#define NO_KEY 0XFF
#define KEY_LO()  P2 &= 0XF0
#define KEY_HI()  P2 |= 0X0F
#define KEY_L(i)  P2 &= ~(1<#define KEY_RD()  ((P2>>4) & 0x0f)

UINT8 OnceKey(void)
{
 UINT8 line = 0;
 UINT8 key = NO_KEY;//key value
 KEY_LO();
 if (KEY_RD() == 0X0F)
 {
  KEY_HI();
  return NO_KEY;
 }

 for (line=0; line<4; line ++)
 {
  KEY_HI();
  KEY_L(line);
  key = KEY_RD();
  switch (key)
  {
   case ROW_FIRST:
    key = 4*line + 0;
    break;
   case ROW_SECOND:
    key = 4*line + 1;
    break;
   case ROW_THIRD:
    key = 4*line + 2;
    break;
   case ROW_FOURTH:
    key = 4*line +3;
    break;
   default :
    key = 0x0f;
    break;
  }
  if (key < 0x10)
  {
   return key;
  }
 }
 return NO_KEY;
}

第二种，线性反转法。就是行和列分别读出。实现代码如下：

#define CVT(i) ((i)==(~1)&0x0f)? 0: ((i)==(~2)&0x0f)? 1: ((i)==(~4)&0x0f)? 2: ((i)==(~8)&0x0f)? 3: 4;

#define KEY0_3HI() P2 |= 0X0F
#define KEY0_3LO() P2 &= 0XF0
#define KEY4_7HI() P2 |= 0XF0
#define KEY4_7LO() P2 &= 0X0F
#define KEY0_3RD() (P2 & 0X0F)
#define KEY4_7RF() ((P2>>4) & 0X0F)

UINT8  OnceKey(void)
{
 UINT8 line = NO_KEY;
 UINT8 row = NO_KEY;
 UINT8 key;
 KEY0_3HI();
 KEY4_7LO();
 line = KEY0_3RD();//读入行的值
 if (0x0f == line)
 {
  key = NO_KEY;
 }
 else
 {
  KEY0_3LO();
  KEY4_7HI();
  row = KEY4_7RD();//读入列的值
  if (0x0f == row)
  {
   key = NO_KEY;
  } 
  else
  {
   key = CVT(line)*4 + CVT(row);
  }
 }
 KEY0_3HI();
 KEY4_7HI();
 return key;