说明：  校电赛选拔程序，利用8051单片机的计数器功能，采集交流信号的下降沿，设计计数器，并在1602上显示，精确位数至4位。
(Electric race school selection process, using 8051 counter function, the falling edge of AC signal acquisition, design counter, and 1602 on display, accurate to four digits.)