说明：  c8051,PCA捕捉模块0的CEX0被设置在按键S3对应的引脚上，按下和松开S3，将会产生下降和上升沿信号，引发捕捉模块动作。
(c8051, PCA Capture Module 0 CEX0 S3 is set the corresponding pin in the key, press and release the S3, will result in decline and the rising edge signal capture module trigger action.)