说明：  两片74HC595级联，形成16位并行数据输出。控制8个数码管显示。显示刷新周期为2ms。 单片机只需3个I/O口发送串行数据到74HC595即可。程序用c51编写。
(74HC595 2 cascade, the formation of 16-bit parallel data output. Control of 8 digital tube display. Display refresh cycle for 2ms. Only three single-chip I/O port to send serial data to 74HC595. Procedures used to prepare c51.)