Electrical – FreeRTOS – Potential issues with Periodic Tasks

There two basic approaches that come to mind.

1) Use a queue as outlined by the OP.

According to the FreeRTOS API documentation for xQueueSend, all of the data from your screenData structure is copied into the queue (see description for the pvItemToQueue argument: "A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area." This means your assumption is incorrect. After the code execution returns from the xQueueSend call, you can modify data without fear of overwriting what was just queued up.

2) Use a mutual exclusion semaphore or mutex.

(read more about FreeRTOS mutex implementation in the API documentation of xSemaphoreCreateMutex) to synchronize access to the display between different tasks.

The main trade-offs to consider in choosing between a queue or a mutex are as follows:

Using a queue will buffer the data and not block any tasks from executing unless your queue becomes full but requires you to allocate more memory (10 * sizeof(screenData) in this case).

Using a mutex does not require the memory allocation but will block execution of tasks if more than one is trying to access the display at the same time.

You can use a semaphore to signal an event. If the event is generic to taskCheck and there is no data associated with the event then you might do it something like this.

void task1(int change1)
{
 int value1 = change1;
 xSemaphoreGive(the_semaphore);
}

void task2(int change2)
{
 int value2 = change2;
 xSemaphoreGive(the_semaphore);
}

void task3(int change3)
{
 int value3 = change3;
 xSemaphoreGive(the_semaphore);
}

void taskCheck()
{
  xSemaphoreTake(the_semaphore, WAIT_FOREVER);
  doSomething();
}

If the_semaphore is a counting semaphore then multiple events could stack up while taskCheck handles them one at a time.

But if the events are specific or contain data then you probably want to use a queue instead of a semaphore.

Read about FreeRTOS queues and semaphores here.