求大神指点 哪错了？想输出一个方波，可就是没反应

#include<reg51.h>
#include<intrins.h>
sbit P0_0=P0^0;
unsigned char tt;
void main()
{
TMOD=0X50;
TH1=(65536-50000)/256;
TL1=(65536-50000)%256;
EA=1;
ET1=1;
TR1=1;
while(1);
}
void timer()interrupt 3
{
TH1=(65536-50000)/256;
TL1=(65536-50000)%256;
tt++;
TF1=0;
if(tt==20)
{
tt=0;
P0_0=~P0_0;
}
}