void externalinterrupt(void) interrupt 0
{
	if(!flag){
		TR0 = 1;       
		flag = !flag;
        }
        if(flag){
	       TR0 = 0;
	       IEN0 = 0x00;
	       flag = !flag;
               timercounts = (TH0*256) + TL0;
               time  = timercounts/921583;
               rpm = (unsigned int)(30/time);
        }