计数结束？
等待设置
K4是否按下按下
_ 输出
_________________________________________________________________
_____________________________ _______________________________________________________________
____________________________________ N
_________________ N_________________________________ Y
______________________________ Y ________________________________________________________________________________ _
________________________________
_________________________________________________________
_______________________________________________________________
___________________________________________________ Y
___________________ __ N
___________________________ ________________________________________________
____________________________ Y
倒计时外部中断流程图：
置工作状态标志
_ 关中断
K2键按下？
K3键按下？
调显示程序
个位数缓冲器加1
十位数缓冲加1
K1键按下？
调显示程序
中断返回
Y
_________________________________
______________________________________
________________________________________ N
____ ________________ Y