# Patent

## Title
Microelectromechanical device having an oscillating mass and a forcing stage, and method of controlling a microelectromechanical device

## Abstract

A microelectromechanical device includes: a body; a movable mass, elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom; a frequency detector, configured to detect a current oscillation frequency of the movable mass; and a forcing stage, capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector, at least in a first transient operating condition.

## Background
BACKGROUND
1. Technical Field
The present disclosure relates to a microelectromechanical device having an oscillating mass and a forcing stage and a method for controlling a microelectromechanical device.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has increasingly spread in various technological sectors and has yielded encouraging results especially in providing inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a fixed body (stator) by springs and movable with respect to the stator according to one or more degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors. Thus, by sensing capacitance, it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Instead, by providing appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. In addition, for providing electromechanical oscillators the frequency response of MEMS inertial structures is exploited, which is typically of a second-order low-pass type with one resonance frequency.
MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to a driving sub-system and is kept in oscillation at the resonance frequency. The other mass is drawn in the (translational or rotational) oscillatory motion and, in the event of rotation of the microstructure with respect to a gyroscopic sensing axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer, which enables detection of the Coriolis force and acceleration and hence makes it possible to trace back to the angular velocity.
In gyroscopes, as likewise in other devices, the movable mass or the system of movable masses is maintained in oscillation at a controlled frequency. This may be accomplished through a driving device coupled to the micromechanical structure so as to form a resonant microelectromechanical loop which vibrates with controlled frequency and amplitude. Clearly, upon turning-on of the device (power-on) or at exit from low-consumption configurations (power-down) a start-up transient occurs before the movable mass or the system of movable masses reaches a stable condition of oscillation.
In the start-up transient, the oscillatory motion is forced through start-up components, which supply a fixed amount of energy, normally by applying one or more sequences of pulses of programmed duration to the movable mass. Once the transient is exhausted, the start-up components are de-activated, and the oscillation is maintained by the microelectromechanical loop that guarantee normal operation.
Sequences of pulses may be generated during start-up transients by a local oscillator embedded in an ASIC (“Application Specific Integrated Circuit”) chip coupled to the micromechanical structure. The overall number of pulses (i.e. the maximum duration of the forcing sequence of pulses) is determined from the residual difference between the oscillation frequency of the oscillator and the microelectromechanical loop. The residual frequency difference, in fact, causes a phase lag at each oscillation cycle between the oscillator output and the natural resonance frequency of the microelectromechanical loop. The overall phase delay therefore increases in time as the number of pulses increases and may lead to a condition in which energy provided by the local oscillator tends to counter rather than favoring oscillation of the microelectromechanical loop. In particular, the overall phase delay cannot exceed π/2 for an efficient forcing.
BRIEF SUMMARY
In order to avoid this condition, energy supply by the local oscillator is stopped after a programmed maximum number of pulses. Thus, start-up transients may be quite long, while it would be desirable to keep them as short as possible.
According to an aspect of the present disclosure there is provided a microelectromechanical device comprising:
a body;
a movable mass, elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom;
a frequency detector, configured to detect a current oscillation frequency of the movable mass; and
a forcing stage, capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector, at least in a first transient operating condition.
According to another aspect of the present disclosure there is provided an electronic system having a microelectromechanical device and a control unit coupled to the microelectromechanical device;
the microelectromechanical device comprising:
a body;
a movable mass, elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom;
a frequency detector, configured to detect a current oscillation frequency of the movable mass; and
a forcing stage, capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector, at least in a first transient operating condition.
According to another aspect of the present disclosure there is provided a method of controlling a microelectromechanical device, comprising:
oscillating a movable mass with respect to a body according to a degree of freedom;
detecting a current oscillation frequency of the movable mass; and
providing energy to the movable mass through forcing signals having a forcing frequency equal to the detected current oscillation frequency, at least in a first transient operating condition.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
For a better understanding of the disclosure, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:
FIG. 1 is a simplified block diagram of a microelectromechanical device in accordance with an embodiment of the present disclosure;
FIG. 2 is a graph showing quantities relating to the microelectromechanical device of FIG. 1;
FIG. 3 is a graph showing quantities relating to a microelectromechanical device in accordance with another embodiment of the present disclosure;
FIG. 4 is a more detailed block diagram of a first component of the microelectromechanical device of FIG. 1;
FIG. 5 is a more detailed block diagram of a second component of the microelectromechanical device of FIG. 1; and
FIG. 6 is a simplified block diagram of an electronic system incorporating a microelectronic device according to one embodiment of the present disclosure.



DETAILED DESCRIPTION
FIG. 1 schematically illustrates a microelectromechanical gyroscope 1 in accordance with an embodiment of the present disclosure. What is hereinafter disclosed, however, is also applicable to different oscillating microelectromechanical devices, such as an electromechanical oscillator for use in telecommunications or an inertial sensor with force-feedback reading.
The gyroscope 1 comprises a microstructure 2, made of semiconductor material, a driving device 3, and a sensing device 5.
The microstructure 2 is made of semiconductor material and comprises a supporting body 6, a driving mass 7, and at least one sensing mass 8. For the sake of simplicity, in the embodiment illustrated herein reference will be made to the case of a uniaxial gyroscope, in which a single sensing mass 8 is present. The following description applies, however, also in the case of multiaxial gyroscopes, which comprise two or more sensing masses for detecting rotations according to respective independent axes.
The driving mass 7 is elastically connected through springs (not shown) to the supporting body 6 so as to be oscillatable about a resting position in accordance with a translational or rotational degree of freedom.
The sensing mass 8 is mechanically coupled to the driving mass 7 so as to be driven in motion according to the degree of freedom of the driving mass 7 itself. In addition, the sensing mass 8 is elastically connected to the driving mass 7 so as to be oscillatable in turn with respect to the driving mass 7 itself, with a respective further translational or rotational degree of freedom. In particular, in the embodiment described herein, the driving mass 7 is linearly movable along a driving axis X, whereas the sensing mass 8 is movable with respect to the driving mass 7 according to a sensing axis Y perpendicular to the driving axis X. It is understood, however, that the type of movement (translational or rotational) allowed by the degrees of freedom and the arrangement of the driving and sensing axes may vary according to the type of gyroscope. In addition, with reference to the movements of the driving mass 7 and of the sensing mass 8, the expressions “according to an axis” and “in accordance with an axis” will be used to indicate movements along an axis or about an axis, according to whether the movements allowed to the masses by the respective degrees of freedom are translational or else rotational, respectively. In a similar way, the expressions “according to a degree of freedom” and “in accordance with a degree of freedom” will be used to indicate translational or rotational movements, as allowed by the degree of freedom itself.
The driving mass 7 (with the sensing mass 8) is connected to the supporting body 6 so as to define a resonant mechanical system with one resonance frequency (according to the driving axis X).
As illustrated schematically in FIG. 1, the driving mass 7 is capacitively coupled to the supporting body 6 by capacitive driving units 10 and capacitive feedback sensing units 12. The capacitive coupling is of a differential type and is determined by the relative position of the driving mass 7 with respect to the supporting body 6. In particular, the capacitive driving units 10 and the capacitive feedback sensing units 12 are accessible from outside the microstructure 2 through driving terminals 13 and feedback sensing terminals 14, respectively.
The sensing mass 8 is capacitively coupled to the supporting body 6 by capacitive signal sensing units 15, accessible from outside by signal sensing terminals 16. Also in this case, the capacitive coupling is of a differential type and is determined by the relative position of the sensing mass 8 with respect to the supporting body 6.
By way of example, but not necessarily, the microstructure 2 may be obtained as described in U.S. Pat. No. 6,928,872 for a uniaxial gyroscope, which is incorporated by reference herein in its entirety. The microstructure of a multiaxial gyroscope could be obtained, for example, as described in detail in U.S. Pat. No. 7,694,563, which is incorporated by reference herein in its entirety.
The driving device 3 is connected to the driving terminals 13 and to the feedback sensing terminals 14 of the microstructure 2 so as to form, with the driving mass 7, a microelectromechanical loop 19. The driving device 3 is configured to maintain the microelectromechanical loop 19 in oscillation at a driving frequency cop close to the resonance frequency of the mechanical system defined by the driving mass 7 (with the sensing mass 8) connected to the supporting body 6.
The sensing device 5 is connected to the sensing terminals 16 and converts signals indicating the displacement of the sensing mass 8 into an output signal SOUT indicating the angular speed of the microstructure 2.
In greater detail, the driving device 3 comprises a reading and filtering stage 20, a variable-gain amplifier 21, an oscillator 23, a comparator 25, a phase-locked-loop (PLL) circuit 26, a controller 27, a start-up stage 30, and a forcing stage 31.
The reading and filtering stage 20 is connected to the feedback sensing terminals 14 of the microstructure 2 and in one embodiment comprises a C-V converter 20a, such as a charge amplifier, a delay module 20b and a continuous-time filter 20c. The C-V converter 20a may be a discrete-time fully-differential charge amplifier, which has inputs coupled to the capacitive feedback sensing units 12 and is configured to sense capacitance thereof. The delay module 20b introduces a controlled phase delay in the microelectromechanical loop 19 in order to achieve an oscillation condition for the phase (i.e. the phase of the microelectromechanical loop 19 at the driving frequency ωD is 2kπ). The continuous-time filter 20c is an analog filter that converts discrete-time signals from the delay module 20b into corresponding continuous-time signals.
The reading and filtering stage 20 supplies a (continuous-time) first feedback signal VFB1, indicating the conditions of oscillation of the driving mass 7. In particular, the first feedback signal VFB1 indicates the velocity of the driving mass 7. In a different embodiment, the first feedback signal VFB1 indicates the position of the driving mass 7. In addition, the reading and filtering stage 20 controls the phase of the first feedback signal VFB1 so as to guarantee the condition of oscillation on the phase for the microelectromechanical loop 19.
The variable-gain amplifier 21 is coupled to the reading and filtering stage 20 for receiving the first feedback signal VFB1 and is selectively connectable to the feedback driving terminal 13 of the microstructure 2 through bypass switches 28 (in effect, two connection lines and a bypass switch 28 for each of the connection lines are present between the variable-gain amplifier 21 and the feedback driving terminal 13; for reasons of simplicity, FIG. 1 represents a multiple line with just one switch).
The oscillator 23 is connected to the start-up stage 30 for supplying a reference clock signal CKR at a reference frequency ωR that is constant and independent of the frequency of oscillation of the driving mass 7. In particular, the reference clock signal CKR is selected so that the reference frequency ωR is near to the driving frequency ωD.
The comparator 25 is coupled to the reading and filtering stage 20 for receiving the first feedback signal VFB1 and is configured to detect the instants of zero crossing of the input. In practice, the output of the comparator 25, which is connected to the PLL circuit 26 and to the start-up stage 30, supplies a natural clock signal CKN, which, in steady-state conditions, is synchronous (in frequency and phase) with the oscillations of the driving mass 7.
The PLL circuit 26 receives the natural clock signal CKN from the comparator 25. An output of the PLL circuit 26 is connected to a clock input 27a of the controller 27 and supplies a main clock signal CKM and a delayed clock signal CK90, respectively in phase and phase-shifted by 90° with respect to the natural clock signal CKN. In practice, the PLL circuit 26 has the function of a frequency detector that senses the current oscillation conditions (frequency and phase) of the driving mass 7 and of the microelectromechanical loop 19. The delayed clock signal CK90 switches in the presence of the peaks of the first feedback signal VFB1. The PLL circuit 26 further supplies a lock signal SLOCK, having a first logic value when the PLL circuit 26 is locked to the oscillation frequency of the microelectromechanical loop 19 and a second logic value otherwise.
The controller 27, for example a P (Proportional), PI (Proportional-Integral) or PID (Proportional-Integral-Derivative) controller, receives the first feedback signal VFB1 and the delayed clock signal CK90 and controls the gain of the variable-gain amplifier 21 through a control signal VC so as to guarantee the condition of oscillation on the amplitude for the microelectromechanical loop 19.
The start-up stage 30 is selectively activatable in response to active values of one or more state signals SST, which may indicate normal or steady operating conditions, power-on conditions or conditions of exit from power-down. State signals SST may be automatically generated by a control unit, herein not illustrated. When activated, the start-up stage 30 controls the forcing stage 31 through a start-up signal SSU, as described hereinafter.
The forcing stage 31 is connectable between the output of the variable-gain amplifier 21 and the driving terminals 13 of the microstructure 2 through start-up switches 33, 34, controlled through an actuation signal SC, supplied by the start-up stage 30. The bypass switches 28 (controlled by the negated actuation signal SCN) enable connection of the output of the variable-gain amplifier 21 directly to the driving terminals 13, excluding the forcing stage 31.
The forcing stage 31 is controlled by the start-up stage 30 through the start-up signal SSU. In particular, the forcing stage 31 is configured to apply forcing signals VF (voltages in one embodiment) to the driving mass 7, in response to the start-up signal SSU.
Here and in what follows, “forcing signals” is generally used to mean sinusoidal signals or sequences of pulses, such as, but not limited to, square-wave pulses, that and are applied to the driving mass 7 for producing an electrostatic force thereon. A forcing frequency ωF of the forcing signals VF is determined by the forcing stage 31 on the basis of the state of the PLL circuit 26 during start-up procedure. Before the PLL circuit 26 locks to the oscillation frequency of the microelectromechanical loop 19 (lock signal SLOCK at the second logic value), the forcing frequency ωF of the forcing signals VF supplied by the forcing stage 31 equals the reference frequency ωR of the reference clock signal CKR. Moreover, a maximum number of cycles of the forcing signals VF is selected so that the phase delay does not counter oscillations of the microelectromechanical loop 19. In one embodiment, the maximum number of cycles is selected so that the overall phase delay of the forcing signals VF with respect to oscillations of the microelectromechanical loop 19 does not exceed π/2.
Once the PLL circuit 26 has locked to the oscillation frequency of the microelectromechanical loop 19 (that is, the lock signal SLOCK has the first logic value and the main clock signal CKM is in phase with oscillations of the microelectromechanical loop 19), the forcing signals VF provided by the forcing circuit 31 are synchronized in frequency and phase with the oscillations of the microelectromechanical loop 19, through the main clock signal CKM. Hence the forcing frequency ωF equals the current oscillation frequency ωC of the microelectromechanical loop 19.
The gyroscope 1 operates as hereinafter described.
In a normal or steady operating mode, the reading and filtering stage 20 and the variable-gain amplifier 21 maintain the microelectromechanical loop 19 in oscillation at the driving frequency ωD by setting appropriate gain and phase conditions, while the forcing stage 31 is excluded and inactive. Oscillation conditions may include unitary loop gain and loop phase of 2kπ (k=0, 1, 2, . . . ) at the driving frequency ωD.
In the case of rotation about a gyroscopic sensing axis, the sensing mass 8 is subjected to a Coriolis acceleration, which is proportional to the angular rate and is transduced into the output signal SOUT by the sensing device 5.
At start-up of the gyroscope 1 or at exit from power-down conditions, a start-up procedure is executed by activating the start-up stage 30 and the forcing stage 31 through the state signals SST. At the same time, the start-up switches 33, 34 and the bypass switch 28 are operated to connect the forcing stage 31 to the capacitive driving units 10 in place of the variable-gain amplifier 21.
The start-up stage 30, through the start-up signal SSU, requests the forcing stage 31 to send forcing signals VF to the driving mass 7, which starts to oscillate with an increasing amplitude.
As illustrated in FIG. 2, at an early stage of the start-up procedure, the PLL circuit 26 has not locked to the oscillation of the driving mass 7 yet (or, generally, to the oscillations of the microelectromechanical loop 19; the lock signal SLOCK has the second logic value, which is low in the example of FIG. 2). In this condition, the forcing stage 31 generates the forcing signals VF at the reference frequency ωR of the reference clock signal CKR. As already mentioned, the maximum number of cycles (i.e. the duration) of the forcing signals VF at the reference frequency ωR is selected to prevent that an overall phase delay φF with respect to oscillations of the driving mass 7 may exceed π/2. The supply of the forcing signals VF is then paused in presence of the second logic value of the lock signal SLOCK and the start-up switches 33, 34 and the bypass switch 28 are operated to connect variable-gain amplifier 21 to the capacitive driving units 10 and to disconnect the forcing stage 31. At this stage, the oscillation of the microelectromechanical loop 19 is controlled by the variable-gain amplifier 21 and the controller 27 and no forcing signals are provided by the forcing stage 31.
The PLL circuit 26 rapidly locks to the oscillations of the microelectromechanical loop 19 (the lock signal SLOCK has the first logic value, high in the example of FIG. 2). In response to the PLL circuit 26 being locked, the forcing stage 31 sets the frequency of the forcing signals VF at the frequency of the main clock signal CKM, that is, at the current oscillation frequency ωC of the microelectromechanical loop 19. Again, the start-up switches 33, 34 and the bypass switch 28 connect the forcing stage 31 to the capacitive driving units 10 in place of the variable-gain amplifier 21.
Thus, forcing signals VF may present an increasing phase delay φF with respect to oscillations of the driving mass 7 only for a very short time interval at the beginning of the start-up procedure. In any case, the phase delay φF does not exceed π/2 because supply of the forcing signals VF is interrupted after the programmed maximum number of cycles.
Once the PLL circuit 26 has locked, the forcing signals VF are synchronous in frequency and phase with the oscillations of the microelectromechanical loop 19 and the phase delay φF remains substantially constant. The forcing stage 31 is configured to maintain the phase delay φF below π/2. In one embodiment (FIG. 2), the forcing stage 31 is configured to cancel the phase delay φF (φF=0) using phase information contained in the main clock signal CKM. In another embodiment, the phase delay φF may be not zero, but it is anyway less than π/2 (FIG. 3).
Since the initial increment of the phase delay φF is arrested as soon as the maximum number of cycles of the forcing signals VF is reached and the forcing signals VF are synchronous with the microelectromechanical loop 19 after the PLL circuit 26 locks, a condition in which the forcing signals VF counter oscillation of the driving mass 7 because of phase mismatch is effectively prevented. Therefore, the supply of forcing signals VF does not need to be interrupted and may be maintained as long as desired to quickly recover from shut-off or power-down condition. For example, forcing signals VF may be provided until the oscillation amplitude of the driving mass 7 exceeds an amplitude threshold ATH. In turn, the amplitude threshold ATH may be a fraction of a target oscillation amplitude, which is to be maintained during normal or steady operating conditions. The amplitude threshold ATH may be selected to provide a safety margin and avoid elongation over the rated target oscillation amplitude, which may cause collisions with the supporting body 6.
FIG. 4 shows an exemplary implementation of the forcing stage 31. In one embodiment, the forcing stage 31 comprises a pulse generator 35 and a two-way switch 36. The pulse generator 35 is selectively activatable by the start-up signals SSU, when start-up conditions or exit from power-down conditions are detected. Moreover, the pulse generator 35 is configured to provide the forcing signals VF and its pulse generation frequency is controlled by a clock signal received on a control terminal 35a. In particular, the pulse generator 35 provides appropriate voltage levels to effectively transfer energy to the driving mass 7. The pulse generator 35 may be temporarily de-activated after the selected maximum number of cycles of the forcing signals VF, when the lock signal SLOCK indicates that the PLL circuit 26 is not locked to the oscillations of the microelectromechanical loop 19, and re-activated afterwards.
The two-way switch 36 receives the reference clock signal CKR and the main clock signal CKM on its inputs and has an output coupled to the control terminal 35a of the pulse generator 35. The two-way switch 36 is controlled by the lock signal SLOCK and is configured to supply the control terminal 35a with the reference clock signal CKR, when the lock signal SLOCK indicates that the PLL circuit 26 is not locked to the oscillations of the microelectromechanical loop 19, and with the main clock signal CKM, when the lock signal SLOCK indicates that the PLL circuit 26 is locked to the oscillations of the microelectromechanical loop 19.
Conditions to terminate the start-up procedure and the supply of the forcing signals VF may be determined by the start-up stage 30, which, in one embodiment illustrated in FIG. 5, comprises a processing stage 40, a threshold generator 41, a comparator 42 and an enable logic gate 43 (e.g. an AND gate). The processing stage 40 receives the first feedback signal VFB1, which is representative of the oscillation conditions of the driving mass 7, and is configured to determine a current oscillation amplitude AC of the microelectromechanical loop 19. The comparator 42 determines an enable signal EN from the comparison of the current oscillation amplitude AC and of the amplitude threshold ATH, which is provided by the threshold generator 41. For example, the comparator 42 is configured to set an enable signal EN to an enable value (e.g. high) when the current oscillation amplitude AC is lower than the amplitude threshold ATH, and to a disable value (low) when the current oscillation amplitude AC reaches or exceeds the amplitude threshold ATH. The enable logic gate 43 receives the enable signal EN and the state signals SST and provide the start-up signal SSU on its output.
Illustrated in FIG. 6 is a portion of an electronic system 100 in accordance with one embodiment of the present disclosure. The system 100 incorporates a microelectromechanical device (for example, but not necessarily, the gyroscope 100) and may be used in devices as, for example, a palm-top computer (personal digital assistant, PDA), a laptop or portable computer, possibly with wireless capacity, a cellphone, a messaging device, a digital music player, a digital camera or other devices designed to process, store, transmit, or receive information. For example, the gyroscope 1 may be used in a digital camera for detecting movements and performing an image stabilization. In other embodiments, the gyroscope 1 is included in a portable computer, a PDA, or a cellphone for detecting a free-fall condition and activating a safety configuration. In a further embodiment, the gyroscope 1 is included in a motion-activated user interface for computers or video-game consoles. In a further embodiment, the gyroscope 1 is incorporated in a satellite-navigation device and is used for simultaneous position tracking in the event of loss of the satellite-positioning signal.
The electronic system 100 can comprise a controller 110, an input/output (I/O) device 120 (for example a keyboard or a display), the gyroscope 1, a wireless interface 140, and a memory 160, of a volatile or nonvolatile type, coupled to one another through a bus 150. In one embodiment, a battery 180 may be used for supplying the system 100. It is to be noted that the scope of the present disclosure is not limited to embodiments having necessarily one or all of the devices listed.
The controller 110 can comprise, for example, one or more microprocessors, microcontrollers, and the like.
The I/O device 120 may be used for generating a message. The system 100 can use the wireless interface 140 for transmitting and receiving messages to and from a wireless communications network with a radiofrequency (RF) signal. Examples of wireless interface can comprise an antenna, and a wireless transceiver, such as a dipole antenna, even though the scope of the present disclosure is not limited from this standpoint. In addition, the I/O device 120 can supply a voltage representing what is stored either in the form of a digital output (if digital information has been stored) or in the form of analog information (if analogue information has been stored).
Finally, it is evident that modifications and variations may be made to the method and to the device described herein, without thereby departing from the scope of the present disclosure.
The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

## Claims

1. A microelectromechanical device comprising:
a body;
a movable mass elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom;
a phase-locked loop configured to detect a current oscillation frequency of the movable mass the phase-locked loop further configured to provide a main clock signal synchronous with oscillations of the moveable mass in a first transient operating condition; and
a forcing stage capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector in the first transient operating condition, the forcing stage generating the forcing signals on the basis of the main clock signal in the first transient operation condition.

2. The microelectromechanical device according to claim 1, wherein the frequency detector is further configured to detect a current oscillation phase of the movable mass and the forcing stage is further configured to provide the forcing signals with a constant phase delay with respect to the current oscillation phase detected by the frequency detector.

3. The microelectromechanical device according to claim 2, wherein the constant phase delay is less than π/2.

4. The microelectromechanical device according to claim 3, wherein the constant phase delay is zero.

5. The microelectromechanical device according to claim 1 further comprising a driving device coupled to the movable mass so as to form a microelectromechanical loop, the driving device configured to maintain the microelectromechanical loop in oscillation at a driving frequency in a steady operating condition.

6. The microelectromechanical device according to claim 5, wherein the microelectromechanical loop includes the frequency detector.

7. The microelectromechanical device according to claim 6, wherein the PLL circuit is configured to be locked to the oscillations of the microelectromechanical loop in the first transient operating condition.

8. The microelectromechanical device according to claim 7, wherein the forcing stage is configured to be controlled by the main clock signal in the first transient operating condition.

9. The microelectromechanical device according to claim 1 further comprising
an oscillator configured to provide a reference clock signal; and
wherein the forcing stage is configured to generate the forcing signals on the basis of the reference clock signal in a second transient operating condition.

10. The microelectromechanical device according to claim 9, wherein the forcing stage is configured to be controlled by the reference clock signal in the first transient operating condition.

11. The microelectromechanical device according to claim 1, wherein the forcing stage comprises a pulse generator configured to be controlled by the frequency detector in the first transient operating condition.

12. The microelectromechanical device according to claim 1, comprising a start-up stage coupled to the forcing stage and configured to terminate supply of the forcing signals based on a comparison between a current oscillation amplitude of the movable mass and a threshold oscillation amplitude.

13. The microelectromechanical device according to claim 1, wherein the forcing signals comprise sequences of square-wave pulses.

14. A microelectromechanical device, comprising:
a body;
a movable mass elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom;
a frequency detector configured to detect a current oscillation frequency of the movable mass; and
a forcing stage capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector, at least in a first transient operating condition;
an oscillator configured to provide a reference clock signal;
wherein the forcing stage is configured to generate the forcing signals on the basis of the reference clock signal in a second transient operating condition; and
wherein the forcing stage is configured to generate the forcing signals in the second transient operating condition with a maximum number of cycles selected so that a phase delay of the forcing signals with respect to oscillations of the movable mass does not exceed π/2.

15. The microelectromechanical device according to claim 14, wherein the frequency detector comprises a PLL circuit.

16. The microelectromechanical device according to claim 15, wherein the PLL circuit is configured to provide a main clock signal synchronous with oscillations of the microelectromechanical loop in the first transient operating condition and the forcing stage is configured to generate the forcing signals on the basis of the main clock signal in the first transient operating condition.

17. An electronic system, comprising:
a control unit; and
a microelectromechanical device coupled to the control unit and including:
a body;
a movable mass elastically coupled to the body and oscillatable with respect to the body according to a degree of freedom;
a frequency detector configured to detect a current oscillation frequency of the movable mass; and
a forcing stage capacitively coupled to the movable mass and configured to provide energy to the movable mass through forcing signals having a forcing frequency equal to the current oscillation frequency detected by the frequency detector in a first transient operating condition and the forcing signals having a reference frequency in a second transient operating condition, the forcing stage configured to terminate the forcing signals having the reference frequency in the second transient operating condition responsive to a phase delay of the forcing signals relative to oscillations of the movable mass exceeding a threshold value.

18. The electronic system according to claim 17, wherein the frequency detector is further configured to detect a current oscillation phase of the movable mass and the forcing stage is further configured during the first transient operating condition to provide the forcing signals with a constant phase delay with respect to the current oscillation phase detected by the frequency detector.

19. The electronic system according to claim 18, wherein the constant phase delay is less than π/2 and wherein the threshold value is π/2.

20. The electronic system according to claim 19, wherein the constant phase delay is zero.

21. The electronic system according to claim 17, comprising a driving device, coupled to the movable mass so as to form a microelectromechanical loop, the driving device configured to maintain the microelectromechanical loop in oscillation at a driving frequency in a steady operating condition.

22. The electronic system according to claim 21, wherein the microelectromechanical loop includes the frequency detector.

23. The electronic system according to claim 21, wherein the frequency detector comprises a PLL circuit.

24. The electronic system according to claim 23, wherein the PLL circuit is configured to provide a main clock signal synchronous with oscillations of the microelectromechanical loop in the first transient operating condition and the forcing stage is configured to generate the forcing signals on the basis of the main clock signal in the first transient operating condition.

25. The electronic system according to claim 24, wherein the PLL circuit is configured to be locked to the oscillations of the microelectromechanical loop in the first transient operating condition.

26. The electronic system according to claim 24, wherein the forcing stage is configured to be controlled by the main clock signal in the first transient operating condition.

27. The electronic system according to claim 17 further comprising:
an oscillator configured to provide the reference clock signal having the reference frequency; and
wherein the forcing stage is configured to generate the forcing signals on the basis of the reference clock signal in the second transient operating condition.

28. The electronic system according to claim 27, wherein the forcing stage is further configured to be controlled by the reference clock signal in the first transient operating condition.

29. The electronic system according to claim 27, wherein the forcing stage is configured to generate the forcing signals in the second transient operating condition with a maximum number of cycles selected so that a phase delay of the forcing signals with respect to oscillations of the movable mass does not exceed π/2.

30. The electronic system according to claim 17, wherein the forcing stage comprises a pulse generator configured to be controlled by the frequency detector in the first transient operating condition.

31. The electronic system according to claim 17, comprising a start-up stage coupled to the forcing stage and configured to terminate supply of the forcing signals based on a comparison between a current oscillation amplitude of the movable mass and a threshold oscillation amplitude.

32. The electronic system according to claim 17, wherein the forcing signals comprise sequences of square-wave pulses.