/**
  **************************************************************************
  * @file     readme.txt 
  * @version  v2.0.4
  * @date     2022-04-02
  * @brief    readme
  **************************************************************************
  */

  this demo is based on the at-start board, in this demo, show how to build
  a device of usb virtual comport,when use this demo,please connect usart2 
  tx pin(pa2) and rx pin(pa3).

