Methods for RTOS Multi-Task Access to the Same UART

Methods for RTOS Multi-Task Access to the Same UART

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Methods for RTOS Multi-Task Access to the Same UART

Author | strongerHuang

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In RTOS multi-task programming, it is common for multiple tasks to access the same hardware (such as UART, I2C, etc.). If not handled properly, it can lead to a “chaotic” situation.
There are many methods to handle the “chaotic” situation. Below, based on FreeRTOS, I will explain two common methods: mutual exclusion and queues, using UART as an example.

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Mutual Exclusion Access Method
Mutex: A variable that can be in one of two states: unlocked and locked.
Principle: Create a mutex. Task A occupies the resource (using UART to send data) when needed. At this point, Task B and other tasks cannot occupy the resource. When Task A finishes using the resource (after sending data), it releases the resource, allowing other tasks to seize the resource.
Create a mutex
Task A occupies the resource
Use the resource (send data)
Task A releases the resource
Higher priority Task B occupies the resource
Use the resource
Task B releases the resource
In turn, higher priority tasks occupy the resource
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Code:

// Create mutex resourceSemaphoreHandle_t xSemaphore = NULL;xSemaphore = xSemaphoreCreateMutex();
void TaskA(void *pvParameters){  for(;;)  {    // Occupy resource    if(xSemaphoreTake(xSemaphore, 10 ) == pdTRUE)    {      // Use resource (send data)      USART_SendNByte();      // Release resource      xSemaphoreGive(xSemaphore);    }  }}
Difference between Semaphore and Mutex:
Semaphore: Multiple tasks synchronize to use a resource;
One task signals others after completing an action, allowing them to execute certain actions;
Mutex: Multi-task mutual exclusion for certain resources;
When one task occupies a resource, other tasks cannot access it until that task releases it.

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Queue Operation Method
The queue operation method is FIFO, first in first out principle. For example: Task A wants to use UART to send a string of data, adding it to the queue; then Task B also wants to use UART to send a string of data.
Then, Task A adds this string of data to the queue, and Task B adds the data it wants to send to the queue.
In another UART sending task, read the data from the queue in FIFO order and send it out.
Create a queue (send data queue)
Create a task (UART send data task)
Task A adds to the queue
Task B adds to the queue
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On the other side, tasks read queue data in order and use UART to send it out.

Code:

QueueHandle_t xQueue;xQueue = xQueueCreate(QUEUE_LENGTH, QUEUE_ITEM_SIZE);
xTaskCreate(UART_Send_Task, "UART_Send", STACK_SIZE, NULL, TASK_PRIORITY, NULL);
void TaskA(void *pvParameters){  for(;;)  {    // Task related operations
    // Add to queue    xQueueSend(xQueue, &TaskA_Buf, 10)  }}
void TaskB(void *pvParameters){  for(;;)  {    // Task related operations
    // Add to queue    xQueueSend(xQueue, &TaskB_Buf, 10)  }}
void UART_Send_Task(void *pvParameters){  for(;;)  {    // Loop read queue BUF    if(xQueueReceive(xQueue, &Buf, 10) == pdTRUE)    {      USART_SendNByte(&Buf);    }  }}

The above two methods are commonly used and relatively simple. I hope they are helpful to everyone.

Note:The code is for learning and understanding principles only. In projects, code should be added, deleted, or modified according to actual situations.

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