51单片机按键问题

//-----------------------------------第一行按键的检测----------------------------
P0=0xfe;
temp=P0;
temp=temp&0xf0;
//如果有按键按下
while(temp!=0xf0)
{
delay(5); //消抖之后如果依然按下，则再次赋值检测
temp=P0;
temp=temp&0xf0;
while(temp!=0xf0) //有键按下
{
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;
}
}
}