采用IO触发测距，给至少10us的高电平信号；
模块自动发送8个40KHz的方波，自动检测是否有信号返回；
有信号返回，通过IO输出一高电平，高电平持续的时间就是超声波从发射到返回的时间．距离=(高电平时间*声速(340m/s))/2;(IO triggered ranging is used to give at least 10us of the high level signal;
The module automatically sends 8 40KHz Fang Bo, automatically detects whether there is a signal to return;
There is a signal to return, and a high level is output through the IO. The high duration is the time from which the ultrasonic wave is transmitted to the return. The distance = (high level time * sound speed (340m/s)) /2;)