用8253对标准脉冲信号进行计数，就可以实现定时功能。用板上的1MHz作为标准信号，将8253可编程计数器/定时器的时间常数设在1000000次，就可以在定时器的管脚上输出1秒针高/1秒钟低的脉冲信号。因为8253每个计数器只有十六位，要用两个计数器才能实现一百万次的计数，实现每一秒输出状态发生一次反转。-With 8253 pairs of standard pulse counting, timing can be achieved. 1MHz with the board as a standard signal, the 8253 programmable counter/timer time constant in one million times, you can pin on the timer output 1 second hand high/low 1 second pulse signal. Since 8253 only sixteen per counter, can be achieved using two counters count a million times, to achieve the output state occurs once every second reversal.