This paper introduces an original architecture to measure and convert into digital data the oscillations of a resonant beam. An electromechanical CMOS magnetic field sensor is considered here for the purpose of a case study. The proposed architecture is based on counting the cycles of an oscillator output, whose frequency depends on the deformation of the mechanical structure. In order to reduce drifts, the architecture implements a differential measure by using both up and down counting within a mechanical vibration period, using the actuation signal for synchronization. Simulation results demonstrate that a high resolution can be achieved with acceptable integration time. Such a signal processing architecture is particularly suitable for low-cost CMOS mechanical structures.