US20080030479A1 - System and method of integrating a touchscreen within an lcd - Google Patents

Abstract

A touchscreen liquid crystal display, method for using a liquid crystal display as a user input, and a mobile electronic device are provided. The touchscreen liquid crystal display comprises: a liquid crystal display having a viewing surface and including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface; a driver circuit coupled to the first and second electrodes for driving the electrodes for selectively controlling a display state of the display pixel elements; and a measurement circuit coupled to the electrodes for measuring display pixel element voltages for at least some of the display pixel elements formed by the first electrode, and for each display pixel element for which a display pixel element voltage is measured, comparing the measured voltages to reference voltages and determining a relative force of the external pressure on the viewing surface based on the measured voltages.

Description

RELATED APPLICATION DATA
• The present application is a continuation of non-provisional U.S. patent application Ser. No. 10/717,877, filed Nov. 20, 2003, which claims priority to provisional U.S. patent application Ser. No. 60/427,963, filed Nov. 21, 2002. The entire contents of non-provisional application Ser. No. 10/717,877 and ofprovisional application 60/427,963 are hereby incorporated by reference.
TECHNICAL FIELD
• The present disclosure relates to the field of liquid crystal displays (LCDs), and more specifically to touchscreen LCDs.
BACKGROUND
• Although there are several types of touchscreens possible, the two most commonly used touchscreens in handheld electronic devices are resistive and capacitive touchscreens.
• Resistive touchscreens use a thin, flexible membrane over a glass substrate. The substrate surface and the facing membrane surface have a transparent metallic coating and are separated by spacers. When a user presses on the outer surface of the membrane, the inner surface of the membrane meets the substrate causing a change in resistance at the point of contact. A touchscreen controller measures this resistance using the membrane and the substrate as a probe. The two resistance measurements provide the x and y coordinates of the point of contact. Resistive touchscreens reduce the reflection and clarity of the LCD because of the added membrane layer and air gap in front of the surface of the LCD. A solution is required that does not require added layers that reduces the LCD visibility.
• Capacitive touchscreens use a metallic coating on a glass sensor. Typically, voltage is applied to the four corners of the sensor. When the screen is not in use, the voltage spreads across the sensor in a uniform field. When the user touches the sensor, the touchscreen controller recognizes a disturbance of the field and sends the x-y coordinate of the point of contact to the CPU of the device. Capacitive touchscreens can only be used with a bare finger or conductive stylus. A touchscreen solution is required that can convert any touch into touchscreen data.
• Resistive and capacitive touchscreens add thickness to the LCD module because of the added layers to provide touchscreen capabilities. With the demand for streamlining and minimizing the size of handheld devices, LCD modules need to be as thin as possible. A touchscreen solution is required to maximize the reflective characteristics of an LCD and to minimize the thickness of an LCD module.
BRIEF DESCRIPTION OF THE DRAWINGS
• In order that the present disclosure may be more clearly understood, one or more example embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:
• FIG. 1 is a sectional drawing showing the structure of an LCD;
• FIG. 2 is a graph depicting the voltage change for a segment of pixels due to an applied force on an LCD;
• FIG. 3 is a front view of the LCD;
• FIG. 4 is a system diagram showing touchscreen circuitry for an integrated LCD touchscreen; and
• FIG. 5 is a flow diagram showing the method for sensing a force applied to the LCD.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
• The present disclosure provides a touchscreen which is integrated into an LCD by using the electrodes that forms the pixels to measure voltage differences to locate a point of contact, and a method for controlling a touchscreen with integrated LCD.
• In accordance with one example embodiment of the present disclosure, there is provided a touchscreen liquid crystal display, comprising: a liquid crystal display having a viewing surface and including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface; a driver circuit coupled to the first and second electrodes for driving the electrodes for selectively controlling a display state of the display pixel elements; and a measurement circuit coupled to the electrodes for measuring display pixel element voltages for at least some of the display pixel elements formed by the first electrode, and for each display pixel element for which a display pixel element voltage is measured, comparing the measured voltages to a reference voltage and determining a relative force of the external pressure on the viewing surface based on the measured voltages.
• In accordance with another example embodiment of the present disclosure, there is provided a method for using a liquid crystal display as a user input, the liquid crystal display having a viewing surface and including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface, the method comprising: (a) selectively driving the first and second electrodes to cause the pixel elements to display an image visible on the viewing surface; (b) sampling voltages between the first and second electrodes; (c) comparing the measured voltages to a reference voltage; and (d) determining a relative force of the external pressure on the viewing surface based on the measured voltages
• In accordance with a further example embodiment of the present disclosure, there is provided a mobile electronic device, comprising: a processor for controlling the operation of the device; a touchscreen liquid crystal display for display an image visible from a viewing surface, comprising: a liquid crystal display including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface; a driver circuit coupled to the first and second electrodes for driving the electrodes for selectively controlling a display state of the display pixel elements; and a measurement circuit coupled to the electrodes for measuring display pixel element voltages for at least some of the display pixel elements formed by the first electrode, and for each display pixel element for which a display pixel element voltage is measured, comparing the measured voltages to a reference voltage and determining a relative force of the external pressure on the viewing surface based on the measured voltages.
• Turning now to the drawings,FIG. 1 depicts an LCD structure. As it is known in the art, theLCD100 consists of a sandwich ofliquid crystal112 between atop glass substrate104 and abottom glass substrate118 withpolarizers102,120 on the external surfaces of theglass substrates104,118. A user would view information on theLCD100 through thetop glass substrate104. Thepolarizers102,120 control the light that enters and leaves theLCD100. Thetop polarizer102 is polarized oppositely or perpendicularly to thebottom polarizer120. Polarized light enters the LCD and twists around theliquid crystal molecules112 so that the light's polarization becomes oppositely polarized and then exits theLCD100. Wherever light passes through all the layers of theLCD100, pixels appear white.
• On the internal surface of thetop glass substrate104 is acolour filter106. A first layer of strips oftransparent electrodes108 is on thetop glass substrate104. A second layer oftransparent electrodes116 is attached on the internal surface of thebottom glass substrate118, perpendicular to the first layer ofelectrodes108. Therefore if the first layer of electrodes ran in a direction parallel to the width (commons) of theglass substrates104,118, then the second layer ofelectrodes116 runs in a direction parallel to the length (segments) of theglass substrates104,118. These transparent electrodes are usually made using Indium-Tin Oxide (ITO). Wherever a strip of ITO from the first layer ofelectrodes108 crosses a second strip of ITO from the second layer ofelectrodes116, a pixel element is formed. Each strip of ITO from the first and second layer ofelectrodes108,116 is typically electrically connected to a drive circuit. At each pixel, the drive circuit can control the voltage, which determines the state of theliquid crystal material112.
• Over the electrodes are twoalignment layers110,114, which is usually a thin polymer film that is rubbed to form grooves (grooves not shown). The grooves in thetop alignment layer110 andbottom alignment layer114 are usually brushed or rubbed so that theliquid crystal112 will twist in order to align with the grooves. The angles at which thealignment layers110,114 are brushed to form the twist in the alignment of liquid crystal molecules are typically set depending on the desired contrast, viewing angle, background colour and any other factor that determines such angles. When the electrodes are driven, a voltage is placed across theliquid crystal112 twisting the molecules out of alignment. The light that enters theLCD100 does not twist and subsequently cannot exit theLCD100. Such pixels appear black.
• The electrical model of a pixel is similar to a capacitor. The intersection of segments and commons of ITO108,116 form capacitor plates and theliquid crystal112 acts as the dielectric of a capacitor. The capacitance is determined as follows:
  C=(k∈₀A)/d  (1)
• where C is the capacitance, k is the dielectric constant, ∈₀is the permittivity of free space, A is the area of the plates, and d is the distance between the plates. The dielectric constant of theliquid crystal112 is determined by the type ofliquid crystal112 used in theLCD100. In the pixel model, the area of the plates is equal to the area of the pixel, and the distance between the plates is the distance between theelectrodes108,116. Voltage V across this capacitor is equal to charge Q over capacitance C (V=Q/C), therefore, voltage is proportional to the distance between the plates.
• When a force is applied to the surface of thetop glass substrate104, such as a press with a finger or stylus, the distance between the top andbottom glass substrates104,118 changes and thus changes the distance between the strips ofITO electrodes108,116. When the distance between the electrodes changes, the capacitance of the pixel changes and the change in capacitance can be detected by the resulting change in voltage at that pixel. Because of the relationship between voltage and distance between the plates, as the electrodes get closer, the pixel voltage will decrease. Using the capacitance change of an LCD pixel to determine the location of an applied force eliminates the need for touchscreen overlays, which add thickness to an LCD module and therefore add thickness to a device which houses such an LCD module. Because overlays also tend to obscure the reflection and clarity of an LCD, eliminating the overlay and using the existing LCD structure improves the visibility for a touchscreen type LCD module. Cost is also reduced since no extra material other than the LCD is required. The solution does not require an extra glass layer, or flex connectors that add to the overall cost of an LCD module.
• FIG. 2 is a graph depicting the voltage change for a segment of pixels due to an example of a simulated applied force on an LCD. In thisexample graph200, thevoltage change202 across a segment of pixels corresponding to a strip of ITO in a 160×160 pixel LCD is indicated. The segment of pixels was numbered from 0 to 159 across an active area of the LCD. The simulated force on the top glass of the LCD was pressed at pixel52 on this segment. The voltage change shows adrop204 in pixel voltage at pixel52. If the voltage at each pixel is compared to a reference voltage, then the location of the applied force can be identified by detecting the pixel location of the minimum voltage (maximum voltage difference). This reference voltage may come from a segment of pixels that is not exposed to an external pressure such as a finger or stylus press. As shown in thegraph200, the voltage drops for several pixels in the vicinity of where the LCD was pressed. These residual voltage drops are due to several factors including but not limited to the size of the finger or stylus, the pixel size, and the deflection of the glass as it is being pressed.
• FIG. 3 shows a front view of an integrated touchscreen LCD assembly. TheLCD assembly300 comprises atop glass302, and abottom glass304. Aseal306 encloses the liquid crystal material between the twoglass substrates302,304. Theseal306 is preferably a glass epoxy seal. Aviewing area310 is the area of an LCD within theseal306 that is visible through a bezel or cutout in a device in which the LCD is housed. Anactive area312 is defined by a conductive area of ITO segments and commons (not shown) within theviewing area310; that is, the area where the images are displayed. Areference ITO segment308 is located outside theactive area312, outside theviewing area310, in close proximity to theseal306. In this example thereference ITO308 is a segment, but thereference ITO308 is not limited to a segment format, and could also be in a common format. When a force is applied to theactive area312 of theLCD300, thereference segment308 is not impacted because of its close proximity to theseal306 and therefore a voltage drop across thereference segment308 is negligible. Thereference segment308 is driven using the same data as any segment line that is being measured.
• FIG. 4 is a system diagram showing touchscreen circuitry for an integrated LCD touchscreen. The system is controlled by an MPU (micro-processing unit)401. Thecircuit400 comprises measuringcircuitry403 and existingLCD driver circuitry402, which is preferably incorporated into an integrated circuit (IC). The components of the measuringcircuitry403 are preferably added to the IC housing theLCD driver circuitry402.
• The existingLCD driver circuitry402 electrically connects to the segments404 andcommons408 of anLCD409, wherein the segment lines404 haveswitches405 to disconnect the pixels of the segment from thedriver402. Theseswitches405 are controlled by alogic controller410. In this example, onesegment406 is disconnected from thedrive circuitry402 at any given time by opening thesegment switch407. TheLCD409, in this example, has 160×160 pixels; therefore there are 160 segment lines (SEG0-SEG159)404 and160 common lines (COM0-COM159)408. A reference segment line (REF SEG)450 is also controlled by thedriver circuitry402 wherein the REF SEG also has aswitch452 controlled by thelogic controller410. The system also preferably comprises a multiplexer (MUX)412, a correlated double sampler (CDS)414, anamplifier416, a sample and hold (S/H)418, a comparator (C)420, an analog-to-digital converter (A/D)422, andseveral registers426,428,430,432,434.
• TheMPU401 communicates with thedriver circuitry402. The driver circuitry preferably comprises anMPU interface440, an LCD controller withRAM442,SEG drivers444,COM drivers446, and adisplay timing circuit448. TheMPU401 communicates with the driver circuitry via theMPU interface440, which converts the MPU data into LCD driver data. TheLCD controller442 takes the data from theMPU401 and combines it with data from thedisplay timing circuit448. Thedisplay timing circuit448 defines the frame frequency of the LCD and determines when the segments and commons are driven. The LCD controller converts the combination of data from theMPU401 and thedisplay timing circuit448 to driver data and sends it to theSEG drivers444 and theCOM drivers446, which respectively drive the SEG lines404 and the COM lines408. The SEG lines and COM lines form the pixels on theLCD409. The LCD controller uses RAM as a frame buffer for representing data that is to be displayed.
• Theswitch407 on a scannedsegment line406 disconnects the pixels on that segment line from theSEG driver444. The voltage of the disconnected segment line may be measured by the measuringcircuitry403. Thelogic control410 determines when theswitches405,407 are opened or closed and only one switch will be opened at a time. Aswitch407 is open preferably for approximately one frame, which is when an entire LCD screen is updated or refreshed. A typical frame frequency for a 160×160 LCD is 65 Hz. TheSEG driver444 drives the REF SEG450 with the same data as the segment that is being sampled.
• Thelogic controller410 performs several functions in thissystem400. As previously mentioned, thelogic controller410 opens asegment switch407 for measurement by the measuringcircuitry403. Thelogic controller410 also addresses theMUX412 to select a sample segment (in this example, segment159406 is sampled and scanned) for scanning such that it is disconnected from theSEG driver444 by opening the sample segment switch (407). Thelogic controller410 provides the clock signal to theCDS414 to define when the sampling occurs. TheCDS414 subtracts the reference segment voltage from the voltage of thesample segment line406. Using aCDS414 is a technique commonly used in the field of CCD (charged coupled device) imaging to process the output signal from a CCD image sensor in order to reduce low-frequency noise from components such as the LCD driver circuit, components within the device housing the LCD, and sources outside the device. Using CDS in CCD imaging is well known in the art.
• TheCDS414 sends the voltage difference to theamplifier416, which increases the signal since the voltage difference from theCDS414 will be very small. The amplified signal is sent to the S/H418. The S/H418 stores the maximum voltage difference measured for all the scanned segments. Thecomparator420 compares the present voltage difference with the maximum voltage difference stored in the S/H418. If the present voltage difference is greater than that stored in the S/H418, then the comparator output is asserted and a new maximum voltage difference is stored by the S/H418. If the present voltage difference is not greater than the stored voltage difference in the S/H418, then no new voltage difference is stored. Thelogic controller410 then scans the next segment until all segments are scanned.
• There are two scanning directions being measured in this example. When thesample segment406 is scanned by thelogic controller410, the measuringcircuit403 sees 160 different output readings for thissample segment406 as thecommon lines408 are driven one by one. Thelogic controller410 then determines thesample segment406 that has the maximum difference from reference segment450. When thelogic controller410 starts scanning segments the location of the force can also be determined along thecommon lines408. TheSEG counter register430 andCOM counter register432 keep track of which segment and common are being measured, respectively. Thelogic controller410 saves the value in theSEG counter430 andCOM counter432 when thecomparator420 triggers thelogic controller410 the counter value for both SEG and COM are saved. These saved values represent the location of the maximum voltage difference.
• If the present voltage difference is higher than the stored voltage difference, theAID422 converts the voltage difference to a value that represents the force applied to the glass and may save it to a register, Z,426. This value may be used for input options. Detecting the amount of pressure used in the applied force can indicate what kind of press was used; for example determining the amount of force applied can indicate if the user had made a full press or a double press. As the force applied to the glass increases, the capacitance at the selected pixels increases and subsequently the voltage difference increase. When the voltage at the selected pixels is compared to the REF SEG450, the difference will be larger than a segment that has no applied force.
• In an idle mode, where there is no force applied to the LCD glass, the measuringcircuit403 preferably scans only one segment at a slow rate. A slow rate is selected to reduce power consumption that scanning may increase. Another reason for a slow rate of scanning is to reduce the impact on the contrast of the LCD. This segment is preferably located in the middle of theLCD408. Therefore, if a 160×160 LCD is used, the middle segment is segment79
• In an alternative embodiment, the measuringcircuitry403 may scan more than one segment when in idle mode. In this embodiment, the measuring circuitry may alternate the scan for an applied force on the LCD glass by scanning one segment per frame in selected areas of theLCD408. For example, if the measuring circuitry scans three segments in the idle mode, the measuring circuitry may scan a segment near one edge of theactive area312 of the LCD in one frame, a segment at the middle of theactive area312 in the next frame, and a segment at the opposite edge of theactive area312 in the next frame.
• When a new maximum voltage difference is measured and saved and thecomparator420 triggers thelogic controller410 from idle mode into scan mode, thelogic controller410 scans the segments of the LCD and compares each segment to the REF SEG450. To minimize power consumption and contrast degradation, a percentage of the segments are preferably scanned. For a 160×160 LCD, the minimum percentage of segments scanned to minimize power consumption and contrast degradation is approximately 10%. In an alternative embodiment, for higher accuracy of determining the location of an applied force, a higher percentage of segments or all the segments are scanned when the logic controller is triggered into the scan mode.
• In a further alternative embodiment, if more than one segment is scanned in idle mode, then when a force is applied to the LCD glass, the segment that is continuously scanned closest to the force has the maximum voltage difference measurement. The measuring circuit may only scan the segments in close proximity to the scanned segment with the lowest voltage difference measurement.
• When a force is applied to the LCD glass, the logic controller sends an interrupt signal to theMPU interface440, which in turn sends the signal to theMPU401. The MPU reads the location value of the applied force and interprets the corresponding input made by the user. The location registers are cleared.
• In an alternative embodiment, the center of deflection of an applied force may be calculated by the device operating system by taking a weighted averaged of all deflections and calculating the centroid of the force. Such a calculation is made to determine the location of an applied force with greater accuracy. In the mode previously described, a location of an applied force can be defined by which segment and common have the lowest voltage. Using a centroid calculation allows the location to be determined to a fraction of a pixel. This method is preferred for applications that require high resolution such as hand writing recognition. The centroid calculations are determined using the following formulae:$\begin{array}{cc}\begin{array}{c}{\mathrm{Seg}}_{\mathrm{centroid}}={\mathrm{<figure-callout\; label="X"\; filenames="US20080030479A1-20080207-D00002.png"\; state="\{\{state\}\}">X</figure-callout>}}_0+\\ \frac{\sum _{\mathrm{com}=0}^{\mathrm{com}}\sum _{\mathrm{seg}=0}^{\mathrm{seg}}\mathrm{SEG\_counter}\times Z\left(\mathrm{seg},\mathrm{com}\right)}{\sum _{\mathrm{com}=0}^{\mathrm{com}}\sum _{\mathrm{seg}=0}^{\mathrm{seg}}Z\left(\mathrm{seg},\mathrm{com}\right)}\end{array}& \left(2\right)\\ \begin{array}{c}{\mathrm{Com}}_{\mathrm{centroid}}={\mathrm{<figure-callout\; label="Y"\; filenames="US20080030479A1-20080207-D00002.png"\; state="\{\{state\}\}">Y</figure-callout>}}_0+\\ \frac{\sum _{\mathrm{com}=0}^{\mathrm{com}}\sum _{\mathrm{seg}=0}^{\mathrm{seg}}\mathrm{COM\_counter}\times Z\left(\mathrm{seg},\mathrm{com}\right)}{\sum _{\mathrm{com}=0}^{\mathrm{com}}\sum _{\mathrm{seg}=0}^{\mathrm{seg}}Z\left(\mathrm{seg},\mathrm{com}\right)}\end{array}& \left(3\right)\end{array}$
• The location of the applied force is found as previously described. The pixels around the location of the lowest recorded voltage are scanned again, for example in a 10×10 matrix around the location. A 10×10 matrix is an example of the size of a typical finger press; however, the matrix is not limited to such a matrix size. Smaller matrix sizes may be used to represent a typical force applied from a stylus press.
• In equation (2), X_Ois the segment number for the starting location of the matrix. Typically, this segment is the leftmost segment of the matrix, but may also be the rightmost segment. SEG counter is the value in theSEG counter register430. Z (seg, corn) is the value representing the amount pressure of the applied force, which is stored in theZ register426.
• In equation (3), Y_Ois the common number for the starting location of the matrix. Typically, this common is the topmost segment of the matrix, but may also be the bottommost segment. COM counter is the value in theCOM counter register432. Z (seg, corn) is the value representing the amount pressure of the applied force, which is stored in theZ register426.
• The centroid calculation is analogous to a center-of-mass calculation for an object if the local mass density is represented, in this case, as the pressure of an applied force.
• FIG. 5 is a flow diagram illustrating themethod500 used to determine the location of an applied force to an LCD. Instep502, the measuringcircuit403 is in idle mode and scans at least one segment in the active area of an LCD. The measuringcircuit403 compares the voltage of the scannedsegment406 to the reference segment450. Instep504, a user applies a force to the LCD glass to make an input to a device housing the LCD. Instep506, a voltage drop is detected across the scannedsegment406. The measuringcircuit403 is triggered into scan mode instep508. The measuringcircuit403 scans a percentage of the segments to find the segment with the maximum drop in voltage. The percentage is preferably between 10% and 100% of segments. The maximum drop in voltage represents the location of the applied force. Instep510, the maximum voltage difference between a scanned segment and the reference segment450 is stored. The segment and common locations of the maximum voltage difference is also stored. Instep512, the measuringcircuit403 sends theMPU401 the location value. Instep514, theMPU401 translates the location into input data. Instep516, theMPU401 clears the location value and returns the measuringcircuit403 to the idle mode. Themethod500 returns to step502 where the measuring circuit continuously scans at least one segment.
• The presently described present disclosure can be applied to display panels of both passive matrix and active matrix type displays. In active matrix type displays the control and measurement circuitry can be conveniently incorporated as part of the display panel.
• It will be appreciated that the above description relates to the preferred embodiment by way of example only. Many variations on the present disclosure will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the present disclosure as described and claimed, whether or not expressly described.

Claims (23)

1. A touchscreen liquid crystal display, comprising:

a liquid crystal display having a viewing surface and including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface;

a driver circuit coupled to the first and second electrodes for driving the electrodes for selectively controlling a display state of the display pixel elements; and

a measurement circuit coupled to the electrodes for measuring display pixel element voltages for at least some of the display pixel elements formed by the first electrode, and for each display pixel element for which a display pixel element voltage is measured, comparing measured voltages to a reference voltage and determining a relative force of the external pressure on the viewing surface based on the measured voltages.

2. The display ofclaim 1, wherein the measurement circuit is configured for determining a type of press causing the external pressure applied to the viewing surface.

3. The display ofclaim 2, wherein determining a type of press comprises determining whether the determined relative force is associated with a full press or double press on the viewing surface causing the external pressure applied to the viewing surface.

4. The display ofclaim 1, further comprising:

a reference electrode in the liquid crystal display overlapping with the first or second electrodes to form reference pixel elements;

wherein the measurement circuit is configured to measure corresponding reference pixel element voltages at corresponding reference pixel elements to obtain the reference voltage.

5. The display ofclaim 4, wherein the reference pixel elements are located outside of a viewable area of the liquid crystal display a sufficient distance so as not to be substantially affected by external pressure applied to the viewing surface.

6. The display ofclaim 4, wherein the measurement circuit is configured to determine in dependence on the measured display pixel element voltages and corresponding reference pixel element voltages a relative displacement between at least some of the first electrodes and the second electrodes in response to external pressure applied to the viewing surface to determine a relative force of the external pressure on the viewing surface based on the measured voltages.

7. The display ofclaim 1, wherein the measurement circuit is configured for operating in a first mode and in a second mode, wherein in the first mode the measurement circuit measures voltages across a subset of the pixel elements until the measured electrical characteristic indicates that external pressure has been applied to the viewing surface, after which the control circuit automatically operates in the second mode, wherein in the second mode the measurement circuit measures voltages across a larger set of the pixel elements and determines the location of the external pressure based thereon.

8. The display ofclaim 1, wherein the measurement circuit is configured for determining a location of the external pressure on the viewing surface based on the measured voltages.

9. The display ofclaim 8, wherein a location of the external pressure is determined based on which measured pixel element voltage varies the greatest from a reference value determined in dependence on the reference voltage.

10. The display ofclaim 4, wherein each of the first and second electrodes is a substantially transparent strip electrode, the first electrodes being arranged substantially parallel to each other, the second electrodes being arranged substantially parallel to each other and substantially orthogonal to the first electrodes for defining the array of pixel elements, each pixel element being associated with one of the first electrodes and one of the second electrodes, the measuring circuit including a sampling circuit for sampling a voltage across each of the pixel elements and a processing circuit for detecting the displacement and a location thereof based on the sampled voltages, the reference electrode being arranged substantially parallel to either the first or second electrode and substantially orthogonal to the other of the first or second electrode.

11. The display ofclaim 10, wherein the reference electrode forms the reference liquid crystal pixel elements with the first electrodes, and the second electrodes are each individually sampled to acquire the measured voltages, the reference electrode being driven with the same data as the second electrode being sampled.

12. The display ofclaim 10, wherein a plurality of scanable electrodes are included among at least one of the first electrodes and the second electrodes, each scanable electrode being connected by an associated switch to the driver circuit, the sampling circuit including a controller for individually controlling each switch, the controller being configured for opening the switch associated with a selected one of the scanable electrodes and causing the voltage across the pixel elements associated with the selected one of the scanable electrodes to be sampled when the switch associated with the selected one of the scanable electrodes is open.

13. The display ofclaim 10, wherein the electrodes are Indium-Tin Oxide (ITO).

14. A method for using a liquid crystal display as a user input, the liquid crystal display having a viewing surface and including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface, the method comprising:

(a) selectively driving the first and second electrodes to cause the pixel elements to display an image visible on the viewing surface;

(b) sampling voltages between the first and second electrodes;

(c) comparing the measured voltages to a reference voltage; and

(d) determining a relative force of the external pressure on the viewing surface based on the measured voltages.

15. The method ofclaim 14, further comprising:

determining a type of press associated with the external pressure applied to the viewing surface in accordance with the determined relative force.

16. The method ofclaim 15, wherein determining a type of press comprises determining whether the external pressure applied to the viewing surface relative force is associated with a full press or double press on the viewing surface.

17. The method ofclaim 14, further comprising:

sampling voltages between a reference electrode in the liquid crystal display and a plurality of first electrodes or second electrodes that the reference electrode overlaps for use as the reference voltage.

18. The method ofclaim 14, further comprising:

determining based on the sampled voltages if any of the first electrodes have been displaced towards the second electrodes.

19. The method ofclaim 18, wherein the sampling step (b) includes sampling voltages at a sub-set of pixel element locations until a determination is made that a displacement of first electrodes has occurred and then sampling voltages at a larger set of pixel element locations and determining based on the sampled voltages from the larger set a relative location of the displacement.

20. The method ofclaim 19, wherein sampling of the sub-set is carried out at a lower rate than sampling of the larger set.

21. The method ofclaim 19, wherein based on the measured voltages from the sub-set a general location of the displacement is determined, and the larger set is selected to include the general location.

22. The method ofclaim 18, further comprising determining the center of deflection of the displaced first electrodes by determining, based on the measured voltages, a weighted average of the deflection at a plurality of the pixel locations and determining a centroid of the deflection based on the weighted average.

23. A mobile electronic device, comprising:

a processor for controlling the operation of the device;

a touchscreen liquid crystal display for display an image visible from a viewing surface, comprising:

a liquid crystal display including a plurality of parallel first electrodes located on one side of a liquid crystal containing area and overlapping with a plurality of parallel second electrodes located on an opposite side of the liquid crystal containing area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface;

a driver circuit coupled to the first and second electrodes for driving the electrodes for selectively controlling a display state of the display pixel elements; and

a measurement circuit coupled to the electrodes for measuring display pixel element voltages for at least some of the display pixel elements formed by the first electrode, and for each display pixel element for which a display pixel element voltage is measured, comparing the measured voltages to a reference voltage and determining a relative force of the external pressure on the viewing surface based on the measured voltages.

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