There is provided a media playback system for monitoring usage of a plurality of media contents. The nonvolatile memory cell includes a phase-change layer, a first and a second gate that are in contact with sides of the phase-change layer to face each other across the phase-in change layer and control a current flowing through the phase-change layer by each gate being arranged to induce the phase transition of the phase-change layer independently of the other. The oscillator control circuit generates a control signal responsive to comparing the selected phases of the output clock signal with the reference signal, and the phase selection circuit may include a modulator to generate phase selection sequences having desired time-average values that enable arbitrary frequency adjustability.