选用8253的计数器2进行100ms的定时，其输出 OUT2与8259的IRQ7相连，当定时到100 ms时产生一个中断信号，在中断服务程序中进行时、分、秒的计数，并送入相应的存储单元；8255的A口接七段数码管的位选信号，B口接数码管的段选信号，时、分的数值通过对8255的编程可送到七段数码管上显示。-2 for selection of 8253' s counter 100ms timing, the output OUT2 and 8259 IRQ7 linked to 100 ms when the timer generates an interrupt signal when the interrupt service routine, the hours, minutes, seconds count, and sent to the appropriate storage unit　8255, A port connection segment digital tube-bit select signal, B port then select the segment of digital control signals, the sub-values ??can be programmed through 8255 to the seven segment LED display.