Rapid and precise interpolation of curves has been a key issue in development of computer numerical control(CNC) systems. With the development of firmware, it is a new attempt to perform the interpolation by means of firmware. This paper presents a method to develop a rapid and precise interpolator that can produce a smooth interpolating curve. An algorithm is first shown to construct a smooth interpolating curve and a way is put forward to calculate the constructed curve with firmware;then details are introduced to design and implement the firmware units that perform rapid and precise calcula...