说明：  设计一个键盘接口电路。要求： （1）用200Hz时钟频率有效地去除按键的抖动； （2）用20Hz时钟频率对键盘进行扫描识别，并用数码管将按键的编号显示出来；
(Design a keyboard interface circuit. Requirements: (1) use the 200Hz clock frequency to effectively remove the shaking of the keys; (2) scan the keyboard with 20Hz clock frequency and display the key number of the key with the digital tube;)