Described is an apparatus for providing spread-spectrum to a clock signal. The apparatus comprises: an oscillator to generate an output clock signal, the oscillator to receive an adjustable reference signal to adjust frequency of the output clock signal; a first circuit to provide a first signal indicative of a center frequency of the output clock signal; a second circuit to generate a switching waveform to provide spread-spectrum for the output clock signal; and a third circuit, coupled to the first and second circuits, to provide the adjustable reference signal according to the first signal and the switching waveform.