#include<reg52.h>
unsigned char i=0x01;

fun()  interrupt 0
{
P0=i^0xff;
if(i<0x80)
{
i<<=1;

}
else
i=0x01;
}

main()
{
P0=0xff;
EA=1;
EX0=1;
IT0=1;           //下降沿中斷允許
while(1);
}