/**
  **************************************************************************
  * @file     readme.txt
  * @brief    readme
  **************************************************************************
  */
  
  this demo is based on the at-start board and AT32-Comm-EV, in this demo, 
  shows how to use usart to achieve rs485 communication.
  
  set-up
  - usart tx      --->   pa2
  - usart rx      --->   pa3
  - usart de     --->   pa1
  

