s52_speed_measure

代码说明：

  S52三个定时器实现两路测速（测频）系统，一个定时器作时间基准，另外两个定时器作输入捕捉用，实现测频。(S52 3 Two Speed Timer (measuring frequency) system, a timer for a time basis, for the other two timers with input capture, to achieve frequency measurement.)