用基于89C52的定时器2的外部捕获功能实现对距离的测量，改善了以往很多用定时器和外部中断所照成的资源浪费和程序系统所照成的中断冲突。-89C52 timer with 2 on the external capture the distance measurement to achieve improved a lot in the past by using the timer and external interrupt according to the waste of resource