Patent Publication Number: US-2023152126-A1

Title: Device and method for touch sensing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Pat. Application No. 17/171,726, filed Feb. 9, 2021, which application claims the benefit of French Patent Application No. 2001298 filed on Feb. 10, 2020, which applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally concerns electronic devices and methods, and more particularly devices comprising a touch sensor and associated methods. 
     BACKGROUND 
     Many touch sensors are known. One of the advantages of touch sensors is that they enable to have a device leave a low-power operating mode when a contact is established. 
     SUMMARY 
     An embodiment provides a method of detection of a touch contact by a sensor, comprising: a first step of comparison of a voltage with a first voltage threshold; and a second step of comparison of the voltage with a second voltage threshold, the second step being implemented if the first voltage threshold has been reached within a duration shorter than first duration threshold, the second voltage threshold being higher than the first voltage threshold. 
     According to an embodiment, the voltage is the voltage across a capacitor. 
     According to an embodiment, during each comparison step, the capacitor is charged by the repeating of the steps of: charging a touch region; and discharging the region into the capacitor. 
     According to an embodiment, the sensor comprises a plurality of touch regions. 
     According to an embodiment, a touch contact is detected when the first comparison step and the second comparison step are carried out and the second voltage threshold has been reached within a duration shorter than a second duration threshold. 
     According to an embodiment, the method comprises at least a third step, preferably two third steps, of comparison of the voltage with the second voltage threshold, if the second voltage threshold has been reached, during the previous step, within a duration shorter than a second duration threshold. 
     According to an embodiment, a touch contact is detected when the first comparison step, the second comparison step, and the at least one third comparison step are carried out and the second voltage threshold has been reached, during the last third comparison, within a duration shorter than the second duration threshold. 
     According to an embodiment, at least one of the voltage thresholds is supplied by a digital-to-analog converter. 
     According to an embodiment, at least one of the voltage thresholds is the threshold of a Schmitt trigger. 
     According to an embodiment, each comparison step is followed by the discharge of the capacitor. 
     Another embodiment provides an operating method of an electronic device, wherein the previously-described touch contact detection method is carried out during a low-power operating mode. 
     According to an embodiment, the detection of a touch contact causes the leaving of the low-power operating mode. 
     According to an embodiment, the touch contact detection method is regularly implemented during the low-power operating mode. 
     Another embodiment provides a touch sensor comprising a comparison circuit configured to compare a voltage with first and second voltage thresholds. 
     According to an embodiment, the sensor comprises a capacitor, the voltage across the capacitor being the voltage compared by the comparison circuit. 
     According to an embodiment, the sensor comprises a contact region configured to be charged and to be discharged into the capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG.  1    shows an example of an electronic device of the type for which the embodiments described in relation with  FIGS.  3  to  5    are intended; 
         FIG.  2    shows an embodiment of a touch sensor; 
         FIG.  3    shows an embodiment of an operating method of the device of  FIG.  2   ; 
         FIG.  4    shows an example of variations of a voltage of the device of  FIG.  1   ; and 
         FIG.  5    shows another embodiment of a touch sensor. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties. 
     For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. 
     Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements. 
     In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures. 
     Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%. 
       FIG.  1    shows an example of an electronic device  10  of the type for which the embodiments described hereinafter in relation with  FIGS.  2  to  5    are intended. 
     Device  10  comprises a touch sensor, that is, a circuit configured to detect the contact of a portion of a body, for example, a human body, for example, the contact of a finger  16 , with a region  12 , located in a package  14 . 
     Device  10  is for example a wireless device, for example, a device powered by a battery or a cell. Further, device  10  is for example a device capable of being inactive for long periods. Device  10  is for example a remote control or a switch. Device  10  is for example a wireless headphone. 
     Region  12  for example corresponds to a button, having its activation, that is, its contact with finger  16 , causing the activation of one or a plurality of functionalities of device  10  and/or the leaving of a low-power operating mode. 
     The power consumption of device  10  is often a significant criterion in the design of device  10 . Thus, during idle periods, device  10  enters the so-called low-power operating mode. In such an operating mode, for example, standby, certain components and circuits of device  10  are not operating and are not powered, to keep energy. 
     During low-power operating periods, the circuit of detection of the contact with region  12  is generally maintained active. Thus, the detection of a contact between finger  16  and region  12  is generally used as a signal commanding the leaving of the low-power operating mode. 
       FIG.  2    shows an embodiment of a touch sensor  20 . 
     Sensor  20  comprises a contact region  22 . Contact region  22  for example corresponds to the region  12  of  FIG.  1   . Sensor  20  is thus configured to detect a physical contact between a user’s finger and region  22 , for example, a user’s finger and region  22 . 
     Region  22  for example corresponds to a conductive layer, or pellet, covered with an insulating layer. Region  22  is preferably a metal pellet, for example, made of copper, covered with an electric insulator, for example, glass. Region  22  is, due to its forming, a portion of a capacitor Cin having a relatively low capacitance value, however sufficiently high to store charges. More particularly, the conductive pellet corresponds to a first electrode of capacitor Cin. When the user comes into contact, preferably by his/her finger, with region  22 , the user’s part in contact with region  22  forms the second electrode of capacitor Cin. Capacitor Cin is thus formed by the assembly comprising region  22  and the user, the capacitance value of capacitor Cin then being greater than the capacitance value of capacitor Cin in the absence of a contact. Thus, the value of capacitance Cin increases when there is a contact with region  22  and the user. In other words, the number of charges capable of being stored in capacitor Cin increases when a user touches region  22 . Hereinafter, region  22 , be it in or not in contact with the user, may be called “capacitor  22 ”. 
     Region  22  is coupled to a node  24  of application of a power supply voltage of sensor  20  via a switch  26 . Preferably, switch  26  is formed by one or a plurality of transistors. More particularly, the region is coupled, preferably connected, to a node  27 . Transistor  26  is coupled, preferably connected, to node  27  by one of its conduction terminals and to node  24  by its other conduction terminal. 
     Further, region  22  is coupled to a node  28  of application of a reference voltage of sensor  20 , preferably the ground, via a switch  30 . Preferably, switch  30  is formed by one or a plurality of transistors. Transistor  30  is coupled, preferably connected, to node  27  by one of its conduction terminals and to node  28  by its other conduction terminal. 
     Thus, capacitor Cin may be charged with the power supply voltage via switch  26 , switches  26  and  30  being respectively turned on and off during the charge. Similarly, capacitor Cin may be discharged onto node  28  via switch  30 , switches  26  and  30  being respectively turned off and on during the discharge. In other words, the charges of capacitor Cin are transferred to node  28  via switch  30 . 
     Sensor  20  further comprises a capacitive element  32 , for example, a capacitor. The capacitance of element  32  is greater than the capacitance of capacitor Cin in the presence of a contact with the user. Preferably, the capacitance of element  32  is equal to at least 1,000 times the maximum capacitance of capacitor Cin, for example, from 1,000 to 10,000 times the maximum capacitance of capacitor Cin. The capacitance of capacitor Cin is for example in the order of a few picofarads, for example, substantially equal to 1 pF. 
     Capacitive element  32  is coupled, preferably connected, to node  28  by one of its terminals, and to a node  34  by another one of its terminals. 
     Node  34  is coupled to node  24  via a switch  36 . Preferably, switch  36  is formed by one or a plurality of transistors. More precisely, transistor  36  is coupled, preferably connected, to node  34  by one of its conduction terminals and to node  24  by its other conduction terminal. 
     Further, node  34  is coupled to node  28  via a switch  38 . Preferably, switch  38  is formed by one or a plurality of transistors. Transistor  38  is coupled, preferably connected, to node  34  by one of its conduction terminals and to node  28  by its other conduction terminal. 
     Sensor  20  further comprises a switch  40 . Switch  40  is coupled to node  27  by one of its terminals and to node  34  by its other terminal. 
     According to an embodiment, sensor  20  may also comprise a switch  41 . Switch  40  is coupled, preferably connected, to node  27  by one of its terminals and to node  34  by its other terminal. Switches  40  and  41  are for example controlled in the same way. In other words, switches  40  and  41  are for example turned off and turned on at the same times. As a variation, switch  41  may be maintained on during the sensor operation. 
     Sensor  20  further comprises a comparator  42 . Comparator  42  receives at an input  44  the voltage of node  34 . Node  34  is thus coupled, preferably connected, to input  44 . Another input  46  is coupled, preferably connected, to a reference voltage generation circuit  48 . The comparator thus compares the voltage on node  34  with a reference voltage delivered by circuit  48 . 
     Circuit  48  is configured to deliver first (S 1 ) and second (S 2 ) reference voltages, or voltage thresholds, of different values, the value of the first voltage being smaller than the value of the second voltage. The value of the first voltage is for example in the range from 50% to 95% of the value of the second voltage, preferably from 75% to 95% of the value of the second voltage, preferably equal to approximately 90% of the value of the second voltage. The value of the first voltage threshold is for example substantially equal to the value of the power supply voltage applied to node  24  divided by 4 and the value of the second voltage threshold is for example, equal to the value of the power supply voltage applied to node  24  divided by 2. 
     For example, circuit  48  comprises a digital—to-analog converter, configured to deliver a voltage of variable value. For example, circuit  48  may deliver a finite number of fixed voltages. 
     According to an embodiment, sensor  20  may comprise a plurality of capacitors Cin, that is, a plurality of distinct regions having their contact with a user capable of being detected by sensor  20 . Each region  22  is coupled, like the region  22  shown in  FIG.  2   , to input  44  by a switch  40 . In other words, input  44  is coupled to a plurality of regions  22 . Further, each region  22  is coupled to nodes  24  and  28  by switches  26  and  30 , as shown in  FIG.  2   . In other words, according to an embodiment, sensor  20  comprises a plurality of regions  22  and as many switches  26 , switches  30 , and switches  40  as regions  22 . Preferably, sensor  20  implements an operating method which will be described in relation with  FIG.  3    for the different regions  22  one after the others. In this case, switch  41  is for example maintained on. 
       FIG.  3    shows an embodiment of an operating method of the device of  FIG.  2   . 
     Sensor or device  20  is considered as being in a low-power operating mode (block  50  - LP). During the low-power operating mode, the touch sensor regularly verifies whether there is a touch contact between the device and a user. 
     Thus, during the low-power mode, the sensor starts a first acquisition (block  52  -START ACQUI1). At the beginning of the acquisition, capacitor  32  is preferably set to a reference charge level, preferably fully discharged. In other words, the charges contained by capacitor  32  are preferably all transferred to node  28 , that is, to ground. Preferably, capacitor  22  is also set to a reference charge level, preferably fully discharged. During this acquisition, capacitor  22  ( FIG.  2   ) is fully charged, and is then discharged into capacitor  32 . The steps of charge and discharge of capacitor  22  are repeated until capacitor  32  is fully charged. 
     During a first phase of the acquisition, capacitors  22  and  32  are discharged. In this example, switches  26  and  36  are off. Further, switch  40  and switch  41  are on. Switch  30  and/or switch  38  is turned on, to be able to discharge capacitors  22  and  32  into node  28 . 
     During a second phase of the acquisition, switch  40  is maintained off, to be able to modify the charge levels of capacitors  22  and  32  independently from each other. During the second phase of the acquisition, switch  26  is then turned on and switches  30 ,  36 ,  38 , and  40  are turned off. Thus, capacitor  22  is fully charged by node  24 . 
     During a third phase of the acquisition, capacitor  22  is discharged into capacitor  32 . For this purpose, switch  40  is turned on. Further, switches  26 ,  30 ,  36 , and  38  are turned off. 
     The different phases of the acquisition may be separated by idle periods, during which switches  26 ,  30 ,  36 ,  38 , and  40  are all off. Such idle periods preferably have a duration shorter than a few hundreds of nanoseconds, for example, shorter than 50 ns. 
     Other off and on configurations of the different switches may of course be implemented to carry out the three distinct phases of the acquisition, that is, the discharge of capacitors  22  and  32 , the charge of capacitor  22 , and the discharge of capacitor  22  into capacitor  32 . For example, during the first phase, capacitors  22  and  32  may be discharged successively rather than simultaneously. For example, during the first phase, switch  40  may be off during the discharge, switches  30  and  38  then being both on. 
     The second and third phases of the acquisition are repeated to charge capacitor  32  until a voltage V32 across capacitor  32  reaches the value of first reference voltage S 1 . 
     During the repeating of the second and third phases of the acquisition, voltage V32 is compared, by comparator  42 , with the first reference voltage S 1  delivered by circuit  48 . 
     When voltage V32 reaches the value of first reference voltage S 1 , the sensor determines the duration D of the charge of capacitor  32 . For example, the sensor determines the duration D between the beginning of the second initial phase and the time at which voltage V32 reaches value S 1 . 
     Duration D is then compared with a threshold value of duration D 1  (block 54-&gt; D 1 ). Capacitor Cin has, when a user is in contact with region  22 , a value greater than in the case where region  22  is not in contact with the user. Thus, the charge of capacitor  32  is achieved within a smaller number of repetitions of the second and third phases when a user is in contact with region  22  than in the case where region  22  is not in contact with the user. Duration D thus has a smaller value when a user is in contact with region  22  than in the case where region  22  is not in contact with the user. 
     Threshold value D 1  is selected so that a value D greater than value D 1  means that there is no touch contact and a value D smaller than value D 1  suggests a touch contact. 
     If value D is greater than value D 1  (branch YES, block  54 ), the device remains in low-power operating mode (block  50 ).The method, and thus the acquisition, are carried out periodically during the low-power operating mode. Thus, the step represented by block  52  is carried out again after a predetermined period, for example, in the range from 200 to 300 ms, for example, substantially equal to 230 ms. 
     If value D is smaller than value D 1  (branch NO, block  54 ), the sensor initializes a counter, for example, by assigning value 2 to a variable i (block  55 , INIT COUNTER) and starts a second acquisition (block  56 , START ACQUI2). Further, the voltage value on input  46  of the comparator is changed by circuit  48  to be equal to the value of second reference voltage S 2 . 
     During the second acquisition, the three acquisition phases are for example carried out as described in relation with the first acquisition. More particularly, the second acquisition comprises a first phase during which capacitors  22  and  32  are discharged. The second acquisition further comprises second and third phases, repeated until voltage V32 across capacitor  32  is equal to the value of second reference voltage S 2 . As previously, the second phase corresponds to the charge, preferably complete, of capacitor  22  and the third phase corresponds to the discharge, preferably complete, of capacitor  22  into capacitor  32 . 
     The turning-on and the turning-off of the different switches are for example such as described in relation with the first acquisition. 
     When voltage V32 reaches the value of second reference voltage S 2 , the sensor determines a duration D′ of the charge of capacitor  32 . For example, the sensor determines the duration D′ between the beginning of the second initial phase of the second acquisition and the time at which voltage V32 reaches value S 2 . 
     Duration D′ is then compared with a threshold value D 2  (block  58  -&gt; D 2 ). Capacitor Cin has, when a user is in contact with region  22 , a value greater than in the case where region  22  is not in contact with the user. Thus, the charge of capacitor  32  is performed within a smaller number of repetitions of the second and third phases when a user is in contact with region  22  than in the case where region  22  is not in contact with the user. Duration D′ thus has a smaller value when a user is in contact with region  22  than in the case where region  22  is not in contact with the user. 
     Threshold value D 2  is selected so that a value D′ greater than value D 2  means that there is no touch contact and a value D′ smaller than value D 2  suggests a touch contact. 
     If value D′ is greater than value D 2  (branch YES, block  58 ), the device remains in low-power operating mode (block  50 ). 
     If value D′ is smaller than value D 2  (branch NO, block  58 ), this suggests a touch contact at the level of region  22 . Preferably, the second acquisition is carried out a plurality of times, preferably three times, to ascertain that the detection is correct and that it is not a false measurement, in other words, a false positive. The value assigned to variable i during the use of the counter is equal to the desired number of acquisitions minus 1. For this purpose, the value of variable i of the counter is compared with the desired number of successive second acquisitions, here three acquisitions (block  60 , COUNTER NULL?). 
     If variable i has a value greater than zero (branch NO, block  60 ), that is, if less than three second acquisitions have been carried out, the value of the variable is decremented by one unit, for example, by a value ‘1’ (block  61 , DECREMENT COUNTER) and a new second acquisition is implemented. 
     If variable i has a value equal to zero (branch YES, block  60 ), this means that, for the three successive second acquisitions, duration D′ is shorter than threshold value D 2 . The sensor thus considers that there effectively is a touch contact at the level of region  22  (block  62  -DETECT). Preferably, the device then leaves the low-power operating mode and elements of the device may then be turned back on. 
     If, during one of the second acquisitions, value D′ is greater than threshold value D 2 , the touch contact detection step is over until the next iteration of the first acquisition. 
     The determination of the different durations D and D′ and the comparison of durations D, D′ with threshold values D 1  and D 2  may for example be performed digitally, for example, by software provided for this purpose. For this purpose, the device for example comprises a microcontroller configured to carry out these operations. Otherwise, such operations may be performed analogically, the sensor for example comprising a counting circuit to determine durations D and D′ and a comparator to compare the durations with threshold values D 1  and D 2 . 
     Similarly, the control signals of switches  26 ,  30 ,  36 ,  38 , and  40  may be determined digitally, for example, by software provided for this purpose, for example, by the microcontroller, or in analog fashion. 
       FIG.  4    shows an example of variations of a voltage of the device  20  of  FIG.  1    over time. More particularly,  FIG.  4    shows an example of variations of voltage V32 across capacitor  32  ( FIG.  2   ). 
     At the origin of the shown time range, that is, at a time to, the device is in the low-power operating mode. Further, there is no touch contact at the level of region  22 . During the low-power operating mode, first acquisitions are periodically carried out. 
     At time to, sensor  20  starts a first acquisition (block  52 ,  FIG.  3   ). Voltage V32 thus increases along the charge of capacitor  32  via capacitor  22 . At a time t1 subsequent to time t0, comparator  42  determines that voltage V32 has reached value S 1 . Duration D, here equal to the difference between t1 and t0, is compared with threshold value D 1  and the sensor determines that, in the present example, duration D is longer than threshold value D 1 . 
     No touch contact is thus detected and the device thus remains in the low-power operating mode. 
     At a time t2, subsequent to time t1, sensor  20  starts another first acquisition. Voltage V32 thus increases along the charge of capacitor  32 . At a time t3 subsequent to time t2, comparator  42  determines that voltage V32 has reached value S 1 . Duration D, here equal to the difference between t3 and t2, is compared with threshold value D 1  and the sensor determines that, in the present example, duration D is greater than threshold value D 1 . 
     No touch contact is thus detected and the device thus remains in the low-power operating mode. 
     At a time t4, subsequent to time t3, another first acquisition is started by the sensor. Voltage V32 thus increases along the charge of capacitor  32 . At a time t5 subsequent to time t4, comparator  42  determines that voltage V32 has reached value S 1 . Duration D, here equal to the difference between t5 and t4, is compared with threshold value D 1  and the sensor determines that, in the present example, duration D is shorter than threshold value D 1 . Such a result suggests that there has been a touch contact at the level of region  22 . In the example of  FIG.  4   , it is a false positive. Indeed, there has been no touch contact. Such a false positive is at least partially due to value S 1 , which is relatively low and thus likely to be reached relatively fast, even when there has been no touch contact. Indeed, the sensitivity of the sensor when the threshold is threshold S 1  is relatively high, higher than when the threshold is threshold S 2 . 
     At time t5, after duration D has been compared with the threshold value and the result of this comparison has suggested the presence of a contact, a second acquisition (block  56 ,  FIG.  3   ) is started by the sensor. 
     During the second acquisition, capacitor  22  is charged and discharged into capacitor  32 , during second and third phases, to reach value S 2 . In the present example, the comparator determines that value S 2  has been reached at a time t6. Duration D′, here equal to the difference between t6 and t5, is compared with threshold value D 2  and the sensor determines that, in the present example, duration D′ is longer than threshold value D 2 . Thus, although the value of duration D of the first acquisition, between times t4 and t5, suggests that there is a touch contact at the level of region  22 , the value of duration D′ indicates that this is not true. Value S 2  is higher, and thus the determination is less sensitive and more accurate. The sensor thus remains in low-power operating mode. 
     Another first acquisition is carried out between times t7 and t8 subsequent to time t6. The duration D of this acquisition implies that there is no touch contact at the level of region  22 . 
     At a time T subsequent to time t8, a touch contact is performed. For example, a user’s finger comes into contact with region  22 . The finger remains in contact with region  22  for all the times of  FIG.  4    subsequent to time T. 
     At a time t9, subsequent to time T, a first acquisition is implemented. Voltage V32 reaches value S 1  at a time t10. The value of duration D, here equal to the difference between time t10 and time t9, is smaller than the value of threshold D 1 , which suggests that there is a touch contact at the level of region  22 . 
     At time t10a second acquisition is implemented. The value of voltage V32 reaches threshold S 2  at a time t11, subsequent to time t10. Duration D′, equal during this acquisition to the difference between time t11 and time t10 is shorter than threshold D 2 . 
     At time t11, another second acquisition is implemented. The value of voltage V32 reaches threshold S 2  at a time t12, subsequent to time t11. Duration D′, equal during this acquisition to the difference between time t12 and time t11, is shorter than threshold D 2 . 
     At time t12, a second acquisition is implemented. The value of voltage V32 reaches threshold S 2  at a time t13, subsequent to time t12. Duration D′, equal during this acquisition to the difference between time t13 and time t12, is shorter than threshold D 2 . 
     To confirm the detection of the touch contact, three second acquisitions are successively carried out. The sensor considers that the touch contact is confirmed when the durations D′ of the third second acquisitions are shorter than threshold D 2 . If the duration D′ of one of the three acquisitions is longer than threshold D 2 , the sensor considers that there is no contact and the device remains in low-power operating mode. The succession of second acquisitions stops when the duration D′ of one of the three second acquisitions is longer than threshold D 2 . For example, if the second one of the three successive second acquisitions has a duration D′ greater than value D 2 , the last acquisition is not carried out. 
     More generally, the number of successive second acquisitions may be different. At least one second acquisition is carried out, preferably three second acquisitions. The larger the number of second acquisitions, the more certain the contact detection. However, the increase of the number of second acquisitions also causes an increase of the energy cost of the detection, and of the duration of the detection. The smaller the number of second acquisitions, the less the detection is power consuming and the less time is taken by the detection. However, the smaller the number of second acquisitions, the less certain the result. Thus, decreasing the number of second acquisitions increases the risk of having a false positive, that is, of detection of a contact while there is no contact. 
     In the example of  FIG.  4   , capacitor  32  is discharged after each acquisition. The value of voltage V32 thus recovers a low value at times t1, t3, t5, t6, t8, t10, t11, t12, and t13. The low value corresponds to a reference value, preferably the value of the voltage on node  28 , preferably equal to 0 V. 
     Although the duration between times t1 and t2, between times t3 and t4, between times t6 and t7, and between times t8 and t9, is shown as being shorter than the duration of one of the acquisitions, such durations between these times are preferably substantially equal to one another. Further, the durations between these times are preferably greater than ten times the duration of a first acquisition. The durations between these times are for example in the range from 200 to 300 ms, for example, substantially equal to 230 ms. 
     Although it is considered, in  FIG.  4   , that the second acquisitions start at the time at which the first or second previous acquisition ends, that is, at time t5, t10, t11, or t12, it is possible for them to be separated by a duration Di, not shown. Duration Di for example corresponds to the duration of comparison of the duration value with threshold D 1  or D 2 . Duration Di is preferably shorter than 5 ms. 
       FIG.  5    shows another embodiment of a touch sensor  70 . Sensor  70  comprises elements identical to the elements of the sensor  20  of  FIG.  2   , referenced in the same way. These elements will not be detailed again. 
     Sensor  70  differs from sensor  20  in that sensor  70  comprises a comparison circuit comprising comparator  42  and an additional comparison element  72 . An input of element  72  is coupled, preferably connected, to node  34 , and thus receives voltage V32. 
     An output of element  72  is coupled to an output node  73  of sensor  70  via a switch  77 . Similarly, the output of comparator  42  is coupled to node  73  via a switch  75 . 
     Comparison element  72  compares the voltage value at its input with a fixed threshold value, specific to the selected comparison element and to its characteristics. For example, element  72  is a Schmitt trigger. 
     In the example of  FIG.  5   , circuit  48  supplies a threshold value, for example, a single threshold value. Preferably, the threshold value supplied by circuit  48  is smaller than the fixed threshold of element  72 . The threshold value supplied by circuit  48  is for example in the range from 75% to 95% of the value of the fixed threshold of element  72 , preferably equal to approximately 90% of the value of the fixed threshold of element  72 . 
     The operation of sensor  70  is similar to the operation of the sensor  20  described in relation with  FIGS.  3  and  4   . During first acquisitions, voltage V32 is compared with the threshold supplied by circuit  48  and with the threshold of element  72 . Since the value of the threshold delivered by circuit  48  is smaller than the value of the threshold of element  72 , voltage  34  first reaches this value. When the value supplied by circuit  48  has been reached, the first acquisition is finished. Further, during the first acquisition, switch  75  is on and switch  77  is off. 
     During second acquisitions, switch  75  is turned off and switch  77  is turned on. Thus, the comparison of element  42  has no further effect and only the comparison with the threshold of comparator  72  has an impact on the sensor. As a variation, during the second acquisitions, comparator  42  may be configured: 
     to receive from circuit  48  a value greater than the threshold value of element  72 ; or   not to compare the voltage on node  34  with a reference voltage. For example, comparator  42  may be deactivated.   

     Thus, each second acquisition ends when the voltage on node  34  reaches the threshold value of element  72 . 
     As in the case of sensor  20 , a touch contact is detected when a first acquisition and at least one second acquisition, preferably three second acquisitions, have been successively carried out, and when the duration D of the first acquisition and the duration D′ of the at least one second acquisition are below thresholds, respectively D 1  and D 2 . 
     Although the embodiments are described in the context of the leaving of a low-power operating mode, that is, the detection of a touch contact causes the leaving of the low-power mode, it should be understood that the detection of a contact may be used in other situations without modifying the described embodiments. For example, region  22  may form a button activating, when a touch contact is detected, one or a plurality of functionalities of device  10 . 
     It could have been chosen to compare the voltage on node  34  with a single threshold value, for example, with a fixed threshold of a Schmitt trigger. Thus, in low-power operating mode, acquisitions would be performed regularly. During each of the acquisitions, capacitor  32  would be charged to reach the threshold value. However, the threshold value would then be relatively significant, for example, equal to the second threshold described in relation with  FIG.  2   , to avoid too large a number of false positives. Thus, acquisitions during the low-power operating mode would be more power consuming than the first acquisitions described in relation with  FIGS.  2  and  5   . 
     An advantage of the described embodiments thus is that the power consumption is decreased with respect to a sensor only comparing the voltage on node  34  with a single value, for example, equal to the value of the second threshold. 
     Another advantage of the embodiments is that the sensor is more accurate and has less risks of false positive than a sensor only comparing the voltage on node  34  with a single value, for example, equal to the value of the first threshold. 
     Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. 
     Finally, the practical implementation of the described embodiments and variants is within the abilities of those skilled in the art based on the functional indications given hereinabove. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.