void externalinterrupt(void) interrupt 0    
{
   if(TR0 == 0 )    // first pulse start timing
   {
      TR0 = 1;    
   }
   else            // next pulse stop timing
   {
      TR0 = 0;        //disable timer0
      //IEN0 = 0x00;    //disable all interrupts Why?
 
      timercounts = (TH0*256) + TL0;  // read timer

      time = timercounts/921583;      // the Bug: 
      rpm = (unsigned int)(30/time);  // 
   }
}