秒表设计频率 f=1.1931816MHz。

定时器0输出方波的频率: fout=1.1931816/65536=18.2Hz

输出方波的周期Tout=1/18.2=54.945ms。

8253A每隔55ms引起一次中断，作为定时信号，5s/55ms=91，这样91次中断就是5秒的时间。 -Stopwatch design frequency f = 1.1931816MHz. Timer 0 output square wave frequency: fout = 1.1931816/65536 = 18.2Hz　output square wave of period Tout = 1/18.2 = 54.945ms. 8253A cause an interrupt every 55ms, as the timing signals, 5s/55ms = 91, so 91 is 5 seconds break.