数码管前三位显示一个跑表，从000到999之间以1 秒速度运行，三个独立键盘实现按下第一个时计时停止，按下第二个时计时开始，按下第三个是计数值清零从头开始。-The top three shows a digital stopwatch, from 000 to 999 seconds, running at 1 , to achieve three separate keyboard, when pressed the first time to stop, when the second s