Abstract:
Detection of an anomalous event in an electronic apparatus includes detecting accelerations acting on the electronic apparatus, establishing a normal-mode range of accelerations that corresponds to normal operation of the apparatus, and detecting the anomalous event when a level of acceleration of the electronic apparatus exits the normal-mode range and remains outside the normal mode range for more than a defined duration. The method includes displacing the normal-mode range toward a current level of acceleration of the electronic apparatus while the level of acceleration remains outside the normal-mode range. Additionally, the normal-mode range is increased toward a maximum size while the level of acceleration remains outside the normal-mode range, and is decreased toward a minimum size while the level of acceleration is within the normal-mode range.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a method and device for detecting anomalous events for an electronic apparatus, in particular a portable apparatus, for example for detecting free-fall events, to which the ensuing treatment will make particular reference, without this implying any loss of generality. 
     2. Description of the Related Art 
     As is known, portable electronic apparatuses can easily be subjected during their normal use to potentially harmful or even destructive events, such as free-fall events, impact, high vibrations, or shock in general. These events are herein defined as “anomalous events”, in so far as they are extraneous to a condition of normal use or operation of portable apparatuses (a so-called “normal mode” condition). In particular, the normal-mode condition is defined by the set of actions (for example, actuation of keys or pushbuttons, displacements, rotations, etc.) and by the external conditions (for example, environmental conditions) normally associated with the use of the portable apparatus. 
     Anomalous events are particularly harmful in the case where the portable apparatuses are provided with a hard-disk unit. In fact, in a hard disk, a read/write head is generally kept at a minimum distance of separation from a storage medium (a magnetic film carried by a rotating disk). In the case of a fall or other shock, the read/write head can collide with the storage medium and thus cause damage to the hard disk and/or irreversible loss of stored data. 
     In order to prevent, or at least limit, occurrence of destructive events, it has been proposed to use detection devices within the portable apparatuses, designed to detect occurrence of a particular anomalous event (for example, a fall or a shock) and to implement suitable actions of protection. For example, once the anomalous event has been detected, it is possible to issue a command for forced “parking” of the read/write head of a hard disk of the portable apparatus, which is brought into a safe area (for example, the position assumed by the head with the apparatus turned off). 
     Detection devices generally operate on the basis of detection of accelerations acting on the portable apparatuses, and are provided, for this purpose, with accelerometric sensors, in particular microelectromechanical (MEMS) inertial sensors made using the semiconductor technologies, which prove advantageous given their small size. 
     As is known, an inertial sensor of a microelectromechanical type in general comprises a mobile mass, suspended above a substrate, and anchored to the substrate and to a corresponding package via elastic elements. Mobile electrodes are fixedly coupled to the mobile mass, and fixed electrodes, capacitively coupled to the mobile electrodes, are fixedly coupled to the substrate. In the presence of an acceleration, the mobile mass undergoes a displacement with respect to the substrate, which brings about a capacitive variation of the capacitor formed between the mobile electrodes and the fixed electrodes. Starting from the capacitive variation, via appropriate processing operations, it is possible to determine the value of the acceleration acting on the sensor. In particular, also in conditions of rest, the inertial sensor detects a non-zero acceleration, due to the effect of the Earth&#39;s gravitational acceleration (g), which determines in any case a displacement of the mobile mass with respect to a position of equilibrium. 
     Detection of anomalous events by the protection devices is generally based on the result of the comparison between an instantaneous absolute value of acceleration, measured by the corresponding inertial sensors, and one or more acceleration thresholds. 
     For example, in the case of the free-fall event, the portable apparatus, and the inertial sensor fixed thereto, are to a first approximation subject only to the Earth&#39;s gravitational acceleration (g). In this condition, both the mobile mass and the package of the inertial sensor are subjected to the same acceleration (g) so that the displacement of the mobile mass with respect to a reference system fixed with respect to the package, also in free-fall conditions, is zero, as also is the acceleration detected. Consequently, a free-fall condition is detected when the resultant acceleration detected is lower than a pre-set threshold, or, alternatively, when the values of the individual components of acceleration detected along a first axis, a second axis, and a third axis of a set of three Cartesian axes fixed with respect to the inertial sensor drop simultaneously below the pre-set threshold. In its simplest embodiment, the algorithm for free-fall detection hence requires only definition of a threshold value and comparison of the instantaneous values of acceleration with this threshold. Usually, a control is added on the time length of the free-fall event detection so as to reject false detections that have a duration shorter than a given time interval (for example, 90 ms). 
     A shock acting on the portable apparatus is detected in a substantially similar way by verifying that a given threshold has been exceeded by the instantaneous acceleration acting on the portable apparatus. 
     BRIEF SUMMARY 
     According to an embodiment of the present invention, a method for detecting anomalous events in an electronic apparatus is provided, that includes detecting accelerations acting on the electronic apparatus, establishing a normal-mode range of accelerations that corresponds to normal operation of the apparatus, and detecting the anomalous event when a level of acceleration of the electronic apparatus exits and remains outside the normal-mode range for more than a defined duration. The method includes displacing the normal-mode range toward a current level of acceleration of the electronic apparatus while the level of acceleration remains outside the normal-mode range. Additionally, the normal-mode range is increased toward a maximum size while the level of acceleration is outside the normal-mode range, and is decreased toward a minimum size while the level of acceleration is within the normal-mode range. 
     According to an embodiment, if the acceleration exits the normal-mode range for a first duration, returns to a normal-mode condition for a second duration, then exits the normal-mode condition again, for a third duration, and if the second duration, in the normal-mode condition, is less than a duration threshold, the first and third durations are combined for the purpose of detecting an anomalous event. 
     According to another embodiment, a device for detecting anomalous events is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present disclosure, preferred embodiments are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  shows a simplified block diagram of a portable apparatus provided with a device for detecting anomalous events, according to an embodiment; 
         FIG. 2  shows a circuit block diagram of the device for detecting anomalous events of  FIG. 1 ; 
         FIGS. 3-5  show representations, in the space of accelerations, of accelerations acting on the portable apparatus of  FIG. 1  and of threshold regions associated therewith; 
         FIG. 6  is a flowchart corresponding to operations implemented by the device for detecting events of  FIG. 2 ; 
         FIGS. 7 and 8  show further representations of accelerations acting on the portable apparatus  1 ; and 
         FIG. 9  is a flowchart corresponding to operations implemented by the device for detecting events of  FIG. 2 , according to a different embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Known methods for detection of anomalous events have a series of problems that may jeopardize their reliability. In particular, two factors of disturbance can alter the identification of an anomalous event: a significant offset of the inertial sensor, i.e., a case in which the sensor indicates some acceleration value while at rest; and, at least in the case of identification of free fall, the rotation of the portable apparatus during the fall (the “roll” effect). 
     The first factor of disturbance is intrinsic to the inertial sensor and reflects upon the final measurements of acceleration, the offset being added algebraically to the current value of the acceleration acting on the sensor. In the case where the offset value is comparable with the value of the pre-set acceleration threshold, there is the risk that the acceleration detected will not drop below the threshold, and that the free-fall event will hence not be identified. This situation is defined as a “missed event” and is particularly dangerous in so far as it does not enable the necessary protections on the portable apparatus to be carried out. Furthermore, there may be offset values such as to cause a detection of an anomalous event, for example a free fall event, even in the case where the portable apparatus is kept in a normal-mode condition. This situation is defined as “false trigger” and, even though it does not jeopardize integrity of the portable apparatus, may jeopardize operation thereof by, for example, repeatedly parking the read/write head of the hard disk during normal operation of the portable apparatus. In addition, the value of the offset can undergo variations over time, for example on account of ageing and/or structural modifications of the inertial sensor, thus rendering a possible compensation thereof problematical. 
     The second factor of disturbance is linked to the position of the inertial sensor on board the portable apparatus and to the displacement of the portable apparatus itself during the anomalous event (for example, during the free fall), and is caused by the centrifugal acceleration that may be generated upon rolling of the portable apparatus. In a known way, the centrifugal acceleration A c  is given by the expression:
 
 A   C   =r ·(2·π·Φ) 2  
 
where r is the distance of the inertial sensor from the center of mass of the portable apparatus, and Φ is the number of revolutions per second performed by the portable apparatus.
 
     The contribution of the centrifugal acceleration A c  is not in general negligible and can alter detection of the anomalous event. For example, considering a distance r of 5 cm and one revolution of the portable apparatus every two seconds, a centrifugal acceleration A c  of approximately 0.80 g is developed. The contribution of the centrifugal acceleration can consequently be such that one or more of the components of acceleration acting on the portable apparatus does not drop below the pre-set threshold in the case of free fall, and the anomalous event is not detected. Likewise, the additional contribution due to the centrifugal acceleration can cause false detections of anomalous events. 
     In order to solve the above problems and to increase the reliability of detection of anomalous events, the use has been proposed of additional electronics in the detection devices such as to eliminate the contribution of the aforesaid factors of disturbance (by means of appropriate processing of the acceleration signals). This solution entails, however, an increase in the complexity and cost of the detection devices and of the corresponding portable apparatuses and, in the case where detection of anomalous events is entirely entrusted to a main microprocessor of the portable apparatus, an even considerable decrease in the corresponding performance. 
     As shown in  FIG. 1 , a portable apparatus  1 , for example a portable computer (laptop), a PDA (Personal Data Assistant), a digital audio player, a mobile phone, a digital camcorder or photographic camera, a satellite navigator, or the like, includes a microprocessor control circuit  2 , configured to control the general operation thereof, and a hard-disk device  3 , operatively coupled to the microprocessor control circuit  2  for writing and reading data. In a per-se known manner, the hard-disk device  3  is provided with a read/write head  4  and a storage medium  5 , associated with the read/write head  4 . 
     The portable apparatus  1  further comprises a detection device  6  for detecting anomalous events associated with the portable apparatus, the detection device being provided for this purpose with: an accelerometer sensor  7 , configured to detect one or more acceleration signals Acc corresponding to accelerations acting on the portable apparatus  1 ; and a processing circuit  8 , connected to the accelerometer sensor  7  and configured to process the acceleration signals Acc in order to detect an anomalous event. For example, the detection device  6  can be configured to generate, when detecting an anomalous event, an interrupt signal INT, and to supply it, continuously and in real time, to the microprocessor control circuit  2  for enabling immediate activation of appropriate actions of protection (for example, parking of the read/write head  4  of the hard-disk device  3 ). Alternatively (and as is indicated schematically by the dashed arrow  9  of  FIG. 1 ), the detection device  6  can be configured to directly carry out appropriate actions of protection, and to issue suitable control signals, e.g., for parking the read/write head  4 . 
     The detection device  6  is integrated within the portable apparatus  1 ; for example, it can be provided in a chip and coupled to a printed circuit set within the portable apparatus  1 , to which also the microprocessor control circuit  2  is coupled. 
     In greater detail, the accelerometer sensor  7  is a MEMS inertial sensor, made, for example, as described in “A Low-g 3 Axis Accelerometer for Emerging Automotive Applications”, B. Grieco et al., AMAA 2004, which detects the components of the acceleration acting on the portable apparatus  1  directed along the three axes X, Y, Z of a set of three Cartesian axes fixed with respect to the portable apparatus  1 , and supplies respective acceleration signals Acc x , Acc y , and Acc z  (designated as a whole as acceleration signals Acc). 
     According to an aspect of the present invention, the detection device  6  is configured to monitor a normal-mode condition of the portable apparatus  1  and to detect an anomalous event as an exit from the normal-mode condition. In particular, as will be described in detail hereinafter, the detection device  6  considers the variations of an acceleration acting on the portable apparatus  1  (for example, variations between a current value and a previous value of acceleration) and detects a residence within the normal-mode condition when these variations are below a pre-set threshold. The threshold value depends, for example, on the type of portable apparatus, on the type of movements (and accelerations), and on the environmental conditions normally associated with its use. Preferably, the detection device  6  also evaluates the duration of residence of the portable apparatus  1  outside the normal-mode condition, and detects an anomalous event when this duration is longer than a pre-set time interval (the value of which is again a function of the type and conditions of use of the portable apparatus). 
       FIG. 2  shows an embodiment of the processing circuit  8  according to a first embodiment of the present invention. 
     In detail, the processing circuit  8  comprises: a first filtering stage  10 , implementing a low-pass (LP) filter of a finite-impulse-response (FIR) type, which is connected to the accelerometer sensor  7  and supplies at output a first filtered acceleration signal Acc 1  (reference is made here, for simplicity, to the acceleration signal Acc as a whole and not to the corresponding acceleration components Acc x , Acc y  and Acc z , but it is clear that similar considerations apply to the acceleration components); a second filtering stage  11 , implementing a low-pass infinite-impulse-response (IIR) filter, which is connected to the accelerometer sensor  7 , supplies at output a second filtered acceleration signal Acc 2 , and has a cut-off frequency lower than that of the aforesaid FIR filter; and a combination stage  12 , which has a first input and a second input, respectively connected to the output of the first filtering stage  10  and of the second filtering stage  11 , and supplies at output a combination signal, which is a function of the first and second filtered acceleration signals. In particular, the combination stage  12  implements a subtractor stage and is configured to supply an acceleration-variation signal ΔAcc (shown in  FIG. 4 ), given by the difference between the first filtered acceleration signal Acc 1  and the second filtered acceleration signal Acc 2 . 
     The processing circuit  8  further comprises: a comparator stage  14 , connected to the output of the combination stage  12  and configured to compare the value of the acceleration-variation signal ΔAcc (in particular a magnitude thereof, given by the combination of the contributions of the components along the three axes X, Y and Z) with an acceleration threshold THA, having a value that can vary, as will be described hereinafter, between a minimum value THA 1  and a maximum value THA 2 , and to supply at output a comparison signal having a logic value; a threshold-variation stage  15 , connected to the output of the comparator stage  14  and configured to vary the value of the acceleration threshold THA as a function of the value of the comparison signal; a counter stage  16 , which is connected to the output of the comparator stage  14 , receives the comparison signal at a count-enable input, and supplies at output a count signal; and a detection stage  18 , which is connected to the output of the comparator stage  14  and of the counter stage  16  and is configured to supply at output the interrupt signal INT according to the value of the comparison signal and of the count signal. 
     The threshold-variation stage  15  is configured to vary dynamically the value of the acceleration threshold THA, gradually between the minimum value THA 1  and the maximum value THA 2 , or vice versa (according to whether the comparison signal switches from a first value, e.g., a low value or logic “0”, to a second value, e.g., a high value or logic “1”, or vice versa). 
     The detection stage  18  is configured to set the interrupt signal INT to a given value (for example, a high logic value) upon detection of an anomalous event. In particular, the anomalous event is detected when the comparison signal has a given value, in particular a high value, and moreover the count signal is higher than a given time threshold THT (which can, for example, be set from outside by a user). 
     Operation of the processing circuit  8  for detecting an anomalous event is now described with reference to  FIGS. 3-5 , which show a three-dimensional space of accelerations in which the acceleration signals Acc and the acceleration-variation signal AAcc are represented. 
     In particular, the instantaneous value of the first filtered acceleration signal Acc 1  is represented by a first point P within the space of accelerations (with co-ordinates corresponding to its acceleration components Acc x , Acc y  and Acc z ), and its magnitude is represented by the distance of the first point P from the origin O of the axes. Variations of the resultant acceleration acting on the portable apparatus  1  correspond to displacements of the first point P within the space of accelerations. The instantaneous value of the second filtered acceleration signal Acc 2  is represented by a second point P′, which also is the center of a spherical region having a radius equal to the minimum value THA 1  of the acceleration threshold THA. The spherical region represents a threshold region indicating a normal-mode condition of the portable apparatus  1  and is consequently referred to in what follows as normal-mode region  20 . Variations of acceleration that keep the first point P within the normal-mode region  20  (and hence the acceleration-variation signal ΔAcc lower than the acceleration threshold THA) are indicative of a normal operation of the portable apparatus  1 . In addition, in a condition of rest of the portable apparatus  1  (as shown in  FIG. 3 ), the second point P′ coincides with the first point P. 
     A variation of acceleration ( FIG. 4 ) of anomalous extent (for example, due to a free-fall condition, to a violent shock, or to high vibrations) causes the point P to exit from the normal-mode region  20 . The acceleration-variation signal ΔAcc exceeds the acceleration threshold THA, and the comparison signal at output from the comparator stage  14  goes to the high value. This event is interpreted by the processing circuit  8  of the detection device  6  as potentially indicating an anomalous event. However, for an anomalous event to be actually detected, residence outside the normal-mode condition has still to be verified for a pre-set period of time (the duration check) equal to the time threshold THT. The counter stage  16  is consequently enabled for counting by switching of the comparison signal. 
     Given that the second point P′ is a result of the filtering operation carried out in the second filtering stage  11  by the IIR filter having a cut-off frequency lower than the FIR filter of the first filtering stage  10 , the second point P′ follows the displacements of the first point P with a certain delay. In general, the first point P represents a current value, whilst the second point P′ represents a previous value of the acceleration acting on the portable apparatus  1 . Thus, the normal-mode region  20 , defined by the second point P′, follows, with a delay, the displacement of the first point P. It should be noted that the rate of displacement of the normal-mode region  20  depends on the implementation of the IIR filter and its different cut-off frequency with respect to that of the FIR filter (in particular, and in a per-se known manner, it depends on a “weight” attributed to a current sample of the acceleration data as compared to that attributed to one or more previous samples). 
     Switching of the comparison signal causes the threshold-variation stage  15  to vary the acceleration threshold THA to the maximum value THA 2 . The normal-mode region  20  is consequently enlarged, and its radius is increased until it becomes (either instantaneously or gradually) equal to the maximum value THA 2 . In addition, switching of the comparison signal enables the counter stage  16  and starts the count signal. When the value of the count signal becomes higher than the time threshold THT, the detection stage  18  identifies an anomalous event and sets (for example to the high logic value) the interrupt signal INT. 
     The normal-mode region  20  then moves until its center (second point P′) coincides again with the first point P ( FIG. 5 ) (corresponding to a new condition of rest of the portable apparatus  1 ). 
     The processing circuit  8  detects re-entry of the first point P within the normal-mode region  20  upon switching of the comparison signal to the low value, and brings the interrupt signal INT back again to the low value. This event causes blocking of the count signal at output from the counter stage  16 , thus defining the total duration of residence in an anomalous operating condition, and also the return of the normal-mode region  20  to the original dimensions, with a radius equal to the minimum value THA 1  of the acceleration threshold THA. 
     In particular, the enlargement of the normal-mode region  20  and the increase of the corresponding radius from the minimum value THA 1  to the maximum value THA 2  advantageously prevents repeated variations of acceleration having a value slightly higher than the minimum value THA 1  (i.e., located at the edge of the spherical region, in the space of accelerations) from being interpreted as anomalous events and giving rise to false-triggering events. In addition, as may be readily understood, variations of acceleration of a high value (and hence potentially dangerous) entail a greater duration of the time interval for re-entry of the first point P within the normal-mode region  20  and hence give rise to the identification of anomalous events. 
     In brief, and with reference to the flowchart of  FIG. 6 , the algorithm described envisages at start an initialization step (block  21 ), in which, in particular, the normal-mode region  20  is centered on the first point P (so that upon turning-on no anomalous operating condition is detected). 
     Next, a continuous monitoring of the value of the acceleration-variation signal ΔAcc is implemented (block  22 ), which is indicative of the variation of acceleration between a current condition and a previous condition of the portable apparatus  1  so as to verify residence of the portable apparatus  1  in a normal-mode condition (block  23 ). 
     As soon as a variation of acceleration is detected higher than (or equal to) the acceleration threshold THA (initially set at the minimum value THA 1 ), which indicates exit from the normal-mode condition, the value of the acceleration threshold is varied gradually from the minimum value THA 1  to the maximum value THA 2  (block  24 ), and a count (CONT) of the duration of residence in the anomalous condition of operation (block  25 ) is started. 
     The procedure then waits for the return of the portable apparatus  1  into the normal-mode condition, while the acceleration-variation signal ΔAcc (block  26 ) is again monitored and compared with the acceleration threshold THA (block  27 ). 
     During residence in the anomalous operation condition, a check is made as to whether the count value (which is incremented as a function of an internal clock) exceeds the time threshold THT (block  28 ). If it does, an anomalous event is detected, and the interrupt signal INT is set to the high value (block  29 ). Then the procedure again waits for the return of the device into a normal-mode condition (blocks  26  and  27 ). 
     Upon entry into the normal-mode condition, the value of the acceleration threshold is gradually varied from the maximum value THA 2  to the minimum value THA 1  (block  30 ), and the interrupt signal INT returns to the low value (block  31 ). The algorithm then returns into a state of waiting for a new anomalous event (block  22 ). 
     The advantages that the device and the method for detecting anomalous events enable are clear from what has been described and illustrated above. 
     In particular, by monitoring exit of the portable apparatus  1  from a normal-mode condition, it is possible to detect a generic anomalous event, unlike traditional methods, which are focused on the identification of a particular type of anomalous event (for example, a free fall or a shock). The safety and possibility of protection of the portable apparatuses in the case of anomalous events is thus increased. In addition, the condition of normal use can advantageously be set and configured from outside, enabling adaptation to the particular type of portable apparatus in which the detection device is incorporated and to the environmental conditions associated with the use of the same portable apparatus. In the circuit embodiment described, it is possible, for example, to vary the minimum and maximum values of the acceleration threshold THA, the value of the time threshold THT, and the parameters and the cut-off frequency of the IIR filter in the second filtering stage  11 . For example, in order to limit the number of false detections, the difference between the maximum value and the minimum value of the acceleration threshold THA will be generally greater when the portable apparatus is used in a noisy environment, or else, the speed of the IIR filter will be generally higher for a portable apparatus, e.g., a mobile phone, which is subject to displacements of a greater amount. 
     Checking the exit from the normal-mode condition enables monitoring (by successive comparisons with one or more thresholds) not of the particular value of the accelerations acting on the portable apparatus  1 , but rather of the variations of the acceleration (or in other words the displacements in the space of accelerations). In this way, the problems associated with the presence of offsets in the accelerometer sensor  7  and with the development of centrifugal acceleration (roll effect) are avoided. In fact, as represented in  FIGS. 7 and 8 , which again refer to the space of accelerations, an offset or a centrifugal acceleration will not alter detection of an anomalous event (in the specific case, a free fall). In detail ( FIG. 7 ), an offset of the accelerometer sensor  7  causes a variation of the acceleration detected in the resting condition (point P off  displaced with respect to the ideal position, designated by P). The offset also causes a change in the position of arrival in the case of free fall, which does not coincide with the origin of the axes (the position in the absence of offsets is represented with a dashed line). The presence of offsets does not alter, however, the degree of the variation of acceleration that occurs upon the free fall. Consequently, the described algorithm, by comparing the variation of acceleration (or the displacement of the point P), and not the absolute value of acceleration, is not altered. Likewise ( FIG. 8 ), the presence of a centrifugal acceleration varies the point of arrival of the point P after free fall, which does not reach the origin of the axes, but a different point, designated by P ROLL . Advantageously, also in this case, the algorithm detects exit of the point P from the normal-mode region  20  (and hence the anomalous event), without being deceived by development of the centrifugal acceleration. 
     It is therefore possible to detect anomalous events associated with the portable apparatus  1  with a high degree of reliability, and hence greatly improve the protection of the portable apparatus. In addition, the circuit complexity and the consequent area occupation of the detection device  6  (in the case of hardware implementation), or the computing power required by the detection algorithm (in the case of software implementation) are extremely limited. 
     Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
     In particular, the algorithm implemented by the detection device  6  can envisage that, upon re-entry into the normal-mode condition, the interrupt signal INT goes back again to the low value not instantaneously, but with a certain delay, given by a further count (which can also be implemented by the counter stage  16 ). This variant is particularly advantageous for remaining in a state of alert and monitoring the anomalous event also in the case where the portable apparatus returns for a short period into a normal-mode condition and then exits again therefrom on account of a further anomalous event that is close in time. This situation can, in fact, indicate an imminent impact or other destructive event (that the algorithm described previously might not detect with a sufficient readiness). 
       FIG. 9  shows a flowchart regarding this variant, which basically coincides with the one described in  FIG. 6 , except as regards a reset procedure of the interrupt signal INT. In detail, upon return of the portable apparatus  1  into the normal-mode condition (output YES from block  27 ), in addition to the variation of the acceleration threshold value from the maximum value THA 2  to the minimum value THA 1  (block  30 ), a further count (CONT′) is started for detecting the duration of residence in the normal-mode condition (block  32 ). Then the value of the acceleration-variation signal ΔAcc is monitored again (block  33 ) so as to verify residence of the portable apparatus  1  in a normal-mode condition (block  34 ). During the residence in this condition, a check is made as to whether the value of the further count exceeds a further time threshold THT′ (block  35 ). If it does, the interrupt signal INT is brought back again to the low value (block  31 , described previously), and the algorithm returns into a state of wait for a new anomalous event (block  22 ). Instead, if a new exit from the normal-mode condition is determined (output NO from block  34 ), the algorithm returns to block  28  so as to continue monitoring of the anomalous event (basically, the momentary return of the portable apparatus  1  into a normal-mode condition is ignored by the algorithm). 
     In addition, the processing circuit  8  may be modified so as to determine not only the occurrence of an anomalous event, but also the type of the anomalous event, by means of additional processing operations on the acceleration signals Acc. For example, in order to identify a free-fall event, the processing circuit  8  can be configured to determine a decrease of the magnitude of the acceleration signal Acc in the current condition, as compared to the previous condition, higher than a given threshold (in the case of free fall, in fact, the components of the acceleration signal Acc ideally go to zero). 
     The variation of the acceleration signal can be determined in a way different from what has been illustrated and described. In addition, filtering of the acceleration signal Acc in the first filtering stage  10  can be omitted. 
     The further control on the duration of residence of the portable apparatus  1  in an anomalous condition of operation can be omitted in given applications and with a given choice of the minimum and maximum values of the acceleration threshold THA. 
     In addition, the processing circuit  8  can be modified so as to implement different acceleration thresholds along the axes X, Y, Z, and the normal-mode region  20  can consequently have a shape different from the spherical shape described and illustrated. 
     As used in the specification and claims, the term residence refers to an acceleration condition of a subject device. For example: a level of acceleration of a device within a defined acceptable range of acceleration may be referred to as residence of the device within a normal-mode condition; and elapsed time during which acceleration of the device is outside a defined normal-mode condition may be referred to as duration of residence outside the normal-mode condition or duration of residence in an anomalous operation condition. 
     The abstract of the present disclosure is provided as a brief outline of some of the principles of the disclosure, and is not intended as a complete or definitive description of any embodiment thereof, nor should it be relied upon to define terms used in the specification or claims. The abstract does not limit the scope of the claims. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet 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.