Patent Publication Number: US-2016224158-A1

Title: Detection of a conductive object during an initialization process of a touch-sensing device

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/520,536, filed on Oct. 22, 2014, which is a continuation of U.S. patent application Ser. No. 13/218,090, filed on Aug. 25, 2011, now U.S. Pat. No. 8,941,393, Issued on Jan. 27, 2015, which claims the benefit of U.S. Provisional Application No. 61/433,148 filed Jan. 14, 2011, all of which are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the field of user interface devices and, in particular, to capacitive sense devices. 
     BACKGROUND 
     Capacitive sense elements may be used to replace mechanical buttons, knobs and other similar mechanical user interface controls. The use of a capacitive sense element allows for the elimination of complicated mechanical switches and buttons, providing reliable operation under harsh conditions. In addition, capacitive sense elements are widely used in modern customer applications, providing new user interface options in existing products. Capacitive sense elements can range from a single button to a large number arranged in the form of a capacitive sense array for a touch-sensing surface. 
     Transparent touch screens that utilize capacitive sense arrays are ubiquitous in today&#39;s industrial and consumer markets. They can be found on cellular phones, GPS devices, set-top boxes, cameras, computer screens, MP3 players, digital tablets, and the like. The capacitive sense arrays work by measuring the capacitance of a capacitive sense element, and looking for a delta in capacitance indicating a touch or presence of a conductive object. When a conductive object (e.g., a finger, hand, or other object) comes into contact or close proximity with a capacitive sense element, the capacitance changes and the conductive object is detected. The capacitance changes of the capacitive touch sense elements can be measured by an electrical circuit. The electrical circuit converts the measured capacitances of the capacitive sense elements into digital values. 
     Because capacitance also changes due to environmental conditions (e.g. changes in humidity and temperature), conventional capacitive sensing algorithms typically use a difference between an immediate value and a filtered baseline average value (initialized by a measurement taken with a non-touch state) to be able to distinguish the presence of a conductive object from an environmental change. 
     Some electronic devices (e.g., printers, set-top cable boxes, blue-ray players) with mechanical buttons use the state of the button as power is applied (i.e., at power up) to enter special diagnostic modes. Certain set-top cable boxes used by various cable television providers in North America require a service technician to place the set-top box into a diagnostic mode (e.g., a test mode, debug mode) by pressing one or more mechanical buttons on the device while simultaneously applying power to the device. 
     Although it is desirable to equip set-top boxes with capacitive sense elements as replacements for the mechanical buttons, to do so requires the device to detect the initial presence of a conductive object proximate to one or more capacitive sense elements at power up (i.e., during the initialization process of the device). However, devices using the conventional capacitive sensing algorithms based on filtered baseline averages would fail to detect the presence of a conductive object during the initialization process because the initial presence of the conductive object would prevent an accurate measurement with a non-touch state of the device (e.g., the set-top cable box described above). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not of limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  is a block diagram illustrating one embodiment of an electronic system having a processing device for detecting a presence of a conductive object during an initialization process of a touch-sensing device. 
         FIG. 2  is a block diagram illustrating one embodiment of an electronic system having a capacitance sensor coupled to a processing device for detecting a presence of a conductive object during an initialization process of a touch-sensing device. 
         FIG. 3  illustrates one embodiment of a method of detecting a presence of a conductive object during an initialization process of a touch-sensing device. 
         FIG. 4  illustrates a top-side view of an embodiment of a printed circuit board including an array of capacitive sense elements and a reference capacitive sense element, with and without an overlay. 
         FIG. 5  illustrates another embodiment of a printed circuit board including an array of capacitive sense elements arranged on a first side the printed circuit board and a reference capacitive sense element arranged on a second side of the printed circuit board. 
         FIG. 6  illustrates a graph of effects of rising temperature and detection of a conductive object on capacitance of a capacitive sense element measured by a capacitive switched relaxation oscillator with software-only temperature compensation. 
         FIG. 7  illustrates a graph showing an exemplary comparison of effects of different environmental conditions on capacitance of a capacitive sense element measured at one environmental condition (Condition A) and at another environmental condition (Condition B) according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A method and apparatus for detecting a presence of one or more conductive objects at power up of a touch-sensing device is described. Power up of the touch-sensing device includes the application of power to the device and any associated initialization process of the device (herein referred to as the “initialization process”). The initialization process may also occur following a reset of the touch-sensing device. In one embodiment, a reference sense element is employed to capture one or more reference sensing parameters during an initialization process of a touch-sensing device. The reference sense element is located or positioned such that a conductive object is not permitted to be present and is therefore not sensed during the initialization process of the touch-sensing device. The one or more calibrated sensing parameters are used to determine if a conductive object is proximate to any of the one or more capacitive sense elements. A measured capacitance value of each of the one or more capacitive senses elements is compared with a baseline capacitance value, which may be stored in a memory (e.g., a non-volatile memory) of the touch-sensing device, for each of the capacitive sense elements to determine a capacitance difference value. The baseline capacitance values represent the capacitance value of each capacitive sense element when not acted upon by a conductive object. The capacitance difference value for each capacitive sense element is compared to a threshold value to detect the presence of a conductive object on the touch-sensing device. The method and apparatus use a capacitance measurement of the reference sense element to determine whether any other capacitive sense elements are activated during the initialization process. In addition, a pattern of the difference values on multiple capacitive sense elements may be analyzed to determine a location of a touch event on an array of capacitive sense elements (e.g., a centroid analysis indicating a specific location of a touch event on the array of slider capacitive sense elements) as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. In an embodiment, a capacitance of each of a reference sense element and one or more capacitive sense elements is measured. A difference value is calculated by comparing the measured capacitance of the reference sense element to the measured capacitance of each of the one or more capacitive sense elements. A presence of a conductive object proximate to one or more of the capacitive sense elements is detected when the difference value is greater than a predetermined threshold value. 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail, but rather in a block diagram in order to avoid unnecessarily obscuring an understanding of this description. 
     Reference in the description 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 invention. The phrase “in one embodiment” located in various places in this description does not necessarily refer to the same embodiment. 
       FIG. 1  is a block diagram illustrating one embodiment of an electronic system  100  for detecting a presence of a conductive object proximate to a capacitive sense element during an initialization process of a touch-sensing device according to an embodiment of the present invention. Electronic system  100  includes processing device  110 , non-volatile memory  120 , and capacitance sensor  130  connected to reference sense element  150  and an array of capacitive sense elements  160 . 
     Processing device  110  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  110  may be one or more separate integrated circuits and/or discrete components. In one exemplary embodiment, processing device  110  is the Programmable System on a Chip (PSoC®) processing device, developed by Cypress Semiconductor Corporation, San Jose, Calif. Processing device  110  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. 
     Although described as non-volatile memory  120 , other types of memory known by those of ordinary skill in the art may be used, such as magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory, or another type of medium suitable for storing electronic instructions. 
     Capacitance sensor  130  may be integrated into the IC of the processing device  110 , or alternatively, in a separate IC. Alternatively, descriptions of capacitance sensor  130  may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code describing capacitance sensor  130 , 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  130 . 
     Capacitance sensor  130  includes an adjustable sensing parameter source  140  controlled by the processing device  110 . One having ordinary skill in the art will appreciate that the adjustable sensing parameter may be any known parameter that affects the output of the capacitance measurement of the reference sense element and capacitive sense elements ( 150  and/or  160 ) connected to the capacitance sensor  130 , including, but not limited to a current supplied by a current digital-to-analog converter (IDAC) (as shown in the exemplary embodiments illustrated in  FIGS. 1 and 2 ), an adjustable comparator reference voltage supplied by a voltage digital-to-analog converter (VDAC), or a variable multiplication factor of the measurement output value. The processing device  110  is configured to control the adjustable sensing parameter source to tune the output of the source, as described in detail below in connection with  FIG. 3 . Capacitance sensor  130  may include analog I/O for coupling to an external component, such as a touch-sense pad, a touch-sense slider, touch-sense buttons, and/or other devices. Capacitance sensor  130  and processing device  110  are described in more detail below. 
     In an embodiment, the capacitive sense elements  160  (collectively referred to as a “capacitive sense array”) and reference sense element  150  may be responsive to the presence of a conductive object (e.g., a stylus or a user&#39;s finger) as well as the effect of one or more environmental factors. The environmental factor may be any attribute or characteristic of an environment of the capacitive sense elements  160  and reference sense element  150  that effects the capacitance of the capacitive sense elements  160  and reference sense element  150 , such as, for example, temperature, humidity, moisture, etc. 
     In one embodiment, the capacitance sensor  130  can be used to measure capacitance using self-capacitance sensing or mutual capacitance sensing as described below. In one embodiment, the capacitance sensor  130  includes a relaxation oscillator as described below with respect to  FIG. 2 . In other embodiments, the capacitance sensor  130  may include other capacitance sensing circuits, such as the CapSense® Sigma-Delta (CSD) and CapSense® Successive Approximation (CSA) circuits, developed by Cypress Semiconductor, San Jose, Calif., charge accumulation circuits, charge transfer circuits, charge integration circuits, or other capacitance sensing circuits as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. 
     In an embodiment, the reference sense element  150  and the capacitive sense elements  160  having similar characteristics (e.g., size, similar overlay, similar sense element to ground clearance, etc.) are utilized to normalize variables that can affect capacitance and improve the ability to calibrate the sense elements. 
     In an embodiment, the capacitance sensor  130 , which is coupled to the reference sense element  150  and capacitive sense elements  160 , is configured to perform self-capacitance sensing. In a self-capacitance sensing mode, the reference sense element  150  and each capacitive sense element  160  sensor element needs only one connection wire to the capacitance sensor  130  circuit. For the self-capacitance sensing mode, the presence of a conductive object or an environmental factor acting on the reference sense element  150  or capacitive sense element  160  increases the sense element capacitance as the conductive object capacitance is added to the sense element capacitance. 
     In an embodiment, the capacitance sensor  130  of the reference sense element  150  and capacitive sense elements  160  is configured to perform mutual capacitance sensing. In mutual capacitance sensing mode, change in the mutual capacitance between electrodes is detected. The reference sense element  150  and each capacitive sense element  160  sensor element each use at least two electrodes: one is a transmitter (TX) electrode (also referred to herein as transmitter electrode) and the other is a receiver (RX) electrode. The intersection of the TX electrode and the RX electrode is called a sense element. When a conductive object touches a sense element or is in close proximity to the sense element, the capacitive coupling between the receiver and the transmitter of the sense element is decreased as the conductive object shunts part of the electric field to ground (e.g., chassis or earth). 
       FIG. 2  illustrates an embodiment of the electronic system  200  including a capacitance sensor  230  coupled to a processing device  210  for detecting a presence of a conductive object during an initialization process of a touch-sensing device. As shown, the electronic system  200  further includes a non-volatile memory  220 , an array of capacitive sense elements  260 , a reference sense element  250 , and a multiplexer array  270 . In this embodiment, the capacitance sensor  230  includes a relaxation oscillator including an adjustable IDAC current source (i.e., the sensing parameter)  240  that is programmable by the processing device  210 . Although  FIG. 2  illustrates the multiplexer array  270  as being external to the relaxation oscillator  230 , one having ordinary skill in the art will appreciate that the multiplexer array  270  may be a part of the relaxation oscillator  230 . 
     The relaxation oscillator  230  is formed by the capacitance to be measured from any of the capacitive sense elements  260  or reference sense element  250 , the adjustable sensing parameter source  240 , a comparator  231 , a reset switch  233 , and a timer element  232 . In an embodiment, a pin (not shown) may be coupled to each of the capacitive sense elements  260  and the reference sense element  250  and the multiplexer array  270 . One having ordinary skill in the art will appreciate that the operation of the relaxation oscillator  230  and components is well known in the art, and that various other arrangements and circuitry may be used in the electronic system  200  of the present invention. The relaxation oscillator is configured to charge and discharge and includes an output signal that is capable of measurement. In an embodiment, the output signal is an input into a counter that counts the number of times the charge/discharge occurs (i.e., the count measurement), where an increase in capacitance results in an increase in counts. In an alternative embodiment, instead of a counter, a digital filter may be employed at the output of the relaxation oscillator to convert the capacitance measurement into a digital value representing the capacitance. 
       FIG. 3  illustrates an embodiment of a method  300  of detecting a presence of a conductive object during an initialization process of a touch-sensing device. During initialization of the touch-sensing device (block  310 ), a capacitance measurement of a reference sense element (i.e., a sense element that is not touched or touchable during the initialization process) is set to a target output value for the reference sense element by adjusting one or more sensing parameters, at block  320 . In an embodiment, the processing device ( 110 ,  210 ) is configured to control the adjustable sensing parameter source ( 140 ,  240 ) to tune the output of the source such that the capacitance measurement of the reference sense element ( 150 ,  250 ) is set to the target output value, where the resulting value of the sensing parameter is referred to as the sensing parameter value. The sensing parameter value is then used to measure a capacitance on each of the multiple sensing elements, at block  330  (i.e., a measurement of the capacitance of each capacitive sense element is taken at the sensing parameter value). In an embodiment, finer granularity of the sensing parameter adjustment results in an identification of a more precise reference point. 
     According to an embodiment of the present invention, during a manufacturing/production phase or initial testing phase of the touch-sensing device, an initial calibration takes place with no touch present and the initial measured capacitances of each of the sensing elements are stored in non-volatile memory of the device. As shown in block  340 , a determination is made whether the device already has stored baseline capacitance values for each capacitive sense element. The stored baseline capacitance values represent the capacitance value of each capacitive sense element when not acted upon by a conductive object. If no baseline capacitance values have been stored, the initial measurements for each sensing element are stored in non-volatile memory, at block  350 . One having ordinary skill in the art will appreciate that storage of the baseline capacitance values in block  350  typically occurs only one time during manufacturing, but may occur during post-production use of the device, for example, in the event that the stored baseline values are no longer stored in non-volatile memory due to an error or problem resulting in a loss of data. 
     If the device already has baseline capacitance values for each capacitive sense element stored in non-volatile memory, those capacitance values are used as baseline capacitance values, in block  360 . If not, the initial measured values for each capacitive sense element are used as baseline capacitance values, at block  370 . At block  380 , the capacitance values for each capacitive sense element measured at the sensing parameter value are compared to the baseline capacitance value stored in non-volatile memory to determine a difference (referred to as the “capacitance difference”). Following block  380  (or block  370 ), normal operation proceeds (i.e., taking a capacitance measurement and updating the baseline average, in block  390 ). 
     Next, the capacitance difference for each capacitive sense element is compared to a threshold value. The presence of a conductive object proximate to a capacitive sense element is detected (i.e., sense element activation) if the capacitance difference is above the threshold value for that capacitive sense element. If the capacitance difference is less than or equal to the threshold value for a capacitive sense element, then no conductive object is detected for that capacitive sense element. As illustrated, method  300  enables the calibration of the sensing parameter to fine tune an output of the reference sense element so that it matches a target measurement output value during each initialization, which results in the calibration of the sensing parameter that are stable over environmental conditions. 
     One having ordinary skill in the art will appreciate that following the initialization process, the baseline values may be tracked according to an any known method, including, but not limited to, a baseline averaging approach or drift compensation using a slew-rate limited reference adjustment. 
       FIG. 4  illustrates an embodiment of the present invention showing an exemplary approach to “hiding” or otherwise rendering the reference sense element  150  not responsive to a presence of a conductive object during an initialization process of the touch-sensing device. As shown, a printed circuit board  410  having disposed thereon a reference sense element  150  and an array of capacitive sense elements  160  disposed thereon. During manufacturing, an overlay is applied to the printed circuit board  410  such that the reference sense element  450  is not responsive to the presence of a conductive object in close proximity or contact with the overlay. In this arrangement, the capacitive sense elements  460  are responsive to the presence of a conductive object in close proximity or contact with the overlay. 
     Alternatively, a two-sided printed circuit board  510  may be utilized, as shown in  FIG. 5 . The printed circuit board  510  includes a first side (Side  1 ) that is externally facing when arranged within the touch-sensing device, and a second side (Side  2 ) that is not accessible by a user during normal operation of the touch-sensing device. As shown, the capacitive sense elements  560  that are intended to be touched/touchable during normal operation of the touch-sensing device are disposed on Side  1  of the printed circuit board  510 , while the reference sense element  550  is “hidden” from the user on Side  2  of the printed circuit board  510 . One having ordinary skill in the art will appreciate that in addition to the embodiments illustrated in  FIGS. 4 and 5 , there are other arrangements and/or techniques that may be utilized in order to hide the reference sense element such that it is not touched/touchable during normal operation of the touch-sensing device. 
     Referring to graph  620  of  FIG. 6 , under software-only compensation, a reference, or baseline  624 , is tracked so the computing device knows when the user interaction is present (e.g., conductive object on or proximate to a button) by comparing the capacitive sensor relaxation oscillators (CSR) raw counts  622  (representing the capacitance due to the presence of a conductive object, such as a user&#39;s finger) with the baseline  624 . If the CSR raw counts  622  exceed the baseline  624  by a threshold value, user interaction is deemed to be present and appropriate actions are taken; otherwise, no action is taken. The baseline  624  is usually established immediately after power-on. If a user presses a button at the moment the touch-sensing device is powered on and being initialized, the software will establish a baseline which includes the capacitance of a finger. Note that the magnitude of raw count change due to temperature in  FIG. 6  can be equal to or greater than the magnitude of raw count change due to the presence of a finger. 
       FIG. 7  illustrates a graph showing an exemplary comparison of effects of an environmental condition (e.g., rising temperature) on capacitance measured without an adjustment of a sensing parameter under one environmental condition (Condition A) and then measured under another environmental condition (Condition B), according to embodiments of the present invention. It also shows the raw count drift between the two environmental conditions on sensing elements with two different capacitances (sense element  1  and sense element  2 ). As shown, environmental changes generally do not cause an offset error in capacitive measurements, but instead generally introduce a gain error. This causes the raw count drift due to the change in environmental conditions to be different for sensors with different capacitances. Thus a capacitive sensor performing a measurement of a sensing element that is unresponsive to touch and without calibration of sensing parameters could detect the change in environmental condition exemplified in  FIG. 6  by observing the drift in raw counts on the no-touch sensing element, however, the magnitude of the raw count drift on other sensing elements due to environmental conditions cannot be determined accurately by this method. Also note that the slope of the measurement output versus capacitance transfer function determines the number of differential output counts resulting from the small change in capacitance due to a finger touch. Therefore, without adjusting the sensing parameter, the touch “signal” resulting from the same touch will have different magnitudes at different environmental conditions. By changing the sensing parameter, according to embodiments of the present invention, the measurement gain can be adjusted to obtain the same output versus capacitance transfer function over different environmental conditions. This is necessary for power up button press detection since the environmental condition on power up and during the initialization process will be unknown. 
     The embodiments described herein are not limited to any particular touch-sensing device, but can be used in other capacitive sense implementations, for example, the sense device may be a touch screen, a touch-sense pad for notebook implementations, a touch-sense slider, or touch-sense buttons (e.g., capacitance sense buttons). In one embodiment, these sense devices may include one or more capacitive sense elements. The operations described herein are not limited to notebook pointer operations, but can include other operations, such as lighting control (dimmer), volume control, graphic equalizer control, speed control, or other control operations requiring gradual or discrete adjustments. It should also be noted that these embodiments of capacitive sense implementations may be used in conjunction with non-capacitive sense elements, including but not limited to pick buttons, sliders (e.g., display brightness and contrast), scroll-wheels, multi-media control (e.g., volume, track advance, etc) handwriting recognition and numeric keypad operation. 
     Certain portions of the embodiments may be implemented as a computer program product that may include instructions stored on a computer-readable medium. These instructions may be used to program a general-purpose or special-purpose processor to perform the described operations. A computer-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The computer-readable storage medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory, or another type of medium suitable for storing electronic instructions. The computer-readable transmission medium includes, but is not limited to, electrical, optical, acoustical, or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, or the like), or another type of medium suitable for transmitting electronic instructions. 
     Additionally, some embodiments may be practiced in distributed computing environments where the computer-readable medium is stored on and/or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the transmission medium connecting the computer systems. 
     Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent and/or alternating manner. 
     The particular features, structures or characteristics described herein may be combined as suitable in one or more embodiments of the invention. In addition, while the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. The embodiments of the invention can be practiced with modification and alteration within the scope of the appended claims. The specification and the drawings are thus to be regarded as illustrative instead of limiting on the invention.