如下图，先在p3口输出0xfe，再读取p3口的电平，如果没有按键按下，temp & 0xf0还是0xf0，如果又第一个键按下，temp & 0xf0还是0xee，其他按键由此类推可得。

//4*4键盘检测程序,按下键后相应的代码显示在数码管上
#include<reg51.h>sbit beep=P2^3;
sbit dula = P2^6;
sbit wela = P2^7;unsigned char i = 100;
unsigned char j, k, temp, key;void delay(unsigned char i)
{for(j = i; j > 0; j--)for(k=125; k>0; k--);
}unsigned char code table[] = {0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d,0x07, 0x7f, 0x6f, 0x77, 0x7c, 0x39, 0x5e, 0x79, 0x71};display(unsigned char num)
{P0 = table[num];dula = 1;dula = 0;P0 = 0xc0;wela = 1;wela = 0;
}void main()
{dula = 0;wela = 0;while(1){//扫描第一行键盘是否按下P3 = 0xfe;	//			1111 1110temp = P3;temp = temp & 0xf0;	//	1111 0000if(temp != 0xf0){delay(10);if(temp != 0xf0){temp = P3;switch(temp){case 0xee:	//1110 1110key = 0;break;case 0xde:	//1101 1110key = 1;break;case 0xbe:	//1011 1110key = 2;break;case 0x7e:	//0111 1110key = 3;break;}while(temp != 0xf0){temp = P3;temp = temp&0xf0;beep = 0;}beep = 1;display(key);P1=0xfe;}}//扫描第二行键盘是否按下P3=0xfd;temp=P3;temp=temp&0xf0;if(temp!=0xf0){delay(10);if(temp!=0xf0){temp=P3;switch(temp){case 0xed:key=4;break;case 0xdd:key=5;break;case 0xbd:key=6;break;case 0x7d:key=7;break;}while(temp!=0xf0){temp=P3;temp=temp&0xf0;beep=0;}beep=1;display(key);P1=0xfc;}}//扫描第三行键盘是否按下P3=0xfb;temp=P3;temp=temp&0xf0;if(temp!=0xf0){delay(10);if(temp!=0xf0){temp=P3;switch(temp){case 0xeb:key=8;break;case 0xdb:key=9;break;case 0xbb:key=10;break;case 0x7b:key=11;break;}while(temp!=0xf0){temp=P3;temp=temp&0xf0;beep=0;}beep=1;display(key);P1=0xf8;}}//扫描第四行键盘是否按下P3=0xf7;temp=P3;temp=temp&0xf0;if(temp!=0xf0){delay(10);if(temp!=0xf0){temp=P3;switch(temp){case 0xe7:key=12;break;case 0xd7:key=13;break;case 0xb7:key=14;break;case 0x77:key=15;break;}while(temp!=0xf0){temp=P3;temp=temp&0xf0;beep=0;}beep=1;display(key);P1=0xf0;}}}
}