void external interrupt(void) interrupt 0{
      if(!flag){ 
          TR0 = 1;           //enable timer0
          flag = 1;
      }    
      else if(flag){
          TR0 = 0;           //disable timer0
          IEN0 =0x00;        //disable all interrupts
          flag =0;
          timercounts = (TH0*256) + TL0;
          time = timercounts/921583;
          rpm = 30/time;
       }