/**
  **************************************************************************
  * @file     readme.txt 
  * @version  v2.0.0
  * @date     2021-11-26
  * @brief    readme
  **************************************************************************
  */

  this demo is based on the at-start board, in this demo, shows how to retarget
  the c library printf function to the usart. this implementation output the printf
  message on the hyperterminal using usart1. using pa9 to transmit data(printf).
