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 1110
    temp = P3;
    temp = temp & 0xf0;	//	1111 0000
    if(temp != 0xf0)
    {
      delay(10);
      if(temp != 0xf0)
      {
        temp = P3;
        switch(temp)
        {
          case 0xee:	//1110 1110
               key = 0;
               break;

          case 0xde:	//1101 1110
               key = 1;
               break;

          case 0xbe:	//1011 1110
               key = 2;
               break;

          case 0x7e:	//0111 1110
               key = 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;
      }
    }
  }
}