
if((xEventGroupWaitBits(xEventGroup, BIT_0, pdTRUE, pdFALSE, portMAX_DELAY) & BIT_0) != 0) {
    // Event BIT_0 occurred
}

This sets BIT_0 event from an interrupt safely.

FreeRTOS

// Setting an event bit from an ISR
BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xEventGroupSetBitsFromISR(xEventGroup, BIT_0, &xHigherPriorityTaskWoken);

Sample Program

This program creates two tasks. One simulates a button press by setting an event bit after 1 second. The other waits for that event and then prints a message when it happens.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "event_groups.h"
#include <stdio.h>

#define BUTTON_PRESS_BIT (1 << 0)

EventGroupHandle_t xEventGroup;

void ButtonTask(void *pvParameters) {
    // Simulate button press after delay
    vTaskDelay(pdMS_TO_TICKS(1000));
    printf("Button pressed! Setting event bit.\n");
    xEventGroupSetBits(xEventGroup, BUTTON_PRESS_BIT);
    vTaskDelete(NULL);
}

void EventHandlerTask(void *pvParameters) {
    printf("Waiting for button press event...\n");
    xEventGroupWaitBits(xEventGroup, BUTTON_PRESS_BIT, pdTRUE, pdFALSE, portMAX_DELAY);
    printf("Button event received! Handling event.\n");
    vTaskDelete(NULL);
}

int main(void) {
    xEventGroup = xEventGroupCreate();
    xTaskCreate(ButtonTask, "ButtonTask", 1000, NULL, 1, NULL);
    xTaskCreate(EventHandlerTask, "EventHandlerTask", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    for(;;);
    return 0;
}

Important Notes

Event bits let tasks communicate without busy waiting.

Always clear event bits after handling to avoid repeated triggers.

Use event groups carefully to avoid race conditions.

Summary

Event-driven architecture lets your program react only when something happens.

FreeRTOS event groups help tasks wait for and signal events efficiently.

This approach saves CPU time and makes your program responsive.

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