Patent Publication Number: US-9846514-B2

Title: Capacitance detecting circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to, and benefits of Chinese Patent Application Serial No. 201210585884.2, filed with the State intellectual Property Office of P. R. C. on Dec. 30, 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure generally relates to the circuit design field, more particularly to a capacitance detecting circuit. 
     BACKGROUND 
     Presently, a touch screen with the advantages of easy and convenient operation and humanization is widely used in various kinds of electronic products such as a mobile phone, a PDA (personal digital assistant), a GPS (global position system), a PMP (MP3, MP4, etc.), a tablet personal computer and the like. 
     Conventionally, an induction unit in the touch screen is judged whether to be touched or not by detecting a self-capacitance thereof. Thus, it becomes more important to accurately detect the self-capacitance generated by the induction unit in the touch screen when the induction unit is touched. 
     However, a conventional method of detecting capacitance has the disadvantage of to detection accuracy. When detecting the capacitance generated by touching the induction unit, an error occurs because of the existence of mutual-capacitance, thereby decreasing detection accuracy. Therefore, it is difficult to correctly judge whether the induction unit is touched or not. 
     SUMMARY 
     We provide a capacitance detecting circuit. The capacitance detecting circuit may comprise: N induction units, where N is an integer larger than equal to 1; a transferring capacitor configured to transfer charges to a capacitor of a scanned induction nit among the N induction units, wherein a first terminal of the transferring capacitor connects to the N induction units, respectively, and a second terminal of the transferring capacitor is grounded; a charging module configured to charge the transferring capacitor, wherein a first terminal of the charging module connects to the first terminal of the transferring capacitor, and a second terminal of the charging module connects to a first power supply; a discharging module configured to discharge the transferring capacitor and comprising a first resistor and a second switch, wherein a first terminal of the discharging module connects to the first terminal of the charging module and the first terminal of the transferring capacitor, respectively, a second terminal of the discharging module is grounded; a controller connected with the charging module, the transferring capacitor and the discharging module, respectively, and configured to control the second switch to switch on when a voltage of the transferring capacitor is larger than a preset threshold voltage, to update a count value and detect a touch on the N induction units according to the count value. 
     With the capacitance detecting circuit, when detecting the capacitance generated by touching the induction unit, influences from mutual-capacitance may be eliminated or reduced, thereby improving detection accuracy. Thus, the induction unit can be judged correctly whether to be touched or not. 
     Additional aspects and advantages of selected, representative examples in the disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of those examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and advantages of the disclosure will become apparent and more readily appreciated from the following descriptions made with reference to the drawings. 
         FIG. 1  is a schematic diagram of a capacitance detecting circuit according to an example. 
         FIG. 2  is a schematic circuit diagram of the capacitance detecting circuit according to an example. 
         FIG. 3  is a schematic circuit diagram of the capacitance detecting circuit according to an example. 
         FIG. 4  is a wave chart of switch controlling signals of the capacitance detecting circuit according to an example. 
         FIG. 5  is a wave chart of switch controlling signals of the capacitance detecting circuit according to another example. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will be made in detail to examples of our methods and circuits. The examples described herein with reference to the drawings are explanatory, illustrative, and used to generally understand the disclosure. The examples shall not be construed to limit the disclosure. The same or similar elements and the elements having same or similar functions are denoted by like reference numerals throughout the descriptions. 
     In the description, terms concerning attachments, coupling and the like such as “connect”, “connects”, “connected”, “interconnect”, “interconnects”, “interconnected”, refer to a relationship in which structures are secured or attached to one another through mechanical or electrical connection, or directly or indirectly through intervening structures, unless expressly described otherwise. 
     In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance, significance or order. 
     As shown in  FIG. 1 , according to an example, a capacitance detecting circuit is pro-vided. The capacitance detecting circuit comprises N induction units ( 1 ,  2 ,  3  . . . N), a transferring capacitor Cmod, a charging module  101 , a controller  102  and a discharging module  100 . 
     When each induction unit is touched, the capacitance thereof is changed. N is an integer larger than or equal to 1. In one example, the capacitance detecting circuit may detect a plurality of induction units simultaneously. 
     As shown in  FIG. 1 , the transferring capacitor Cmod is configured to transfer its charges to a self-capacitor of a scanned induction unit among the N induction units ( 1 ,  2 ,  3  . . . N). A first terminal of the transferring capacitor connects to first terminals of the N induction units, respectively, a second terminal of the transferring capacitor is grounded, and second terminals of the N induction units are commonly grounded. In general, when the induction unit is touched, the capacitance thereof may be increased. For example, when it is scanned that induction units  1 ,  2 ,  3  among the N induction units are touched, a self-capacitance of each of the scanned induction units  1 ,  2 ,  3  is changed (for example increased), and the transferring capacitor Cmod transfers its charges to the self-capacitor of each of the scanned induction units  1 , respectively, to pull down a voltage of the transferring capacitor Cmod. 
     The charging module  101  is configured to charge the transferring capacitor Cmod. A first terminal of the charging module  101  connects to the first ten terminal of the transferring capacitor Cmod, and a second terminal of the charging module  101  connects to a first power supply VD. The charging module  101  comprises a first switch SW 1  and a current source I connected in series. 
     The discharging module  100  is configured to discharge the transferring capacitor Cmod and comprises a first resistor R 1  and a second switch SW 2 . A first terminal of the discharging module  100  connects to the first terminal of the charging module  101  and the first terminal of the transferring capacitor Cmod respectively, and a second terminal of the discharging module  100  is grounded. 
     A controller  102  connects to the charging module  101 , the transferring capacitor Cmod and the discharging module  100 , respectively. The controller  102  is configured to control the second switch SW 2  to switch off when a voltage of the transferring capacitor Cmod is larger than a preset threshold voltage Vth, to update a count value and detect a touch on the N induction units according to the count value. That is to say, the controller  102  controls the second switch SW 2  to switch on when the voltage of the transferring capacitor Cmod is larger than the preset threshold voltage Vth; controls the second switch SW 2  to switch off when the voltage of the transferring capacitor Cmod is smaller than the preset threshold voltage Vth; and updates a count value according to the touch on the induction unit to detect the touch on the N induction units. 
     Specifically, the controller  102  controls the first switch SW 1  to switch on to make the charging module  101  charge the transferring capacitor Cmod. The controller  102  controls the second switch SW 2  to switch on when the voltage of the transferring capacitor Cmod is larger than the preset threshold voltage Vth. The controller  102  controls the second switch SW 2  to switch off when the voltage of the transferring capacitor Cmod is smaller than the preset threshold voltage Vth. The voltage of the transferring capacitor Cmod is pulled down when the charges of the transferring capacitor Cmod are transferred to a scanned induction unit. The larger the capacitance of the scanned induction unit is, the lower the voltage of the transferring capacitor Cmod is so that the smaller the count value of a counter is, that is to say, the count value of the touched induction unit is smaller than that of a non-touched induction unit, thus detecting the touch on the N induction units according to the count value. 
     In one example as shown in  FIG. 1 , the capacitance detecting circuit further comprises N selecting assemblies ( 1 ,  2 ,  3  . . . N) in which each selecting assembly corresponds to one induction unit and connects between the one induction unit and the transferring capacitor Cmod. Each selecting assembly controls to connect or disconnect a corresponding induction unit with the transferring capacitor Cmod. 
     In one example, the each selecting assembly comprises a third switch SW 3  and a fourth switch SW 4 . The third switch. SW 3  connects between the corresponding induction unit and the transferring capacitor Cmod. A first terminal of the fourth switch SW 4  connects to the corresponding induction unit and a second terminal of the fourth switch SW 4  is grounded. The third switch SW 3  and the fourth switch SW 4  are switched on alternately. 
     Moreover, as shown in  FIG. 2 , the each selecting assembly comprises a fifth switch SW 5 , a first terminal of the fifth switch SW 5  connects to the corresponding induction unit, a second terminal of the fifth switch SW 5  connects to a second power supply VTH, and a voltage of the second power supply VTH is the preset threshold voltage Vth. Of course, in another example, the voltage of the second power supply VTH may be other values. 
     Further, in one example as shown in  FIG. 2 , the capacitance detecting circuit further comprises a pre-charging module  201  configured to pre-charge the transferring capacitor Cmod. A first terminal of the pre-charging module  201  connects to the first terminal of the transferring capacitor Cmod, and a second terminal of the pre-charging, module  201  connects to the first power supply VD. The pre-charging module  201  comprises a sixth switch SW 6  and a second resistor R 2  connected in series. That is to say, the pre-charging module  201  pre-charges the transferring capacitor Cmod until the voltage of the transferring capacitor Cmod closely reaches the preset threshold Vth before charging the transferring capacitor Cmod. 
     Also as shown in  FIG. 2 , the capacitance detecting circuit further comprises a pre-discharging module  202  configured to pre-discharge the transferring capacitor Cmod before the pre-charging module pre-charges the transferring capacitor. A first terminal of the pre-discharging module  202  connects to the first terminal of the transferring capacitor Cmod, and a second terminal of the pre-discharging module  202  is grounded. The pre-discharging module  202  comprises a seventh switch SW 7  and the pre-discharging module  202  pre-discharges the transferring capacitor Cmod by grounding a first terminal of the seventh switch SW 7 . In the example, pre-charging and pre-discharging the transferring capacitor Cmod helps ensure that states of the transferring capacitor Cmod are the same for each detection, thus increasing the detection accuracy. 
     In one example as shown in  FIG. 2 , the controller comprises: a comparator CMP, a trigger DFF, a counter  206 , a detector  203  and a first selector  204 . A first input terminal of the comparator CMP is applied with the preset threshold voltage Vth and a second input terminal of the comparator CMP connects to the first terminal of the transferring capacitor Cmod. An input terminal of the trigger DFF connects to an output terminal of the comparator CMP that generates a triggering signal according to a comparison result from the comparator CMP. The trigger DEE further comprises a clock signal CNT_CLK input. Au input terminal of the counter  206  connects to an output terminal of the trigger DFF that counts according to the triggering signal. The detector  203  connects to the counter  206  and is configured to detect the touch on the N induction units according to the count value of the counter  206 . An input terminal of the first selector  204  connects to the output terminal of the trigger DFF that generates a first control signal according to the triggering signal. The first control signal is configured to control the discharging module  100  to discharge to the transferring capacitor Cmod. 
     In addition, the controller  102  further comprises a second selector  205 . An input terminal of the second selector  205  connects to the output ten the comparator CMP that generates a second control signal according to the comparison result. The second control signal is configured to control the pre-charging module  201  to pre-charge the transferring capacitor Cmod. 
     Specifically, as shown in  FIG. 2 , the first selector  204  is an AND gate, a first input terminal of the AND gate connects to the output terminal of the trigger DFF, and a second input terminal of the AND gate is applied with a switch enable signal SWITCH EN. During the pre-charging and pre-discharging, the switch enable signal SWITCH EN is low to control the second switch SW 2  of the discharging module  100  to switch off. During the detection after pre-charging, the switch enable signal SWITCH EN is high, and a signal from an output terminal of the AND gate controls the second switch SW 2  of the discharging module  100  to switch on or switch off. The switch enable signal SWITCH EN returns to a lower position after detection is completed. The second selector  205  is a NOR gate, a first input terminal of the NOR gate connects to the output terminal of the comparator CMP, and a second input terminal of the NOR gate is applied with a pre-charging signal PRE_CHRG. 
     In an example, when there is only one induction unit in the capacitance detecting circuit, as shown in  FIG. 3 , a capacitance detection process can be described as follows. 
     The seventh switch SW 7  is switched on to pre-discharge the transferring capacitor Cmod. Second, the seventh switch SW 7  is switched off and the sixth switch SW 6  is switched on so that the first power supply VD pre-charges the transferring capacitor Cmod by the second resistance R 2  until the voltage of the transferring capacitor Cmod closely reaches the preset threshold voltage Vth. The first switch SW 1  keeps off during the pre-discharging and the pre-charging. The sixth switch SW 6  and the seventh switch SW 7  are switched off after the pre-charging is completed. At this time, the first switch SW 1  is controlled to switch on so that the first power supply VD pre-charges the transferring capacitor Cmod via a current source I. When the voltage of the transferring capacitor Cmod is larger than a reference voltage of the comparator CMP (i.e., the preset threshold voltage Vth), the trigger DR controls the second switch SW 2  to switch on according to the result output from the comparator CMP so that the transferring capacitor Cmod is discharged via the first resistance R 1 . When the voltage of the transferring capacitor Cmod is smaller than the preset threshold voltage Vth, an output of the comparator CMP is turned over and the trigger DEF controls the second switch SW 2  to switch off according to the output result from the comparator CMP. During a charge transferring process of the transferring capacitor Cmod, the charges of the transferring capacitor Cmod are transferred to a capacitor Cx of the induction unit via the third switch SW 3 . When the third switch SW 3  is switched off, the voltage of the transferring capacitor Cmod is pulled down to be smaller than the preset threshold voltage Vth, which enables the output of the comparator CMP to be turned over to switch off the second switch SW 2 . At this time, the transferring capacitor Cmod is charged via the current source I until the voltage thereof is larger than the preset threshold voltage Vth, which enables the output of the comparator CMP to be turned over to switch on the second switch SW 2 . The third switch SW 3  and the fourth switch SW 4  are controlled by a two-phase non-overlapped clock signal (that is, when the third switch SW 3  is controlled to switch on, the fourth switch SW 4  is controlled to switch off; and when the third switch SW 3  is controlled to switch off, the fourth switch SW 4  is controlled to switch on), the charges on the capacitor Cx transferred from the transferring capacitor Cmod are discharged to the ground by the fourth switch SW 4 . The fifth switch is a synchronous switch. The fifth switch SW 5  is kept off when the induction unit is being scanned. 
     The capacitance of the capacitor Cx, the duty ratio of a rectangular wave output from the trigger DFF and the count value of the counter  206  when the induction unit is touched are different from those when the induction unit is not touched. When detection is finished, the above-mentioned processes are repeated. 
     Therefore, the detector  203  may judge whether the induction unit is touched according to the count value of the counter. For example, it is assumed that when the induction unit is not touched, the count value of the counter is 1000; and when the induction unit is touched, the count value of the counter is 500. Thus, it may judge whether the induction unit is touched or not according to the count value of the counter. 
     In one example, a wave chart of switch controlling signals under a non-fullscreen synchronous mode is shown in  FIG. 4 , where a high level means that the switch is controlled to switch on, and a low level means that the switch is controlled to switch off. When an M th  induction unit is scanned the fifth switch SW 5  of an M th  selecting assembly is controlled to switch off by the controller  102 , the third switch SW 3  and the fourth switch SW 4  of the M th  selecting assembly are controlled to switch on alternately by the controller  102 , and the third switch SW 3 , the fourth switch SW 4  and the fifth switch. SW 5  of other selecting assemblies except the M th  selecting assembly are controlled to switch off by the controller  102 . 
     In another example, a wave chart of the switch controlling signals under a fullscreen synchronous mode is shown in  FIG. 5 , where the high level means that the switch is controlled to switch on, and the low level means that the switch is controlled to switch off. When an M th  induction-unit is scanned, the fifth switch SW 5  of an M th  selecting assembly is controlled to switch off by the controller  102 , the third switch SW 3  and the fourth switch SW 4  of the M th  selecting assembly are controlled to switch on alternately by the controller  102 , the third switch SW 3  of other selecting assemblies except the M th  selecting assembly is controlled to switch off by the controller  102 , and the fourth switch SW 4  and the fifth switch SW 5  of other selecting assemblies except the M th  selecting assembly are controlled to switch on alternately by the controller  102 . That is to say, when the M th  induction unit is scanned under the fullscreen synchronous mode, the fifth switch SW 5  of a currently scanned induction unit is controlled to switch off, while the fifth switches SW 5  and the third switches SW 3  of other synchronous but non-scanned induction units have a same on-off state, that is, the third switch SW 3  and the fourth switch SW 4  of the scanned induction unit are switched on alternatively, and the fourth switches SW 4  and the fifth switches SW 5  of other non-scanned induction units are switched on alternatively. In this way, when the currently scanned induction unit (i.e., the M th  induction unit) is touched, the charges of the transferring capacitor Cmod is transferred to the capacitor (self-capacitor) Cx of the M th  induction unit via the third switch SW 3  of the M th  selecting assembly. The capacitors Cx of the synchronous but non-scanned induction units are charged with a specified voltage via the corresponding fifth switches SW 5 . In the example, the specified voltage value is equal to the preset threshold voltage Vth. Therefore, the capacitance Cx of the current scanned induction unit and that of the synchronous but non-scanned induction units are charged with the preset threshold voltage Vth, and are also discharged to ground via the corresponding fourth switch SW 4  to eliminate the influence of mutual capacitance, thus improving the detection accuracy. 
     With the capacitance detecting circuit, when detecting capacitance generated by touching the induction unit, the influence of a mutual-capacitance may be eliminated, thus improving a detection accuracy, thereby the induction unit can be judged correctly whether to be touched or not. 
     In addition, each function cell of the examples may be integrated in a processing module, or these cells may be separate physical entities, or two or more cells are integrated in a processing module. The integrated module may be realized in a form of hardware or in a form of software function modules. When the integrated module is realized in the form of software function module and is sold or used as a standalone product, the integrated module may be stored in a computer readable storage medium. 
     The storage medium mentioned above may be read-only memories, magnetic disks or CD and the like. 
     Although explanatory examples have been shown and described, it will be appreciated by those skilled in the art that the above examples cannot be construed to limit the disclosure and changes, alternatives and modifications can be made in the examples without departing from spirit, principles and scope of the disclosure.