51單片機按鍵問題

//-----------------------------------第一行按鍵的檢測----------------------------
P0=0xfe;
temp=P0;
temp=temp&0xf0;
//如果有按鍵按下
while(temp!=0xf0)
{
delay(5); //消抖之后如果依然按下，則再次賦值檢測
temp=P0;
temp=temp&0xf0;
while(temp!=0xf0) //有鍵按下
{
switch(temp)
{
case 0xe0:
beep=0;
keyNum=1;
break;
case 0xd0:
beep=0;
keyNum=2;
break;
case 0xb0:
beep=0;
keyNum=3;
break;
case 0x70:
beep=0;
keyNum=4;
break;
}
}
}