#include <reg51.h>
sbit SW    = P1^7;
sbit IND   = P1^0;
sbit WAVE  = P2^5;
void timer0(void) interrupt 1
  {
    WAVE = ~WAVE;	
  }
void serial0() interrupt 4
  {
    if(TI == 1)
      {
        SBUF = 'A';	
        TI = 0;		
      }
    else
      {
        RI = 0;		
      }
  }
void main()
  {
    SW = 1;		
    TH1 = -3;		
    TMOD = 0x22;	
    TH0 = 0xA4;		
    SCON = 0x50;   
    TR0 = 1;
    TR1 = 1;			
    IE = 0x92;		
    while(1)		
      {
        IND = SW;	
      }
  }