说明：  一般我们单片机同一端口有8个引脚,用来作键盘的话只可以实现4*4的矩阵,本源码介绍了如何在不使用芯片进行键盘扩展时的键盘扫描方法,以20(5*4)键为例.
(Generally we have the same port single-chip 8-pin, used as a keyboard can be achieved if only 4* 4 matrix, the source describes how the non-use of chips in the keyboard to expand the keyboard scanning method, 20 (5* 4) key as an example.)