本程序为PCA捕捉例程，判断4个(S1-S4)按键是否按下然后触发中断对LED灯 D2\D3取反同时也完成按键扫描的过程-The procedures for PCA capture routine, to determine four (S1-S4) button is pressed and then trigger the interruption of LED lights D2 \ D3 negated also completed the process of key scan