/**
  **************************************************************************
  * @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, pb7 connect to a
  key, press the key then pb6 output a pulse. pulse width = 2.048 ms.
