# Patent

## Title
Capacitive field sensor with sigma-delta modulator

## Abstract

A capacitive sensor includes a switching capacitor circuit, a comparator, and a charge dissipation circuit. The switching capacitor circuit reciprocally couples a sensing capacitor in series with a modulation capacitor during a first switching phase and discharges the sensing capacitor during a second switching phase. The comparator is coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response. The charge dissipation circuit is coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional application of U.S. Non Provisional application Ser. No. 12/167,100 filed on Jul. 2, 2008; now U.S. Pat. No. 8,089,289 issued Jan. 3, 2012 which claims the benefit of U.S. Provisional Application No. 60/947,865, filed on Jul. 3, 2007, the contents of both which are incorporated herein by reference.


TECHNICAL FIELD
This disclosure relates generally to electronic circuits, and in particular but not exclusively, relates to capacitance sensing circuits.
BACKGROUND INFORMATION
Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. In general, capacitive sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. A capacitive sensor permits eliminating complicated mechanical switches and buttons, providing reliable operation under harsh conditions. Capacitive sensors are widely used in the modern consumer applications, providing new user interface options in the exiting products (cell phones, digital music players, personal digital assistances, etc.).
One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed). Switches SW1 and SW2 are operated in a non-overlapping manner repeating the transfer of charge from Cx to Csum.
Capacitance sensor 100 is operated to measure the capacitance of Cx in the following manner. In an initial stage, Csum is reset by discharging Csum by temporarily closing switch SW3. Then, switches SW1 and SW2 commence operating in the two non-overlapping phases that charge Cx and transfer the charge from Cx into Csum. The voltage potential on Csum rises with each charge transfer phase, as illustrated in FIG. 1B. The voltage on Csum can by calculated according to equation 1.




  
    
      
        
          V
          Csum
        
        =
        
          
            V
            dd
          
          ⁡
          
            (
            
              1
              -
              
                e
                
                  
                    -
                    N
                  
                  ⁢
                  
                    Cx
                    Csum
                  
                
              
            
            )
          
        
      
    
    
      
        (
        
          Equation
          ⁢
          
            
          
          ⁢
          1
        
        )
      
    
  




where Vcsum represents the voltage on Csum, N represents the cycle count, Cx and Csum represent capacitance values, and Vdd represents a power supply voltage. Accordingly, the capacitance of Cx can be determined by measuring the number of cycles (or time) required to raise Csum to a predetermined voltage potential.

The charge transfer method is advantageous due to its relative low sensitivity to RF fields and RF noise. This relative noise immunity stems from the fact that the sensing capacitor Cx is typically charged by a low-impedance source and the charge is transferred to a low-impedance accumulator (i.e., the summing capacitor Csum). However, conventional capacitance sensors have the disadvantage that that voltage on the summing capacitor Csum rises versus time/cycles in an exponential manner (see FIG. 1B and Equation 1). The exponential relationship between the accumulated voltage potential on Csum and the charge transfer time/cycles requires some linearization if the capacitance of Cx is calculated as a function of the voltage potential on Csum after a predetermined time or number of cycles.



BRIEF DESCRIPTION OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
FIG. 1A is a circuit diagram illustrating a conventional capacitance sensor circuit.
FIG. 1B is a graph illustrating the exponential relationship between voltage on a summing capacitor and charge transfer cycles.
FIG. 2 is circuit diagram of a capacitive sensor with a sigma-delta modulator, in accordance with an embodiment of the invention.
FIG. 3 is a diagram illustrating operation of a capacitive field sensor, in accordance with an embodiment of the invention.
FIG. 4A is a timing diagram illustrating non-overlapping clock signals, in accordance with an embodiment of the invention.
FIG. 4B includes two phase diagrams illustrating operation of a switching capacitor circuit, in accordance with an embodiment of the invention.
FIG. 5 is a flow chart illustrating operation of a capacitive sensor with a sigma-delta modulator, in accordance with an embodiment of the invention.
FIG. 6 is circuit diagram of a capacitive sensor with a sigma-delta modulator, in accordance with an embodiment of the invention.
FIG. 7 is a circuit diagram illustrating pin-out connections for implementing a single field sensor interface, in accordance with an embodiment of the invention.
FIG. 8 is a circuit diagram illustrating pin-out connections for implementing a multi-field sensor interface time sharing a single sigma-delta modulator, in accordance with an embodiment of the invention.
FIG. 9 is a functional block diagram illustrating a demonstrative processing system for implementing a capacitive sense user interface, in accordance with an embodiment of the invention.
FIGS. 10A-C are circuit diagrams illustrating alternative dissipation circuit implementations within a sigma-delta modulator, in accordance with embodiments of the invention.



DETAILED DESCRIPTION
Embodiments of an apparatus and method for a capacitive sensor with a sigma-delta modulator are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
FIG. 2 is a circuit diagram illustrating a capacitive sensor 200, in accordance with an embodiment of the invention. Capacitive sensor 200 is capable of converting the measurement of the capacitance of sensing capacitor (Cx) into the measurement of the duty cycle of a feedback pulse signal (FB SIG). Furthermore, the relationship between the duty cycle of FB SIG and the capacitance of Cx is substantially linear. Capacitive sensor 200 may also be regarded as a switching capacitor current to duty cycle converter.
The illustrated embodiment of capacitance sensor 200 includes a switching capacitor circuit 205, sigma-delta modulator 210, a measurement circuit 215, logic 217, and a control circuit 220. The illustrated embodiment of switching capacitor circuit 205 includes sensing capacitor (Cx), switches SW1, SW2, and SW3, a diode D1, and a modulation capacitor (Cmod). The illustrated embodiment of sigma-delta modulator 210 includes a comparator (CMP) 225, a latch 230, a clock source 235, a discharge resistor (Rd), and a discharge switch SW4. Collectively, the discharge resistor Rd and discharge switch SW4 may be referred to as a charge dissipation circuit 227. While component values of switching capacitor circuit 205 and sigma-delta modulator 210 may vary based on the particular application, in general, the capacitance of Cmod will be substantially larger than the capacitance of Cx. Since Cmod acts to accumulate charge transferred from Cx over multiple cycles, it is often referred to as a summing capacitor or an integrating capacitor. In one embodiment, comparator 225 is an analog voltage comparator.
FIG. 3 is a diagram illustrating operation of a capacitive field sensor 300, in accordance with an embodiment of the invention. Capacitive field sensor 300 may be used to implement a user interface of an electronic device. Capacitive field sensor 300 is one possible physical implementation of sense capacitor Cx. The illustrated embodiment of capacitive field sensor 300 is made of two interlocking combs 305 and 310 on a printed circuit board (PCB) substrate. Each comb has a capacitance represented as CA while the finger has a variable capacitance represented as CF. The sense capacitance Cx represents the capacitance divider circuit 315 formed when the finger is brought into proximity with capacitive field sensor 300.
During a finger touch event, part of the electric field is shunted to ground. From a simplified equivalent schematic point of view, this can be illustrated as adding the finger capacitance CF, forming the capacitive voltage divider circuit 315. The finger capacitance changes the transmission coefficient of capacitance divider circuit 315. It is this overall change in capacitance that is sensed by capacitive sensor 200 and converted into a measurement of the duty cycle of a signal FB_SIG output from latch 230 and measured by measurement circuit 215. In one embodiment, logic 217 includes hardware and/or software logic for deciding when a significant change in the duty cycle of FB_SIG should be recognized as a valid finger interaction with capacitive field sensor 300.
FIGS. 4A and 4B illustrate the two non-overlapping phase operation of switching capacitor circuit 205, in accordance with an embodiment of the invention. In one embodiment, during operation of capacitive sensor 200, two configuration phases of switching capacitor circuit 205 are cycled through to perform capacitive sensing. The two phases include: a series charging phase (control signal Phi1 asserted) and a discharge phase (control signal Phi2 asserted).
In one embodiment, control signals Phi1 and Phi2 (see FIG. 2) are generated by control circuit 220 based on a single clock signal CLK. As illustrated in FIG. 4A, Phi1 and Phi2 are generated as non-overlapping pulse signals sufficiently spaced to prevent cross conduction or latch up between SW1, SW2, and SW3. As illustrated in FIG. 4B, during the series charging phase (Phi1=‘1’; Phi2=‘0’), Phi1 close circuits SW1 and open circuits SW2 and SW3. This configuration couples sensing capacitor Cx in series with modulation capacitor Cmod. A charging current Icharge flows from the power source Vdd to ground through Cx, D1, and Cmod causing Cx and Cmod to charge. During the charging phase, diode D1 conducts Icharge in a forward biased operating regime.
During the discharging phase (Phi2=‘1’; Phi1=‘0’), Phi1 open circuits SW1 and close circuits SW2 and SW3. This configuration disconnects the power source Vdd, while coupling both terminals of sensing capacitor Cx to ground to discharge the sensing capacitor. This configuration also reverse biases diode D1, which prevents Cmod from discharging. Accordingly, the voltage Umod at node N1 is held during the discharging phase. When a finger is moved in proximity to field sensor 300, the variable capacitance of Cx is increased causing less charge to be passed to Cmod during each series charging phase. The greater charge captured by Cx during the charging phase is discharged to ground during the discharge phase. Therefore, the larger Cx, the greater the number of switching cycles of SW1, SW2, and SW3 to charge Cmod to a given voltage.
During operation, the charge on Cmod accumulates via the technique described above until the voltage Umod at node N1 reaches Vref. At this point, the output MOD_SIG from CMP_225 toggles, which is latched and fed back to control switch SW4 as feedback signal FB_SIG. FB_SIG causes switch SW4 to close circuit. Discharge circuit 227 discharges Cmod through Rd until Umod drops below Vref, causing MOD_SIG to toggle once again. Latch 230 introduces a small delay into the feedback path prior to open circuiting SW4. This latch delay is controlled by clock source 235. Once SW4 is open circuited, the switching of SW1, SW2, and SW3 recharges Cmod once again. The voltage Umod continuously dithers back and forth about Vref generating a square wave at the output latch 230. This square wave is analyzed by measurement circuit 215 to determine the duty cycle or percentage of time FB_SIG is high versus low. This percentage averaged over time is representative of the capacitance or capacitance change of sensing capacitor Cx.
FIGS. 10A-C are circuit diagrams illustrating alternative implementations of charge dissipation circuit 227 within a sigma-delta modulator, in accordance with embodiments of the invention. FIG. 10A illustrates a sigma-delta modulator 211 having a charge dissipation circuit 228, which replaces SW4 and discharge resistor Rd of charge dissipation circuit 227 with a current source ID controlled by feedback pulse signal FB_SIG. When FB_SIG is a logic HIGH, the current source sinks a current ID from modulation capacitor Cmod to ground. When FB_SIG is logic LOW, the current source is disabled.
FIG. 10B illustrates a sigma-delta modulator 212 having a charge dissipation circuit 229 including a switching capacitor resistor circuit with a gated clock source. When FB SIG is logic HIGH, the clock signal CLK is applied to the switches SW5 and SW6 with non-overlapping pulses (e.g., such as clock signals Phi1 and Phi2 generated by control circuit 220), causing a discharging current to flow to ground from modulation capacitor Cmod. At a logic LOW value for FB_SIG, the clock signal CLK is gated and switching capacitor circuit including Ccomp does not sink current from modulator capacitor Cmod.
FIG. 10C illustrates a sigma-delta modulator 213 having a charge dissipation circuit 231 where the non-overlapping clock phases Phi1 and Phi2 are applied constantly to switches SW5 and SW6, but SW5 and SW6 are selectively connected in series between Umod and either Vref or ground by the multiplexor MUX, depending on the value of the feedback pulse signal FB SIG. The principle of operation of charge dissipation circuit 231 is similar to charge dissipation circuit 229 in that SW5, SW6, and Ccomp operate as a switching capacitor resistor circuit.
FIG. 5 is a flow chart illustrating operation of a capacitive sensor 200 in further detail, in accordance with an embodiment of the invention. The order in which some or all of the process blocks appear in process 500 should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel.
In a process block 505, capacitance sensor 200 is powered on and the output of CMP 210 (MOD_SIG) is initially low, assuming Cmod is initially discharged and the voltage Umod is less than Vref. In this state, MOD_SIG is logic “LOW”. On the next rising clock edge output from clock source 235, latch 230 latches the value of MOD_SIG to its output. This output is fed back to the control terminal of switch SW4 as feedback signal FB_SIG. A logic LOW open circuits SW4 decoupling node N1 from ground (process block 510) and permitting Cmod to accumulate charge.
With power provided to switching capacitor circuit 205, switches SW1, SW2, and SW3 commence operation (process block 515). Switches SW1, SW2, and SW3 switch under control of a control signals Phi1 and Phi2 generated by control circuit 220, as discussed above. As switching capacitor circuit 205 begins charging Cmod, the voltage potential Umod at node N1 begins to rise gradually. Cmod continues to accumulate charge until Umod reaches Vref, as determined by CMP 225 (decision block 520). When Umod reaches or passes Vref, CMP 225 toggles its output (MOD_SIG) to a logic “HIGH” (process block 525).
In a process block 530, latch 230 latches the value of MOD_SIG to its output as FB_SIG. Latching is synchronized to a clock signal output by clock source 235. FB_SIG is fed back to discharge switch SW4. The toggled value is a logic HIGH, which close circuits discharging switch SW4 and commences discharge of Cmod through Rd (process block 535). Cmod is discharged until Umod drops back below Vref, as determined by CMP 225 (decision block 540), at which point CMP 225 toggles MOD-SIG (process block 545). Discharge switch SW4 is once again open circuited after MOD_SIG is latched and process 500 repeats from process block 510.
After an initial transitory startup phase, capacitance sensor 200 enters its steady state phase where the voltage potential Umod on Cmod oscillates or dithers about Vref. This oscillation about Vref creates the modulation signal MOD_SIG upon which the feedback pulse signal FB_SIG is based. Once operating in the steady state phase, the duty cycle of the FB SIG is directly proportional to the capacitance or capacitance change of Cx.
Accordingly, in a process block 550, the duty cycle of FB_SIG is measured by measurement circuit 215. In one embodiment, measurement circuit 215 may include a clock gated by FB_SIG and a counter to count a number of clock cycles occurring while FB_SIG is HIGH for a given period of time. Furthermore, there can be other methods to extract the multi-bit digital values from the bit stream data output by the sigma-delta modulator, such as various types of the digital filters or otherwise. Finally, in a process block 555, the measured duty cycle is used to determine the capacitance Cx or capacitance change ACx of the sensing capacitor. Logic 217 may use this digital code to determine whether a user finger has interacted with a capacitive field sensor within a user interface. In one embodiment, measurement circuit 215 may output a digital code indicative of the capacitance or capacitance change of Cx. In one embodiment, capacitive sensor 200 operates as a Cmod charge current (i.e., Icharge in FIG. 4B) to digital code converter. Of course, the charge current of Cmod is related to the variable capacitance of the field sensor Cx.
FIG. 6 is a circuit diagram of a capacitive sensor 600 including a sigma-delta modulator, in accordance with an embodiment of the invention. Capacitive sensor 600 is an alternative embodiment to capacitive sensor 200, but operates using the same principles. The illustrated embodiment of capacitive sensor 600 includes a switching capacitor circuit 605, a sigma-delta modulator 210, measurement circuit 215, and a clock source 620. The illustrated embodiment of switching capacitor circuit 605 includes sensing capacitor Cx, a filter resistor Rfilt, diode D1, modulation capacitor Cmod, a discharge switch SW5, and an inverter INV1. In one embodiment, clock source 620 is a pseudo-random signal (PRS) generator for generating a pseudo-random pulse signal. Other signal generators maybe used, such as a pulse width modulator; however, a PRS generator provides greater electromagnetic noise immunity. Additionally, other frequency spreading techniques can also be used to implement clock source 620, such as frequency sweeping, frequency hopping, changing frequency in the pseudo random order, etc.
Sigma-delta modulator 210 and measurement circuit 215 operate as discussed above in connection with capacitive sensor 200. Similarly, switching capacitor circuit 605 operates to sequentially charge Cmod, just as switching capacitor circuit 205, with a slight variation on its specific implementation. When clock source 620 outputs a logic HIGH, diode D1 is forward biased and switch SW5 is open circuited. The open circuited SW5 connects Cmod in series with Cx and clock source 620. The forward biased D1 permits a charging current to flow through sensing capacitor Cx and filter resistor Rfilt into modulation capacitor Cmod. While clock source 620 is logic HIGH, switching capacitor circuit 605 is in the “charging phase.”
When clock source 620 transitions to a logic LOW, switch SW5 is closed circuited. The closed circuited SW5 connects node N2 to ground. This couples sensing capacitor Cx to ground through Rfilt and reverse biases diode D1. With Cx coupled to ground it discharges, while the reversed biased diode D1 prevents discharge from modulation capacitor Cmod and Cmod retains its voltage Umod. While clock source 620 is logic LOW, switching capacitor circuit 605 is in the “discharge phase.”
During the discharge phase, filter resistor Rfilt and switch SW5 coupled to ground creates a high frequency cutoff low pass filter (LPF). This LPF increases noise immunity to high frequencies. The LPF prevents high amplitude, ultra high frequency noise from erroneously flipping the bias state of diode D1 and causing false triggering.
FIG. 7 is a circuit diagram illustrating pin-out connections for implementing a single field sensor interface 700 within an integrated circuit, in accordance with an embodiment of the invention. Integrated circuit (“IC”) 705 includes sigma-delta modulator 210, clock source 620, inverter INV1, and switch SW5 integrated on a single die. The following components including: sensing capacitor Cx, filter resistor Rfilt, discharge resistor Rd, modulation capacitor Cmod, and diode D1 are externally coupled to IC 705. In one embodiment, inverter INV 1 may be implemented in software or firmware using a look up table (“LUT”).
FIG. 8 is a circuit diagram illustrating pin-out connections for implementing a multi field sensor interface 800 within an integrated circuit, in accordance with an embodiment of the invention. IC 805 couples multiple field sensors Cx1 and Cx2 to a single general purpose input/output (“GPIO”) pin 810. Field sensors Cx1 and Cx2 time share a single GPIO 810, clock source 620, and sigma-delta modulator 210. However, each externally coupled sensor includes its own externally coupled filter resistor (e.g., Rfilt1, Rfilt2) and its own internal switch SW5 (e.g., SW5A, SW5B). Each field sensor Cx1 or Cx2 is scanned one at a time via appropriate switching of the select switches SEL1 and SEL2. Select switches SEL1 and SEL2 either activate the control terminals of switches SW5A and SW5B thereby grounding the corresponding field sensors Cx1 or Cx2, or connect the control terminal to the output of inverter INV1. Although FIG. 8 illustrates just two field sensors Cx1 and Cx2, it should be appreciated that a large number of field sensors can thus timeshare GPIO pin 810.
FIG. 9 is a functional block diagram illustrating a demonstrative system 1100 for implementing a capacitive sense user interface, in accordance with an embodiment of the invention. The illustrated embodiment of system 1100 includes a processing device 1110, a capacitive sense pad 1120, a capacitive sense linear slider 1130, a capacitive sense radial slider 1140, a host processor 1150, an embedded controller 1160, and non-capacitance sensor elements 1170. Processing device 1110 may include analog and/or digital general purpose input/output (“GPIO”) ports 1107.
GPIO ports 1107 may be programmable. GPIO ports 1107 may be coupled to a Programmable Interconnect and Logic (“PIL”), which acts as an interconnect between GPIO ports 1107 and a digital block array of processing device 1110 (not illustrated). The digital block array may be configured to implement a variety of digital logic circuits (e.g., DAC, digital filters, digital control systems, etc.) using, in one embodiment, configurable user modules (“UMs”). The digital block array may be coupled to a system bus. Processing device 1110 may also include memory, such as random access memory (RAM) 1105 and program flash 1104. RAM 1105 may be static RAM (“SRAM”), and program flash 1104 may be a non-volatile storage, which may be used to store firmware. Processing device 1110 may also include a memory controller unit (“MCU”) 1103 coupled to memory and the processing core 1102.
Processing device 1110 may also include an analog block array (not illustrated). The analog block array is also coupled to the system bus. The analog block array also may be configured to implement a variety of analog circuits (e.g., ADC, analog filters, etc.) using, in one embodiment, configurable UMs. The analog block array may also be coupled to the GPIO 1107.
As illustrated, capacitance sensor 1101, which includes an implementation of capacitance sensor 200, 600, 700, or 800 may be integrated into processing device 1110. Capacitance sensor 1101 may include analog 110 for coupling to an external component, such as capacitive sense pad 1120, capacitive sense linear slider 1130, capacitive sense radial slider 1140, and/or other capacitive sense devices. Capacitive sense pad 1120, capacitive sense linear slider 1130, and/or capacitive sense radial slider 1140 may each include one or more sensing capacitors Cx to implement the individual capacitive sense buttons therein.
Processing device 1110 may include internal oscillator/clocks 1106 and communication block 1108. The oscillator/clocks block 1106 provides clock signals to one or more of the components of processing device 1110. Communication block 1108 may be used to communicate with an external component, such as a host processor 1150, via host interface (I/F) line 1151. Alternatively, processing device 1110 may also be coupled to embedded controller 1160 to communicate with the external components, such as host 1150. Interfacing to the host 1150 can be through various methods. In one exemplary embodiment, interfacing with the host 1150 may be done using a standard PS/2 interface to connect to embedded controller 1160, which in turn sends data to the host 1150 via low pin count (LPC) interface. In some instances, it may be beneficial for processing device 1110 to do both touch-sensor pad and keyboard control operations, thereby freeing up the embedded controller 1160 for other housekeeping functions. In another exemplary embodiment, interfacing may be done using a universal serial bus (USB) interface directly coupled to host 1150 via host interface line 1151. Alternatively, processing device 1110 may communicate to external components, such as host 1150 using industry standard interfaces, such as USB, PS/2, inter-integrated circuit (I2C) bus, or system packet interfaces (SPI). Host 1150 and/or embedded controller 1160 may be coupled to processing device 1110 with a ribbon or flex cable from an assembly, which houses the sensing device and processing device.
In one embodiment, processing device 1110 is configured to communicate with embedded controller 1160 or host 1150 to send and/or receive data. The data may be a command or alternatively a signal. In an exemplary embodiment, system 1100 may operate in both standard-mouse compatible and enhanced modes. The standard-mouse compatible mode utilizes the HID class drivers already built into the Operating System (OS) software of host 1150. These drivers enable processing device 1110 and sensing device to operate as a standard cursor control user interface device, such as a two-button PS/2 mouse. The enhanced mode may enable additional features such as scrolling (reporting absolute position) or disabling the sensing device, such as when a mouse is plugged into the notebook. Alternatively, processing device 1110 may be configured to communicate with embedded controller 1160 or host 1150, using nonOS drivers, such as dedicated touch-sensor pad drivers, or other drivers known by those of ordinary skill in the art.
Processing device 1110 may reside on a common carrier substrate such as, for example, an integrated circuit (IC) die substrate, a multi-chip module substrate, or the like. Alternatively, the components of processing device 1110 may be one or more separate integrated circuits and/or discrete components. In one exemplary embodiment, processing device 1110 may be a Programmable System on a Chip (PSoCTM) processing device, manufactured by Cypress Semiconductor Corporation, San Jose, Calif. Alternatively, processing device 1110 may be one or more other processing devices known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”), or the like. In an alternative embodiment, for example, processing device 1110 may be a network processor having multiple processors including a core unit and multiple microengines. Additionally, processing device 1110 may include any combination of general-purpose processing device(s) and special-purpose processing device(s).
Capacitance sensor 1101 may be integrated into the IC of processing device 1110, or alternatively, in a separate IC. Descriptions of capacitance sensor 1101 may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code describing capacitance sensor 1101, or portions thereof, may be generated using a hardware descriptive language, such as VHDL or Verilog, and stored to a machine-accessible medium (e.g., CD-ROM, hard disk, floppy disk, etc.). Furthermore, the behavioral level code can be compiled into register transfer level (“RTL”) code, a netlist, or even a circuit layout and stored to a machine-accessible medium. The behavioral level code, the RTL code, the netlist, and the circuit layout all represent various levels of abstraction to describe capacitance sensor 1101.
In one embodiment, electronic system 1100 may be used in a notebook computer. Alternatively, system 1100 may be used in other applications, such as a mobile handset, a personal data assistant (PDA), a keyboard, a television, a remote control, a monitor, a handheld multi-media device, a handheld video player, a handheld gaming device, or a control panel.
The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a machine (e.g., computer) readable medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or the like.
A machine-accessible medium includes any mechanism that provides (e.g., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-accessible medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.).
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

## Claims

1. A capacitive sensor, comprising:
a switching capacitor circuit to reciprocally couple a sensing capacitor in series with a modulation capacitor during a first switching phase and to discharge the sensing capacitor during a second switching phase;
a comparator coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response; and
a charge dissipation circuit coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.

2. The capacitive sensor of claim 1, wherein the switching capacitor circuit includes a diode coupled between the sensing capacitor and the modulation capacitor.

3. The capacitive sensor of claim 2, wherein the diode is coupled to prevent the modulation capacitor from discharging during the second switching phase while the sensing capacitor is coupled to discharge.

4. The capacitive sensor of claim 2, wherein the switching capacitor circuit further comprises:
a clock source;
an inverter coupled to the clock source; and
a switch coupled between a ground and a circuit node located between the sensing capacitor and the diode, wherein the switch is responsive to one of the inverter or the clock source, and wherein a first terminal of the sensing capacitor is coupled to an opposite one of the inverter or the clock source than the switch.

5. The capacitive sensor of claim 4, wherein the switching capacitor circuit further comprises a resistor coupled between the sensing capacitor and the diode and wherein the circuit node is located between the resistor and the diode.

6. The capacitive sensor of claim 2, wherein the switching capacitor circuit comprises:
a first switch coupled between a first terminal of the sensing capacitor and a first supply voltage;
a second switch coupled between the first terminal and a ground; and
a third switch coupled between a second terminal of the sensing capacitor and the ground.

7. The capacitive sensor of claim 6, further comprising a control circuit coupled to control the first, second, and third switches, wherein the control circuit close circuits the first switch during the first switching phase and close circuits the second and third switches during the second switching phase.

8. The capacitive sensor of claim 1, further comprising:
a latch coupled between the dissipation circuit and the comparator to latch the modulation signal to its output as a feedback signal fed back to the dissipation circuit; and
a measurement circuit coupled to measure a duty cycle of the feedback signal, wherein the duty cycle is indicative of a capacitance of the sensing capacitor.

9. The capacitive sensor of claim 1, wherein the charge dissipation circuit comprises a current source coupled to selectively discharge the modulation capacitor in response to the modulation signal.

10. The capacitive sensor of claim 1, wherein the charge dissipation circuit comprises:
a switching capacitor resistor circuit coupled between a terminal of the modulation capacitor and a ground; and
a clock source gated in response to the modulation signal and coupled to control the switching capacitor resistor circuit.

11. The capacitive sensor of claim 1, wherein the charge dissipation circuit comprises: a switching capacitor resistor circuit coupled to the modulation capacitor and having two switches selectively coupled in series between the modulation capacitor and either a ground or the reference responsive to the modulation signal.

12. A system for implementing a user interface, comprising:
a switching capacitor circuit to reciprocally couple a field capacitor of the user interface in series with a modulation capacitor during a first switching phase and to discharge the field capacitor during a second switching phase;
a modulation circuit coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a feedback signal to control selective discharge of the modulation capacitor; and
a measurement circuit coupled to measure a duty cycle of the feedback signal.

13. The system of claim 12, further comprising logic coupled to determine whether a user interaction with the field capacitor occurred based upon the duty cycle measured by the measurement circuit.

14. The system of claim 12, further comprising a diode coupled between the field capacitor and the modulation capacitor, the diode oriented to prevent discharge of the modulation capacitor during the second switching phase.

15. The system of claim 14, wherein the switching capacitor circuit comprises:
a clock source;
an inverter coupled to the clock source; and
a switch coupled between a ground and a circuit node, wherein the circuit node is located between the field capacitor and the diode, wherein the switch is coupled to be responsive to one of the inverter or the clock source, and wherein the field capacitor is coupled to an opposite one of the inverter or the clock source than the switch.

16. The system of claim 15, wherein the field capacitor and the modulation capacitor are externally coupled to an integrated circuit including the clock source, the modulation circuit, and the measurement circuit.

17. The system of claim 16, further comprising:
an input/output (“I/O”) pin of the integrated circuit through which the field sensor is coupled; and
a plurality of field sensors coupled to the I/O pin to timeshare the modulation circuit and the clock circuit.