#include <reg51.h> //18．            "嘀、嘀、……"報警聲
unsigned int t02s; 
unsigned char t05ms; 
bit flag; 
  
void main(void) 
{ 
  TMOD=0x01; 
  TH0=(65536-500)/256; 
  TL0=(65536-500)%256; 
  TR0=1; 
  ET0=1; 
  EA=1; 
  while(1); 
} 
  
void t0(void) interrupt 1 using 0 
{ 
  TH0=(65536-500)/256; 
  TL0=(65536-500)%256; 
  t02s++; 
  if(t02s==400) 
    { 
      t02s=0; 
      flag=~flag; 
    } 
  if(flag==0) 
    { 
      P1_0=~P1_0; 
    } 
}