In the embodiment of the invention, apparatus and method provide flexibility in generating a stimulation waveform (that comprises at least a stimulation pulse) to an electrode of an Implantable Neuro Stimulator (INS). The waveform is synthesized for each rate period interval. A portion of the rate period interval is identified, in which the portion comprises at least one phase. During each phase, the stimulation waveform is synthesized by a waveform controller and a waveform generator. The waveform controller utilizes waveform parameters, e.g. the initial value, final value, and step time duration, to form a digital signal to the waveform generator. The waveform generator utilizes the digital signal to form the synthesized waveform.