Timepiece including a mechanical movement whose operation is controlled by an electronic device

A timepiece comprising a mechanical movement which is provided with a mechanical oscillator formed by a balance and a balance spring, said mechanical oscillator having a predefined desired frequency F0c and being arranged to time the running of the mechanical movement, said timepiece also including a control device arranged to be capable of controlling the mean frequency of the mechanical oscillator and including an auxiliary time base, formed by an auxiliary electronic oscillator and providing a reference signal, the balance spring being at least partially formed by a piezoelectric material and by at least two electrodes arranged to be capable of having therebetween a voltage U(t) induced by said piezoelectric material when the latter is subjected to mechanical stress during an oscillation of the mechanical oscillator, the two electrodes being electrically connected to the control device which is arranged to be capable of varying the impedance of the control system, which is formed by said piezoelectric material, said at least two electrodes and the control device;

• characterized in that the control device is arranged to be capable of momentarily varying the electrical resistance generated by said control device between said two electrodes, in order to generate, at least at times, control pulses that are distinct and each have a certain duration each control pulse including a momentary decrease in said electrical resistance relative to a nominal electrical resistance, which is generated by the control device between said two electrodes outside said distinct control pulses, the control device being arranged to be capable of applying a plurality of said control pulses during each of said times, such that, between the starts of any two successive control pulses among each plurality of control pulses, there is a temporal distance DT equal to a number N multiplied by half of a determined control period Treg for each of said times, i.e. a mathematical relation DT=N·Treg/2, where N is a positive integer number greater than zero, the control period Treg and the number N being selected to allow synchronization of the mechanical oscillator at a control frequency Freg=1/Treg during each of said times, the control device being arranged to determine, with the reference time base, the start of each of said control pulses, in order to satisfy said mathematical relation between said temporal distance and the control period, and thus to determine the control frequency.