用动态扫描方法和定时器1在数码管的前三位显示出秒表， 精确到1 秒，即最后一位显示1 秒，一直循环下去 设时钟频率为12M-With dynamic scanning method and Timer 1 in the top three shows digital stopwatch, accurate to 1 of the second and final one shows 1 seconds, the clock has been set down cycle freque