FreeRTOS – Event Groups Explained

This article introduces <span>FreeRTOS</span> another synchronization mechanism for multitasking – Event Groups (<span>Event Group</span>). Unlike semaphores, event groups can achieve one-to-many and many-to-many synchronization.

API Introduction

Program Implementation

Objective: Create 3 tasks using an event group, with the relationship being: (Event 1 OR Event 2) AND Event 3. Once the relationship is established, log information will be printed.

• Creating tasks and event groups with STM32CubeMX

• Program Implementation

Create tasks and event groups, as shown in the automatically generated code by STM32CubeMx.

/* Definitions for defaultTask */
osThreadId_t defaultTaskHandle;
const osThreadAttr_t defaultTask_attributes = {
  .name = "defaultTask",
  .priority = (osPriority_t) osPriorityNormal,
  .stack_size = 128 * 4
};
/* Definitions for myTask01 */
osThreadId_t myTask01Handle;
const osThreadAttr_t myTask01_attributes = {
  .name = "myTask01",
  .priority = (osPriority_t) osPriorityLow,
  .stack_size = 128 * 4
};
/* Definitions for myTask02 */
osThreadId_t myTask02Handle;
const osThreadAttr_t myTask02_attributes = {
  .name = "myTask02",
  .priority = (osPriority_t) osPriorityLow,
  .stack_size = 128 * 4
};
/* Definitions for myTask03 */
osThreadId_t myTask03Handle;
const osThreadAttr_t myTask03_attributes = {
  .name = "myTask03",
  .priority = (osPriority_t) osPriorityLow,
  .stack_size = 128 * 4
};
/* Definitions for myEvent01 */
osEventFlagsId_t myEvent01Handle;
const osEventFlagsAttr_t myEvent01_attributes = {
  .name = "myEvent01"
};

Implement event group related operations.

/* USER CODE BEGIN Header_StartDefaultTask */
/**
* @brief Function implementing the defaultTask thread.
* @param argument: Not used
* @retval None
*/
/* USER CODE END Header_StartDefaultTask */
void StartDefaultTask(void *argument)
{
  /* USER CODE BEGIN defaultTask */
  uint32_t eventflags = EVENT_1_BIT | EVENT_2_BIT | EVENT_3_BIT;
  uint32_t eventreceivedflags;
  /* Infinite loop */
  for(;;)
  {
    // wait for event
    eventreceivedflags = osEventFlagsWait(myEvent01Handle,eventflags,               
                                          osFlagsWaitAny | osFlagsNoClear,               // not wait for all bits ready, check the flag manually.
                                          osWaitForever           
                                            );
    
    // Condition(Event1 Or Event2) And Evnet3
    if(((eventreceivedflags & (EVENT_1_BIT | EVENT_2_BIT)) != 0) &&  
       ((eventreceivedflags & EVENT_3_BIT) != 0)) {  
         
         osEventFlagsClear (myEvent01Handle, eventflags);
         printf("all conditions are ready!!!!!!\r\n");
         
       }    
    osDelay(200);
  }
  /* USER CODE END defaultTask */
}

/* USER CODE BEGIN Header_StartTask01 */
/**
* @brief Function implementing the myTask01 thread.
* @param argument: Not used
* @retval None
*/
/* USER CODE END Header_StartTask01 */
void StartTask01(void *argument)
{
  /* USER CODE BEGIN myTask01 */
  /* Infinite loop */
  for(;;)
  {
    osEventFlagsSet (myEvent01Handle,EVENT_1_BIT);
    printf("Task 1 is ready!!!!\r\n");
    osDelay(500);
  }
  /* USER CODE END myTask01 */
}

/* USER CODE BEGIN Header_StartTask02 */
/**
* @brief Function implementing the myTask02 thread.
* @param argument: Not used
* @retval None
*/
/* USER CODE END Header_StartTask02 */
void StartTask02(void *argument)
{
  /* USER CODE BEGIN myTask02 */
  /* Infinite loop */
  for(;;)
  {
    osEventFlagsSet (myEvent01Handle,EVENT_2_BIT);
    printf("Task 2 is ready!!!!\r\n");
    osDelay(1000);
  }
  /* USER CODE END myTask02 */
}

/* USER CODE BEGIN Header_StartTask03 */
/**
* @brief Function implementing the myTask03 thread.
* @param argument: Not used
* @retval None
*/
/* USER CODE END Header_StartTask03 */
void StartTask03(void *argument)
{
  /* USER CODE BEGIN myTask03 */
  /* Infinite loop */
  for(;;)
  {
    osEventFlagsSet (myEvent01Handle,EVENT_3_BIT);
    printf("Task 3 is ready!!!!\r\n");
    osDelay(1500);
  }
  /* USER CODE END myTask03 */
}

/* Private application code --------------------------------------------------*/

Program Execution Results

Thus, the introduction to the event group of the <span>FreeRTOS</span> operating system (<span>Event Group</span>) is complete.