EP3084750B1 - Electrowetting display device, and control method thereof - Google Patents
Electrowetting display device, and control method thereof Download PDFInfo
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- EP3084750B1 EP3084750B1 EP14825331.3A EP14825331A EP3084750B1 EP 3084750 B1 EP3084750 B1 EP 3084750B1 EP 14825331 A EP14825331 A EP 14825331A EP 3084750 B1 EP3084750 B1 EP 3084750B1
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Classifications
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- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
- G09G3/3433—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices
- G09G3/348—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices based on the deformation of a fluid drop, e.g. electrowetting
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
- G09G2310/0202—Addressing of scan or signal lines
- G09G2310/0213—Addressing of scan or signal lines controlling the sequence of the scanning lines with respect to the patterns to be displayed, e.g. to save power
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
- G09G2310/0243—Details of the generation of driving signals
- G09G2310/0254—Control of polarity reversal in general, other than for liquid crystal displays
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/06—Details of flat display driving waveforms
- G09G2310/061—Details of flat display driving waveforms for resetting or blanking
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- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
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- G09G2360/16—Calculation or use of calculated indices related to luminance levels in display data
Definitions
- Electrowetting display apparatuses having a display controller and a display device are known.
- the display elements of such a display device include two immiscible fluids.
- the configuration of the fluids can be controlled by applying a voltage to the display element, thereby forming a display effect.
- the display elements can be controlled to display the data, for example video images.
- European patent publication number 2 476 489 A1 describes an active matrix electrowetting on dielectric device having an AC drive scheme.
- International patent publication number 2013/092646 A1 describes a method of driving an electrowetting display device including a pixel including applying a first pixel voltage to the pixel during a first portion of a display period and applying a second pixel voltage to the pixel during a second portion of the display period, the first and second pixel voltages having different polarities.
- US patent publication number 2011/032276 A1 describes a method of driving a display device including providing a reference signal with a fixed voltage level to a storage capacitor and providing a threshold signal to a pixel element with different voltage level than the fixed voltage level.
- International patent publication number 2011/051413 A1 describes a method of driving a display element of an electrowetting display device including applying a voltage with a signal level configured throughout a display period such that a fluid of the display element adjoins at least a non-zero minimum area of a surface of a support plate.
- FIG. 1 shows a diagrammatic cross-section of part of an example of an electrowetting device.
- the device is an electrowetting display device 1 including a plurality of electrowetting cells, which are display elements 2, one of which is shown in the Figure.
- the lateral extent of the display element is indicated in the Figure by two dashed lines 3, 4.
- the display elements comprise a first support plate 5 and a second support plate 6.
- the support plates may be separate parts of each display element, but the support plates may be shared in common by the plurality of display elements.
- the support plates may include a glass or polymer substrate 6, 7 and may be rigid or flexible.
- the display device has a viewing side 8 on which an image or display formed by the display device can be viewed and a rear side 9.
- the first support plate 5 defines the rear side 9 and the second support plate 6 defines the viewing side; alternatively, the first support plate may define the viewing side.
- the display device may be of the reflective, transmissive or transflective type.
- the display device may be of a segmented display type in which the image may be built up of segments, each segment including several display elements.
- the display device may be an active matrix driven display device, a direct drive display device or a passively driven display device.
- the plurality of display elements may be monochrome. For a colour display device the display elements may be divided in groups, each group having a different colour; alternatively, an individual display element may be able to show different colours.
- a space 10 between the support plates is filled with two fluids: a first fluid 11 and a second fluid 12 at least one of which may be a liquid.
- the second fluid is immiscible with the first fluid.
- the second fluid is electrically conductive or polar and may be water or a salt solution such as a solution of potassium chloride in water.
- the second fluid may be transparent, but may instead be coloured, white, absorbing or reflecting.
- the first fluid is electrically non-conductive and may for instance be an alkane like hexadecane or may be an oil such as silicone oil.
- the first fluid absorbs at least a part of the optical spectrum.
- the first fluid may be transmissive for a part of the optical spectrum, forming a colour filter.
- the first fluid may be coloured by addition of pigment particles or a dye.
- the first fluid may be black, i.e. absorb substantially all parts of the optical spectrum, or reflecting.
- a reflective first fluid may reflect the entire visible spectrum, making the layer appear white, or part of it, making it have a colour.
- the support plate 5 includes an insulating layer 13.
- the insulating layer may be transparent or reflective.
- the insulating layer 13 may extend between walls of a display element.
- layers of the insulating layer may extend uninterrupted over a plurality of display elements 2, as shown in the Figure.
- the insulating layer has a surface 14 facing the space 10 of the display element 2. In this example the surface 14 is hydrophobic.
- the thickness of the insulating layer may be less than 2 micrometres and may be less than 1 micrometre.
- the insulating layer may be a hydrophobic layer; alternatively, it may include a hydrophobic layer 15 and a barrier layer 16 with predetermined dielectric properties, the hydrophobic layer 15 facing the space 10, as shown in the Figure.
- the hydrophobic layer is schematically illustrated in Figure 1 and may be formed of TeflonĀ®AF1600.
- the barrier layer 16 may have a thickness, taken in a direction perpendicular the plane of the substrate, between 100 nanometres and 150 nanometres and may be made of an inorganic material like silicon oxide or silicon nitride or a stack of these (for example, silicon oxide - silicon nitride - silicon oxide) or an organic material like polyimide or parylene.
- the barrier layer may comprise multiple layers having different dielectric constants.
- Wettability relates to the relative affinity of a fluid for the surface of a solid. Wettability may be measured by the contact angle between the fluid and the surface of the solid. The contact angle is determined by the difference in surface tension between the fluid and the solid at the fluid-solid boundary. For example, a high difference in surface tension can indicate hydrophobic properties.
- Each element 2 includes an electrode 17 as part of the support plate 5. In examples shown there is one such electrode 17 per element.
- the electrode 17 is separated from the fluids by the insulating layer 13; electrodes of neighbouring display elements are separated by a non-conducting layer. In some examples, further layers may be arranged between the insulating layer 13 and the electrode 17.
- the electrode 17 can be of any desired shape or form.
- the electrode 17 of a display element is supplied with voltage signals by a signal line 18, schematically indicated in the Figure.
- a second signal line 19 is connected to an electrode 25 that is in contact with the conductive second fluid 12. This electrode may be common to all elements, when they are fluidly interconnected by and share the second fluid, uninterrupted by walls.
- the display element 2 can be controlled by a voltage V applied between the signal lines 18 and 19.
- the electrodes 17 on the substrate 7 are coupled to a display control apparatus. In a display device having the display elements arranged in a matrix form, that is arranged in rows and columns, the electrodes can be coupled to a
- the first fluid 11 in this example is confined to one display element by walls 20 that follow the cross-section of the display element.
- the cross-section of a display element may have any shape; when the display elements are arranged in a matrix form, the cross-section is usually square or rectangular.
- the walls are shown as structures protruding from the insulating layer 13, they may instead be a surface layer of the support plate that repels the first fluid, such as a hydrophilic or less hydrophobic layer.
- the walls may extend from the first to the second support plate but may instead extend partly from the first support plate to the second support plate as shown in FIG. 1 .
- the extent of the display element, indicated by the dashed lines 3 and 4 is defined by the centre of the walls 20.
- the area of the surface 14 between the walls of a display element, indicated by the dashed lines 21 and 22, is called the display area 23, over which a display effect occurs, to be observed from the viewing side 8.
- the first fluid 11 forms a layer over the extent of the display area 23 and the display element is in a closed state.
- the first fluid will contract, the contraction being stronger for higher voltages.
- the display is now in an open state.
- a fully contracted first fluid is shown in Figure 1 by reference 24.
- the display effect depends on an extent that the first and second fluids adjoin the surface defined by the display area, in dependence on the magnitude of the applied voltage V described above.
- the magnitude of the applied voltage V therefore determines the configuration of the first and second fluids within the electrowetting cell.
- FIG. 2 shows schematically a first example electrowetting display apparatus 201.
- the display apparatus includes a display driving system 202 and a display device 203. Data to be displayed is input via an input line 204 to the display driving system.
- the display driving system processes the data and outputs signals on signals lines 218 for driving the display device 203.
- the display driving system 202 includes a display controller 206 and a display driver 207.
- the display controller includes at least one processor 208 for processing the data entered via the input line 204.
- the processor is connected to at least one memory 209 which may include computer program instructions configured to, with the at least one memory and the at least one processor, cause the display controller to perform a method according to embodiments described herein.
- a computer program product comprising a non-transitory computer-readable storage medium may be provided, the computer readable instructions being executable by a computerized device to cause the computerized device to perform a method of driving according to embodiments described herein.
- the display controller prepares the data for use in the display device.
- the output of the processor 208 is connected by line 210 to the display driver 207, which includes driver stages that transform signals to the appropriate voltages for the display device 203.
- the display driver may also change a serial signal input to it into parallel signals for controlling the voltages on electrodes of the display device 203.
- FIG 2 shows the display device 203 in planar view.
- the display device includes a plurality of electrowetting cells 211, represented by the small squares of the grid.
- the electrowetting cells 211 may have the construction of the electrowetting cell as shown schematically in Figure 1 .
- the lower support plate of the display device 203 includes electrodes 217, which may be separately controllable for each cell, as shown in Figure 1 or which may be connected for a plurality of cells such that the plurality of cells is driven simultaneously.
- Figure 2 shows hatched electrodes 217 that each cover a plurality of cells.
- An electrode 225 is electrically connected to the shared second fluid of the display device, which in turn is connected by a common signal line 219 to the display driver 207.
- a display effect can be obtained in each electrowetting cell by controlling the voltage between the electrode 225 and the electrode 217 of that cell.
- the display driver 207 and possibly the display controller 206 may be integrated in a circuit that may be mounted on one of the support plates 5, shown in Figure 1 .
- the electrowetting cells 211 and electrodes 26 in the display device 3 in Figure 1 constitute a numeric display device for displaying a number from 0 to 19.
- the numeric display device shown in Figure 1 is a simple example of a display device of the direct drive type. Many other electrode configurations are feasible, for example to show letters, symbols or images, either in black and white or colour.
- Each electrode is directly connected to a driver stage (not shown in Figure 1 ) in the display driver 7 that controls the voltage on the electrode.
- the electrodes in the electrowetting cells of the direct drive displays are connected to driver stages all the time during which the electrowetting cells show a display effect.
- the group of electrowetting cells controlled by one electrode 26 acts as a display element; the constituting electrowetting cells may be called sub-display elements.
- the voltage on each electrode 26 is permanently and simultaneously controlled by the display driver 7.
- FIG. 3 shows schematically a second example electrowetting display apparatus 31.
- the display apparatus includes a display driving system and a display device 32.
- the display driving system includes a display controller 33, a display row driver 34 and a display column driver 35.
- Data to be displayed is input via an input line 36 to the display driving system.
- the display controller includes a processor 37 for processing the data entered via the input line 36.
- the processor is connected to at least one memory 38.
- the display controller prepares the data for use in the display device.
- An output of the processor 37 is connected by line 39 to the display row driver 34, which includes row driver stages 40 that transform signals to the appropriate voltages for the display device 32.
- Row signal lines 41 connect the row driver stages to rows of the display device 32, providing a row selection signal to each row of the display device.
- Another output of the processor 37 is connected by line 42 to the display column driver 35, which includes column driver stages 43 that transform signals to the appropriate voltages for the display device 32.
- Column signal lines 44 connect the column driver stages to the columns of the display device 32, providing a column signal to each column of the display device.
- the display drivers may comprise a distributor, not shown in Figure 3 , for distributing data input to the display driver over a plurality of outputs connected to the driver stages.
- the distributor may be a shift register.
- Figure 3 shows the signal lines only for those columns and rows of the display device that are shown in the Figure.
- the row drivers may be integrated in a single integrated circuit.
- the column drivers may be integrated in a single integrated circuit.
- the integrated circuit may include the complete driver assembly.
- the integrated circuit may be integrated on the support plate 5 or 6 of the display device.
- the integrated circuit may include the entire display driving system.
- the display device 32 comprises a plurality of display elements arranged in a matrix.
- Figure 3 shows display elements for five rows, labelled k to k+4 and four columns labelled 1 to 1+3.
- the total number of rows and columns for common display devices may range between a few hundred and a few thousand.
- the display elements, also called pixels, of column 1 are labelled m to m+4.
- Each display element may have the same construction as the electrowetting cell 20, 21, 22 in Figure 2 .
- Each display element of the display device 32 includes an active element in the form of one or more transistors.
- the transistor may be a thin-film transistor.
- the transistor operates as a switch.
- the electrodes of the display element are indicated as a capacitor Cp having electrodes 17 and 25.
- a line connecting the electrode 25 of the capacitor to ground is the common signal line 19 and the line connecting the electrode 17 of the capacitor to the transistor is the signal line 18 shown in Figure 1 .
- the display element may include an optional capacitor Cs for storage purposes or for making the duration of the holding state or the voltage applied to the element uniform across the display device. This capacitor is arranged in parallel with Cp and is not separately shown in Figure 3 .
- the column drivers provide the signal levels corresponding to the input data for the display elements.
- the row drivers provide the signals for selecting the row of which the elements are to be set in a specific display effect. Selecting a row means putting a signal on the signal line of the row that switches a transistor of the display elements of the row to a closed state.
- the selection of rows is part of the addressing of display elements in an active matrix display device. A specific display element is addressed by applying a voltage to the column in which the specific display element is located and selecting the row in which the specific display element is located.
- the transistor of a display element When the transistor of a display element receives a pulse on its row selection signal, the transistor becomes conducting and it passes the signal level of its column driver to the electrode 17 of the electrowetting cell. After the transistor has been switched off, the voltage over the cell will be substantially maintained until the transistor is switched on again by the next row selection signal for the display element. The time during which the transistor is switched off is called the holding state of the element.
- the electrodes of the electrowetting cells are connected to the driving stages briefly at the start of a period during which they show a certain display effect. During this connection, a voltage related to the desired display effect is applied to the electrodes.
- the voltage on the electrodes is substantially maintained by one or more capacitors during the period during which the electrowetting cell shows the display effect.
- the method is called 'active', because the display element contains at least one active element, for example a transistor.
- Figure 4 shows a diagram of an example method of driving the display elements in an active matrix display device.
- the method displays images during a series of frames, for example, an image is displayed within the duration of one frame.
- all display elements of a display device may be addressed; in a matrix all rows of a matrix of a display device are addressed or selected during a frame.
- Figure 4 shows two column signals VI and Vl+1 and five row selection signals Vk ... Vk+4 as a function of time t for two consecutive frames p and p+1.
- the display elements may be controlled by a DC driving scheme, in which all voltages applied to the electrodes 17 and 25 of a display element that are indicative of a display effect shown by the display element have the same polarity over time, i.e. within a frame and in subsequent frames.
- a DC driving scheme in which all voltages applied to the electrodes 17 and 25 of a display element that are indicative of a display effect shown by the display element have the same polarity over time, i.e. within a frame and in subsequent frames.
- Such a method is called DC driving, or direct-current driving.
- Voltages applied to the electrowetting cell that are not indicative of a display effect may have the same or a different polarity as the voltages indicative of a display effect.
- An example of voltages not indicative of a display effect are voltages that are applied to the electrodes of the display cell for a very short period of time, such that the voltages does not cause a display effect that can be seen by the eye of an observer, such as the voltages used to apply a reset pulse to the display element.
- a reset pulse may be provided to avoid backflow.
- Backflow is the tendency of the first fluid in the electrowetting cell to flow back to a configuration of a closed state of the display element in spite of a voltage for an open state being applied.
- a reset pulse may for example reduce the applied voltage to zero for a sufficient duration of time to reduce backflow but still sufficiently short not to provide an observable display effect.
- Figure 5 shows an example of use of a reset pulse. It shows a diagram of the voltage Ve between electrode 29 and electrode 26 of an electrowetting cell for several consecutive frames. The frames are numbered along the horizontal axis. The first two frames show DC driving. Frame 1 does not have a reset pulse, frame 2 has a reset pulse 50 at the start of the frame.
- the reset pulse in this example is a short excursion of the applied voltage to zero.
- the application of a reset requires two addressing acts within a relatively short period.
- a frame will comprise a first subframe to address all display elements and set them to a reset voltage and a second subframe to address all display elements again and set them to the voltage for the required display effect; the first and second subframes are relatively close together because of the short duration of the reset pulse.
- Embodiments of the method described below make a selection between a DC driving scheme and an AC driving scheme.
- more drive schemes may be added out of which a selection can be made.
- Some drive schemes that can be applied simultaneously may be selected together.
- the voltages applied to the electrodes of a display element and indicative of a display effect of the display element include at least two voltages having different polarities.
- Such a scheme may change the polarity of the voltage applied to the electrodes at regular intervals. Since the movement of the first fluid within the electrowetting cell is faster for AC driving than for DC driving, the display element will show a better response to data representing large and frequent changes of the display effect when using AC driving compared with using DC driving.
- the above method that can select between DC driving and AC driving may consume less power than a method using AC driving.
- AC driving may properly display both large and frequent changes and small and slow changes of the display effect, AC driving may consume more energy than DC driving.
- the above method uses AC driving for proper response to large and frequent changes and DC driving for low power consumption during display of small and slow changes.
- An AC driving scheme can be implemented as shown in frames 3, 4 and 5 in Figure 5 , where the polarity of the voltage applied changes for each subsequent frame.
- Another embodiment of AC driving is shown in frames 6 to 8 of Figure 5 .
- a frame is divided in two subframes, indicated by suffixes a and b; the same voltage level is applied to the electrowetting cell in both subframes, however the polarity changes between the subframes.
- the display effect will be substantially the same in both subframes, because the display effect is hardly affected by the polarity of the applied voltage.
- the number of subframes per frame can be increased to 3, 4 or more.
- Frames 9 and 10 show a transition of a DC driving scheme to an AC driving scheme.
- the driving scheme uses two subframes per frame.
- the display element is DC driven in frame 9 by applying the same voltage, indicative of a display effect, having the same polarity in both subframes 9a and 9b.
- the display control switches from DC driving to AC driving.
- subframe 10a a voltage level having a certain polarity is applied to the display element; in subframe 10b the same voltage level is applied having the opposite polarity.
- the change of polarity may also occur on the transition from one frame to the next; for example, the polarity may change at the transition from subframe 9b to 10a, making the voltage in subframe 10a have a negative polarity.
- the subframes may be used for displaying interframe data.
- the display controller can form intermediate data or interframe data by interpolating between subsequent frames.
- the interframe data can be set on the display elements during a subframe.
- the subframes within a frame need not have the same level of applied voltage anymore.
- the AC driving scheme can be implemented in the direct drive method as shown in Figure 2 by causing the display driver 207 to change the polarity of the voltage on the signal lines 218 when moving to the next frame or subframe.
- the implementation in the active matrix method can be made by changing the polarity of the column signals VI and Vl+1 in frame p+1 of Figure 4 .
- the voltage applied to the display elements M to M+4 will change polarity between frame p and p+1.
- each frame can be divided into two or more subframes, each subframe applying the same voltage level and a polarity inverted with respect to that of the previous subframe.
- the voltages are no longer applied with alternating polarity but the voltages are applied with the same polarity.
- the voltages When switching from DC driving to AC driving, the voltages will no longer be applied with the same polarity but at least two voltages are applied with changing polarities.
- the selection of a driving scheme for the display element is made in dependence on a characteristic of the data representing the display effect for display by the display element.
- the characteristic of the data used for the selection of the driving scheme may be the frame rate of driving a display element or the rate at which new data for display on a display element is input on the input line of the display apparatus.
- the frame rate is indicative of a rate of consecutively addressing a display element to change a display effect of the display element.
- Figure 6 shows schematically an example of a so-called DC-AC driving method.
- the DC driving scheme will be used when the frame rate is smaller than a predefined frame rate and the AC driving scheme will be used when the frame rate is larger than or equal to the predefined frame rate.
- the predefined frame rate may for example be 20 Hz.
- the selection of the driving scheme may be made in the display controller 206 or 33. It is also possible that the user of the display apparatus can activate a control of the display apparatus and manually set the driving scheme to DC driving for displaying static content, such as pages of a book, and to AC driving for displaying dynamic content, such as a video.
- the characteristic of the data used for the selection of the driving scheme may be a difference value, representing a change in display effect between subsequent display effects of a display element.
- the display controller receives data representing a first display effect and data representing a second display effect for display by a display element.
- the display controller compares data representing the first display effect and data representing the second display effect and determines a difference value, which is indicative of a change of the display effect of the display element.
- the selection of the driving method may be based on this difference value. For example, when the changes in display effect are large, the AC driving scheme may be selected and when the changes in display effect are small, the DC driving scheme may be selected.
- the change in display effect can be determined for all display elements between two subsequent frames.
- Two frames may be considered subsequent if one frame follows the other frame and any number of frames, including zero, may be between them.
- two frames may be considered consecutive if one frame follows the other frame immediately, without other frames between them.
- Data of a first frame representing display effects is stored in the memory 38 of the display controller 33. This data is compared with data of a second, subsequent frame representing display effects, in the processor 37 of the display controller.
- the difference between the display effects in the two frames can be used as the characteristic used for selecting AC or DC driving. In an example, the selection should be carried out such that AC driving is not used when only a few display elements show a large difference in display effect when most of the display elements change display effect by a small amount.
- the difference value indicative of a change in display effects between two frames can be determined in various ways.
- the difference value may be determined on the basis of the magnitude of the changes in the display effect between the first and second frame or on relative changes in the display effect between the first and second frame.
- the display state may be any one of 256 display effects in the form of grey levels, for example display effect intensity levels, numbered from 1 to 256, 1 being a closed state and 256 being a fully open state of the display element.
- the processor 37 may calculate the magnitude of the change in display effect between two frames for all display elements in a frame.
- a predefined difference value can be that the display effect changes by more than a predefined change for a predefined number of display elements of the display device.
- a change from DC to AC driving may be made if the difference value is larger than the predefined difference value, for example 65% of the display elements of the display device change grey level by 60 or more between two frames.
- a change from AC to DC driving may be made if the difference value is less than the predefined difference value for two frames, or, in this example, the number of display element that change grey level by 60 or more is less than half of the display elements.
- the quantity for selection may also be the number of display elements of which the grey state changes by more than a certain relative amount, for example 40%.
- a predefined difference value can be that the display effect changes by more than 40% for half of the display elements. If the difference value between the frames is that for example 60% of the display elements change their display effect by more than 40%, a change may be made by choosing an AC driving scheme.
- FIG. 7 shows a histogram of the magnitude of the changes in display effect between two frames.
- the parameter ni along the vertical axis is the number of display elements of which the display effect changes in a certain range i.
- the horizontal axis shows five ranges of increasing change of display effect.
- the actual histogram of changes may be compared with a predefined histogram for determining a difference value to base the selection of driving scheme on.
- the histogram may also be used to derive statistical parameters, such as average and spread, on which the selection may be based.
- the difference value indicative of a change in display effects between two frames may also be expressed in the form of a motion estimate, such as a motion vector.
- a motion vector represents a change of position of an object in an image between subsequent frames of the image.
- a motion estimate can be determined by a method such as the block-matching algorithm, phase correlation and frequency domain methods, pixel recursive algorithms and optical flow; these techniques are known. Such a method usually forms a field of motion vectors within the image.
- the difference value between frames can be expressed as for example the length of the largest motion vector or the average length of the motion vectors in the image.
- the DC driving method When the difference value in terms of a motion estimate is below a certain predefined value, the DC driving method may be selected; when the difference value is larger than the predefined value, the AC driving method may be selected.
- the above described embodiments of the first class may apply the same driving scheme to all display elements of the display device. It is also possible to drive different parts of the display device with different driving schemes. For example, if part of the image displayed is static and part is dynamic, the dynamic part can be driven using the AC driving scheme and the static part using the DC driving scheme.
- the dynamic part of the image corresponds to certain rows and certain columns. When these rows are selected, the voltage applied to these columns should apply an alternating voltage, for example as shown in frames 6 to 8 of Figure 5 .
- the display elements located in the static part of the image are driven as shown in frames 1 and 2 of Figure 5 .
- the display column driver 35 When driving an active matrix display device such as shown in Figure 3 , the display column driver 35 provides voltages on each column signal line 44.
- Figure 4 shows by way of example the varying voltages VI and Vl+1 for columns 1 and 1+1 for display elements on a few consecutive rows.
- the driver stage 43 for that column must output a high-frequency signal having a high voltage. This may occur when a checkerboard pattern or a pattern with dark and light squares or squares with different colours, is displayed. It is desirable to reduce the relatively high power consumption.
- an electrowetting display device may have a plurality of display elements arranged in an active matrix having rows and columns, a specific display element being addressed by applying a voltage to the display elements along the column of the specific display element and selecting the row of the specific display element.
- the method of driving the electrowetting display device may comprise: determining a first group of rows for which voltages to be applied to display elements in a predefined column or group of columns are within a first range, an extent of the first range being smaller than an extent of a range over which the voltages are controllable; and selecting the rows in the first group consecutively.
- FIG. 8 shows schematically stages of a so-called row-interleave driving method. Stages 61 and 62 show the method in the previous paragraph. During the selection of the rows within the first group, the voltage on the column signal line varies only within the first range. Since the number of transitions between high and low voltage are reduced, the power consumption of the column driver 35 is reduced.
- Figure 9a shows a diagram of voltages versus time within one frame for an embodiment of the row-interleaved driving method.
- the first stage of the method requires determining a first group of rows where the voltages to be applied in a column are within a first, predefined range.
- display elements m, m+2 and m+4 in column l require a voltage falling within a first range 80.
- the extent of the first range is smaller than the extent of the voltage range from Vmin till Vmax over which the applied voltage can be controlled. Common values for Vmin and Vmax are 0 and 30 V, respectively.
- the first range in this example extends from 0 to 5 V and is at the end of the voltage range; that is, an end of the first range coincides with an end of the voltage range.
- the first range may also extend from 0 to 15 V. However, the first range may be located anywhere in the voltage range from Vmin till Vmax.
- the display elements m, m+2 and m+4 are situated on rows k, k+2 and k+4 (see Figure 3 ). These rows therefore belong to the first group.
- the rows of the first group are selected consecutively. This means that rows not belonging to the group are selected before and/or after selection of the rows of the first group.
- Figure 8a shows the selection of the rows of the first group by the consecutive pulses for rows k, k+2 and k+4. In this example the other rows, i.e. k+1 and k+3, are selected after the last row (k+4) of the first group has been selected.
- the voltage on the column signal line 44 of column l varies within the bounds of the first range during selection of the rows in the first group and the power consumption of the column driver 35 will be relatively low.
- the method is also called the row-interleaved driving method, where the interleaved refers to the changes in timing of selecting the rows.
- Figure 9b shows the voltage on the column signal line 44 of column l if the row-interleaved driving method is not used and the rows are selected in the order in which they are arranged in the matrix. The same voltages are applied to the five display elements m to m+4 in Figure 9a and 9b . It is apparent that the method of Figure 9b requires substantially more large changes in voltage than the method of Figure 9a . Hence, the power consumption of the column driver 35 is smaller for the method of Figure 9a than for the method of Figure 9b .
- Figure 9c shows another embodiment of the row-interleaved driving method, similar to the embodiment of Figure 9a , but wherein a second, predefined range 81 is used.
- the second range is at the upper end of the voltage range over which the applied voltage is controllable; however, it may be located anywhere in the voltage range from Vmin till Vmax.
- the first stage of the method requires determining a first group of rows where the voltages to be applied in a column are within the first range and determining a second group of rows where the voltages to be applied in that column are within the second range.
- display elements m and m+2 in column 1 require a voltage falling within the first range 80 and display elements m+1 and m+3 in column 1 require a voltage falling within the second range 81.
- the extent of the second range is smaller than the extent of the voltage range from Vmin till Vmax.
- the second range in this example extends from 25 to 30 V. Hence, rows k and k+2 belong to the first group and rows k+1 and k+3 belong to the second group.
- the rows k and k+2 of the first group are selected first, next a row k+4 not belonging to either the first or second group is selected and subsequently the rows k+3 and k+1 are selected.
- the rows within each group are selected consecutively.
- the order of the selection within a group may be changed and the order of the groups and other rows may also be changed.
- the rows may be selected in order of increasing or decreasing applied voltage.
- the rows may also be selected in the order in which they are arranged in the matrix.
- the voltage Vl applied to the column signal line 44 as shown in Figure 9c has fewer large changes than for a non-row-interleaved method.
- the stages of determining the rows belonging to the second group and selecting these rows are shown in Figure 8 as stages 63 and 64, respectively.
- the method of determining which rows fall in a first group may be applied for a certain column.
- the method may also be amended by determining the average of the voltages to be applied to the display elements in each row.
- the display elements over which the averaging in a row is made are arranged in a group of columns. This group may include all columns of the display device or a selection of the columns.
- the rows having an average of the voltages within a first range belong to the first group. Other methods of determining the rows belonging to a group are possible.
- the display elements may be divided into sub-display elements, each sub-display element designed for displaying a particular colour, for example red, green, blue and white (RGBW).
- the sub-display elements of a display element may be divided over different rows.
- Two exemplary layouts are shown in Figures 10a and 10b.
- Figure 10a shows two display elements 90, each having four sub-display elements distributed over two rows and two columns.
- the display element 91 in Figure 10b has three sub-display elements distributed over three adjacent rows. If the display device having the layout of Figure 10a displays a uniform magenta colour, e.g.
- the R and B sub-display elements will be switched on and the G and W sub-display elements switched off.
- the voltage for columns l and l+1 will be castellated when the rows are scanned in the order in which they are arranged in the matrix.
- the row-interleaved driving method will provide a substantially smoother voltage and, hence, lower power consumption of the display column driver 35.
- An electrowetting display device can be driven by an analog driving scheme.
- an analog driving scheme a voltage is applied to a display element that is indicative of the display effect.
- Examples of analog driving are AC driving and DC driving.
- a pulse width modulation (pwm) scheme may use a first voltage during a first period and a second voltage during a second period for driving the display element.
- the first period may be before the second period and the second period may be before the first period.
- the first voltage is higher than the second voltage.
- the durations of the first period and the second period determine the display effect observed.
- the first voltage may be equal to Vmax and the second voltage equal to Vmin, where Vmax and Vmin are a maximum and a minimum voltage of a voltage range over which the applied voltage can be controlled. Since the movement of the first fluid within the electrowetting cell is faster when driving with large changes in applied voltage, the display element has an improved response to data representing large and frequent changes of the display effect when using pulse width modulation driving compared with using analog driving.
- analog driving requires less power than pwm driving
- a method using an analog driving scheme or a pwm driving scheme depending on the input data requires less power than driving with a pwm driving scheme for all input data.
- Figure 11 shows a diagram of the analog-pwm driving method. It shows the voltage Ve applied to the electrodes of a display element as a function of time for three frames f1 ... f3 Each frame has three subframes, which are labelled fla, f1b, f1c for frame f1. The durations of the three subframes are in the ratio 1:2:4. The display elements of the display device are addressed three times during each frame, once for each subframe.
- Frame f1 shows an analog driving scheme, in which the voltage Ve is applied to the display element for the duration of the frame. At the start of each subframe in frame f1 the same voltage is applied to the display element.
- This is an example of DC driving, where all voltages applied to the display element indicative of a display effect have a same polarity.
- the subframes within a frame have equal length, which may be used for an analog driving scheme and a pwm driving scheme.
- a different applied voltage causes a different configuration of the fluids in the display element and, hence, a different display effect.
- the number of different display effects or grey levels that can be obtained by analog driving depends on the number of voltage levels that can be output by the drivers.
- a driver may for example be able to output 64 voltage levels, corresponding to 6 bit depth.
- AC driving may also be used, alternating the polarity of the applied voltage, for example between frames or between subframes.
- Frames f2 and f3 show a pulse width modulation (pwm) driving scheme.
- the applied voltage is low, and the first fluid in the display element will be covering the entire display area of the display element and the display element will be in an off or closed state.
- the applied voltage is high, causing the first fluid to contract and showing an open state of the display element.
- the voltages applied in the pwm scheme are not indicative of the display effect. The display effect depends on the period the first voltage is applied and the period the second voltage is applied.
- frame f2 The display effect in frame f2 is closed during 5/7 of the frame period and open during 2/7 of the frame period. Since the duration of the frame is relatively short, an eye of an observer will average the impression of both states. Hence, frame f2 will show grey level of 3 on the scale of 1 (closed) to 8 (open). Frame f3 shows a grey level of 6. The three subframes can display 8 grey levels, or a 3 bit depth.
- the voltage changes in Ve of Figure 11 can be implemented in a display apparatus of the direct-drive type by programming the display controller 206 and the display driver 207 in Figure 2 such that the required voltages are set on the signal lines 5, which are connected to the electrodes of the display elements, the timing corresponding to the subframes in each frame.
- the implementation in a display apparatus of the active matrix type provides for addressing of the display elements during the subframes in each frame.
- the choice of analog or pwm driving can be realized by an appropriate choice of voltages on the column signal lines for the subframes.
- the change from analog driving to pulse-width modulation driving and back is made in dependence on a characteristic of the data representing a display effect, input into the display apparatus.
- the characteristic of the data is similar to the characteristic of embodiments in the first class described above.
- the characteristic of the data may for example be a frame rate or a difference value.
- the characteristic may be obtained in the same manner as described above for embodiments of the first class and have similar selection criteria.
- the determination of the characteristic for a display device having a plurality of display elements may be performed as described above for the first class.
- An electrowetting display device can be driven by a DC driving scheme.
- the data input in the display apparatus requires large and frequent changes of the display effect of display element, the quality of an image displayed is reduced. It is desirable to improve the quality of the images displayed.
- the embodiments described below improve the quality of the images displayed.
- an electrowetting display device has at least one display element for displaying a display effect during a display period.
- the method may comprise for example receiving data representing a first display effect for display by the at least one display element; and receiving data representing a second display effect for display by the at least one display element subsequent to display of the first display effect.
- Data representing the first display effect and data representing the second display effect may be compared to determine a difference value indicative of a change of the display effect.
- a driving scheme for the at least one display element can be selected in dependence on the difference value. The selection can be made from at least a first driving scheme and a second driving scheme. In the first driving scheme a voltage indicative of a display effect is applied to the at least one display element for a first number of times during the display period.
- a voltage indicative of a display effect is applied to the at least one display element for a second number of times during the display period.
- the second number is different from the first number.
- the first number may be larger than the second number and the second number may be larger than the first number.
- the method applies a number of times the voltage indicative of the display effect to the display element or to the electrodes of the display element, where the value of the number depends on the difference value.
- the number may have any integer value larger than zero.
- the method can be configured such that an increasing step in display effect causes a larger number of applications of a voltage to the display element, so the required charge and voltage are attained on the electrodes of the display element.
- Figure 12 shows a diagram of the multiple data write driving method.
- the voltage Ve applied to the electrodes of a display element is shown as a function of time for six frames f10 ... f15.
- a display effect of the display device is displayed during a display period 100, which in this example is the same as the duration or length of a frame.
- the change in voltage between frames f10 and f11 is indicative of a display effect of the display element.
- the change in applied voltage between frames f10 and f11 is indicative of a change in display effect.
- the magnitude of the change in display effect can be taken as the difference value between frames f10 and f11.
- the difference value between frames f10 and f11 is smaller than a predefined value and, hence, during frame f11 the display element is driven using the first driving scheme with the first number equal to one.
- the change in display effect has a different sign than in the transition from frame f10 to f11.
- These changes can be treated similarly if the difference value is the magnitude of the change of the display effect, making the difference value independent of the sign of the change.
- the sign can be taken into account, for example by taking into account only positive changes of the display effect.
- the transition from frame f12 to f13 shows a relatively large change in display effect.
- the difference value is now larger than the predefined value and the second driving scheme is selected for frame f13.
- the value of the second number in this example is two, causing two applications of the voltage during the display period.
- the first application 101 is at the start of the display period
- the second application 102 is after the first application but within the display period.
- the second and further applications of a voltage are effective when they are grouped near the start of the display period, as shown in Figure 12 .
- the still larger change in display state at the transition from frame f14 to frame f15 causes three applications 103, 104, 105 of the voltage to the display element.
- the method can be implemented in a display apparatus of the active matrix type by using subframes for each second or further application of the voltage. For example, in frame f13 the display elements of the display device are addressed and the appropriate voltages applied to the column signal lines. Shortly after this addressing action, a subframe causes as next addressing action, applying the same voltages to the column signal lines.
- the change from applying the voltage to a display element once or more times in a display period is made in dependence on a difference value, characteristic of the data representing a display effect and which is input into the display apparatus.
- the difference value is similar to the difference value of embodiments in the first class described above.
- the difference value may for example be a change of the display effect or a number of display elements of the display device that have a change in display state larger than a predefined change.
- the predefined change is for example 60 on a grey scale from 1 to 256.
- the difference value may be obtained in the same manner as described above for embodiments of the first class and have similar selection criteria.
- the determination of the characteristic for a display device having a plurality of display elements may be performed as described above for the first class.
- the above embodiments are to be understood as illustrative examples. Further variations not part of the invention may be envisaged.
- the variations may include a driving scheme having a higher frame rate, which may be selected, either separately or in combination with one of the above-mentioned driving schemes.
Description
- Electrowetting display apparatuses having a display controller and a display device are known. The display elements of such a display device include two immiscible fluids. The configuration of the fluids can be controlled by applying a voltage to the display element, thereby forming a display effect. When data is input to the display controller, the display elements can be controlled to display the data, for example video images. European
patent publication number 2 476 489 A1 describes an active matrix electrowetting on dielectric device having an AC drive scheme. International patent publication number2013/092646 A1 describes a method of driving an electrowetting display device including a pixel including applying a first pixel voltage to the pixel during a first portion of a display period and applying a second pixel voltage to the pixel during a second portion of the display period, the first and second pixel voltages having different polarities.US patent publication number 2011/032276 A1 describes a method of driving a display device including providing a reference signal with a fixed voltage level to a storage capacitor and providing a threshold signal to a pixel element with different voltage level than the fixed voltage level. International patent publication number2011/051413 A1 describes a method of driving a display element of an electrowetting display device including applying a voltage with a signal level configured throughout a display period such that a fluid of the display element adjoins at least a non-zero minimum area of a surface of a support plate. - It is desirable to reduce the power consumption of the display apparatus.
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FIG. 1 shows schematically an example display device; -
FIG. 2 shows schematically a cross-section of an example display apparatus; -
FIG. 3 shows schematically an example display apparatus; -
Fig. 4 shows a diagram of an active matrix driving method; -
Fig. 5 shows a voltage diagram of a DC-AC driving method; -
Fig. 6 shows schematically an embodiment of the method; -
Fig. 7 shows a histogram of change in display effect between two frames; -
Fig. 8 shows schematically stages of a row-interleave driving method; -
Figs. 9a and9c show diagrams of a row-interleaved driving method; -
Fig. 9b shows a diagram of a non-row-interleaved driving method; -
Figs. 10a and 10b show a layout of sub-display elements for colour rendering; -
Fig. 11 shows a diagram of an analog-pulse-width modulation driving method; and -
Fig. 12 shows a diagram of a multiple data write driving method. - The following detailed description will first describe general driving methods, in which concepts common to various embodiments will be presented. The following detailed embodiments are grouped into four classes, each group setting out features of embodiments for a class. The claims relate to embodiments of the first class and the invention is as set forth in the claims; the other classes are provided as comparative examples useful to understand the invention but not forming part of the same.
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FIG. 1 shows a diagrammatic cross-section of part of an example of an electrowetting device. In this example the device is anelectrowetting display device 1 including a plurality of electrowetting cells, which aredisplay elements 2, one of which is shown in the Figure. The lateral extent of the display element is indicated in the Figure by twodashed lines first support plate 5 and asecond support plate 6. The support plates may be separate parts of each display element, but the support plates may be shared in common by the plurality of display elements. The support plates may include a glass orpolymer substrate 6, 7 and may be rigid or flexible. - The display device has a viewing side 8 on which an image or display formed by the display device can be viewed and a
rear side 9. In the Figure thefirst support plate 5 defines therear side 9 and thesecond support plate 6 defines the viewing side; alternatively, the first support plate may define the viewing side. The display device may be of the reflective, transmissive or transflective type. The display device may be of a segmented display type in which the image may be built up of segments, each segment including several display elements. The display device may be an active matrix driven display device, a direct drive display device or a passively driven display device. The plurality of display elements may be monochrome. For a colour display device the display elements may be divided in groups, each group having a different colour; alternatively, an individual display element may be able to show different colours. - A
space 10 between the support plates is filled with two fluids: afirst fluid 11 and asecond fluid 12 at least one of which may be a liquid. The second fluid is immiscible with the first fluid. The second fluid is electrically conductive or polar and may be water or a salt solution such as a solution of potassium chloride in water. The second fluid may be transparent, but may instead be coloured, white, absorbing or reflecting. The first fluid is electrically non-conductive and may for instance be an alkane like hexadecane or may be an oil such as silicone oil. - The first fluid absorbs at least a part of the optical spectrum. The first fluid may be transmissive for a part of the optical spectrum, forming a colour filter. For this purpose the first fluid may be coloured by addition of pigment particles or a dye. Alternatively, the first fluid may be black, i.e. absorb substantially all parts of the optical spectrum, or reflecting. A reflective first fluid may reflect the entire visible spectrum, making the layer appear white, or part of it, making it have a colour.
- The
support plate 5 includes aninsulating layer 13. The insulating layer may be transparent or reflective. Theinsulating layer 13 may extend between walls of a display element. To avoid short circuits between thesecond fluid 12 and electrodes arranged under the insulating layer, layers of the insulating layer may extend uninterrupted over a plurality ofdisplay elements 2, as shown in the Figure. The insulating layer has asurface 14 facing thespace 10 of thedisplay element 2. In this example thesurface 14 is hydrophobic. The thickness of the insulating layer may be less than 2 micrometres and may be less than 1 micrometre. - The insulating layer may be a hydrophobic layer; alternatively, it may include a
hydrophobic layer 15 and abarrier layer 16 with predetermined dielectric properties, thehydrophobic layer 15 facing thespace 10, as shown in the Figure. The hydrophobic layer is schematically illustrated inFigure 1 and may be formed of TeflonĀ®AF1600. Thebarrier layer 16 may have a thickness, taken in a direction perpendicular the plane of the substrate, between 100 nanometres and 150 nanometres and may be made of an inorganic material like silicon oxide or silicon nitride or a stack of these (for example, silicon oxide - silicon nitride - silicon oxide) or an organic material like polyimide or parylene. The barrier layer may comprise multiple layers having different dielectric constants. - The hydrophobic character of the
surface 14 causes thefirst fluid 11 to adhere preferentially to the insulatinglayer 13, since the first fluid has a higher wettability with respect to the surface of the insulatinglayer 13 than thesecond fluid 12. Wettability relates to the relative affinity of a fluid for the surface of a solid. Wettability may be measured by the contact angle between the fluid and the surface of the solid. The contact angle is determined by the difference in surface tension between the fluid and the solid at the fluid-solid boundary. For example, a high difference in surface tension can indicate hydrophobic properties. - Each
element 2 includes anelectrode 17 as part of thesupport plate 5. In examples shown there is onesuch electrode 17 per element. Theelectrode 17 is separated from the fluids by the insulatinglayer 13; electrodes of neighbouring display elements are separated by a non-conducting layer. In some examples, further layers may be arranged between the insulatinglayer 13 and theelectrode 17. Theelectrode 17 can be of any desired shape or form. Theelectrode 17 of a display element is supplied with voltage signals by asignal line 18, schematically indicated in the Figure. Asecond signal line 19 is connected to anelectrode 25 that is in contact with the conductivesecond fluid 12. This electrode may be common to all elements, when they are fluidly interconnected by and share the second fluid, uninterrupted by walls. Thedisplay element 2 can be controlled by a voltage V applied between thesignal lines electrodes 17 on the substrate 7 are coupled to a display control apparatus. In a display device having the display elements arranged in a matrix form, that is arranged in rows and columns, the electrodes can be coupled to a matrix of control lines on the substrate 7. - The
first fluid 11 in this example is confined to one display element bywalls 20 that follow the cross-section of the display element. The cross-section of a display element may have any shape; when the display elements are arranged in a matrix form, the cross-section is usually square or rectangular. Although the walls are shown as structures protruding from the insulatinglayer 13, they may instead be a surface layer of the support plate that repels the first fluid, such as a hydrophilic or less hydrophobic layer. The walls may extend from the first to the second support plate but may instead extend partly from the first support plate to the second support plate as shown inFIG. 1 . The extent of the display element, indicated by the dashedlines walls 20. The area of thesurface 14 between the walls of a display element, indicated by the dashedlines display area 23, over which a display effect occurs, to be observed from the viewing side 8. - When no voltage is applied, the
first fluid 11 forms a layer over the extent of thedisplay area 23 and the display element is in a closed state. When a voltage is applied to theelectrodes Figure 1 by reference 24. - The display effect depends on an extent that the first and second fluids adjoin the surface defined by the display area, in dependence on the magnitude of the applied voltage V described above. The magnitude of the applied voltage V therefore determines the configuration of the first and second fluids within the electrowetting cell. When switching the electrowetting cell from one fluid configuration to a different fluid configuration the extent of second fluid adjoining the display area surface may increase or decrease, with the extent of first fluid adjoining the display area surface decreasing or increasing, respectively.
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Figure 2 shows schematically a first example electrowetting display apparatus 201. In this example of a so-called direct drive type, the display apparatus includes adisplay driving system 202 and adisplay device 203. Data to be displayed is input via aninput line 204 to the display driving system. The display driving system processes the data and outputs signals onsignals lines 218 for driving thedisplay device 203. Thedisplay driving system 202 includes adisplay controller 206 and adisplay driver 207. The display controller includes at least oneprocessor 208 for processing the data entered via theinput line 204. The processor is connected to at least onememory 209 which may include computer program instructions configured to, with the at least one memory and the at least one processor, cause the display controller to perform a method according to embodiments described herein. Further, a computer program product comprising a non-transitory computer-readable storage medium may be provided, the computer readable instructions being executable by a computerized device to cause the computerized device to perform a method of driving according to embodiments described herein. - The display controller prepares the data for use in the display device. The output of the
processor 208 is connected byline 210 to thedisplay driver 207, which includes driver stages that transform signals to the appropriate voltages for thedisplay device 203. The display driver may also change a serial signal input to it into parallel signals for controlling the voltages on electrodes of thedisplay device 203. -
Figure 2 shows thedisplay device 203 in planar view. The display device includes a plurality ofelectrowetting cells 211, represented by the small squares of the grid. Theelectrowetting cells 211 may have the construction of the electrowetting cell as shown schematically inFigure 1 . The lower support plate of thedisplay device 203 includeselectrodes 217, which may be separately controllable for each cell, as shown inFigure 1 or which may be connected for a plurality of cells such that the plurality of cells is driven simultaneously.Figure 2 shows hatchedelectrodes 217 that each cover a plurality of cells. Anelectrode 225 is electrically connected to the shared second fluid of the display device, which in turn is connected by acommon signal line 219 to thedisplay driver 207. A display effect can be obtained in each electrowetting cell by controlling the voltage between theelectrode 225 and theelectrode 217 of that cell. - The
display driver 207 and possibly thedisplay controller 206 may be integrated in a circuit that may be mounted on one of thesupport plates 5, shown inFigure 1 . Theelectrowetting cells 211 and electrodes 26 in thedisplay device 3 inFigure 1 constitute a numeric display device for displaying a number from 0 to 19. The numeric display device shown inFigure 1 is a simple example of a display device of the direct drive type. Many other electrode configurations are feasible, for example to show letters, symbols or images, either in black and white or colour. Each electrode is directly connected to a driver stage (not shown inFigure 1 ) in the display driver 7 that controls the voltage on the electrode. The electrodes in the electrowetting cells of the direct drive displays are connected to driver stages all the time during which the electrowetting cells show a display effect. The group of electrowetting cells controlled by one electrode 26 acts as a display element; the constituting electrowetting cells may be called sub-display elements. During the display of a display effect, such as for providing a static or dynamic image, the voltage on each electrode 26 is permanently and simultaneously controlled by the display driver 7. -
Figure 3 shows schematically a second exampleelectrowetting display apparatus 31. In this example of a so-called active matrix drive type the display apparatus includes a display driving system and adisplay device 32. The display driving system includes adisplay controller 33, adisplay row driver 34 and adisplay column driver 35. Data to be displayed is input via aninput line 36 to the display driving system. The display controller includes aprocessor 37 for processing the data entered via theinput line 36. The processor is connected to at least onememory 38. The display controller prepares the data for use in the display device. - An output of the
processor 37 is connected byline 39 to thedisplay row driver 34, which includes row driver stages 40 that transform signals to the appropriate voltages for thedisplay device 32.Row signal lines 41 connect the row driver stages to rows of thedisplay device 32, providing a row selection signal to each row of the display device. - Another output of the
processor 37 is connected byline 42 to thedisplay column driver 35, which includes column driver stages 43 that transform signals to the appropriate voltages for thedisplay device 32.Column signal lines 44 connect the column driver stages to the columns of thedisplay device 32, providing a column signal to each column of the display device. - The display drivers may comprise a distributor, not shown in
Figure 3 , for distributing data input to the display driver over a plurality of outputs connected to the driver stages. The distributor may be a shift register.Figure 3 shows the signal lines only for those columns and rows of the display device that are shown in the Figure. The row drivers may be integrated in a single integrated circuit. Similarly, the column drivers may be integrated in a single integrated circuit. The integrated circuit may include the complete driver assembly. The integrated circuit may be integrated on thesupport plate - The
display device 32 comprises a plurality of display elements arranged in a matrix.Figure 3 shows display elements for five rows, labelled k to k+4 and four columns labelled 1 to 1+3. The total number of rows and columns for common display devices may range between a few hundred and a few thousand. The display elements, also called pixels, ofcolumn 1 are labelled m to m+4. Each display element may have the same construction as theelectrowetting cell Figure 2 . - Each display element of the
display device 32 includes an active element in the form of one or more transistors. The transistor may be a thin-film transistor. The transistor operates as a switch. The electrodes of the display element are indicated as a capacitorCp having electrodes electrode 25 of the capacitor to ground is thecommon signal line 19 and the line connecting theelectrode 17 of the capacitor to the transistor is thesignal line 18 shown inFigure 1 . The display element may include an optional capacitor Cs for storage purposes or for making the duration of the holding state or the voltage applied to the element uniform across the display device. This capacitor is arranged in parallel with Cp and is not separately shown inFigure 3 . The column drivers provide the signal levels corresponding to the input data for the display elements. The row drivers provide the signals for selecting the row of which the elements are to be set in a specific display effect. Selecting a row means putting a signal on the signal line of the row that switches a transistor of the display elements of the row to a closed state. The selection of rows is part of the addressing of display elements in an active matrix display device. A specific display element is addressed by applying a voltage to the column in which the specific display element is located and selecting the row in which the specific display element is located. - When the transistor of a display element receives a pulse on its row selection signal, the transistor becomes conducting and it passes the signal level of its column driver to the
electrode 17 of the electrowetting cell. After the transistor has been switched off, the voltage over the cell will be substantially maintained until the transistor is switched on again by the next row selection signal for the display element. The time during which the transistor is switched off is called the holding state of the element. In this active matrix driving method the electrodes of the electrowetting cells are connected to the driving stages briefly at the start of a period during which they show a certain display effect. During this connection, a voltage related to the desired display effect is applied to the electrodes. After an electrowetting cell is disconnected from the driver stage, the voltage on the electrodes is substantially maintained by one or more capacitors during the period during which the electrowetting cell shows the display effect. The method is called 'active', because the display element contains at least one active element, for example a transistor. -
Figure 4 shows a diagram of an example method of driving the display elements in an active matrix display device. The method displays images during a series of frames, for example, an image is displayed within the duration of one frame. During a frame all display elements of a display device may be addressed; in a matrix all rows of a matrix of a display device are addressed or selected during a frame.Figure 4 shows two column signals VI and Vl+1 and five row selection signals Vk ... Vk+4 as a function of time t for two consecutive frames p and p+1. - When row k is selected by a pulse on row selection signal k, as shown at the start of frame p in
Figure 4 , the transistor in each display element of row k becomes conducting and the voltages on each of thecolumn signal lines 44 will be put on the electrode 26 of each display element in row k. Subsequently, thedisplay column driver 35 changes the voltages on the column signal lines to the values required forrow k+ 1. When row k+1 is selected by a pulse on row selection signal k+1, the voltages are put on the electrode 26 of the display elements ofrow k+ 1. All rows of the display device will be selected in a similar manner in frame p. The process of selecting the rows starts again in the followingframe p+ 1. - The display elements may be controlled by a DC driving scheme, in which all voltages applied to the
electrodes - A reset pulse may be provided to avoid backflow. Backflow is the tendency of the first fluid in the electrowetting cell to flow back to a configuration of a closed state of the display element in spite of a voltage for an open state being applied. A reset pulse may for example reduce the applied voltage to zero for a sufficient duration of time to reduce backflow but still sufficiently short not to provide an observable display effect.
Figure 5 shows an example of use of a reset pulse. It shows a diagram of the voltage Ve between electrode 29 and electrode 26 of an electrowetting cell for several consecutive frames. The frames are numbered along the horizontal axis. The first two frames show DC driving.Frame 1 does not have a reset pulse,frame 2 has areset pulse 50 at the start of the frame. The reset pulse in this example is a short excursion of the applied voltage to zero. The application of a reset requires two addressing acts within a relatively short period. In an active matrix method a frame will comprise a first subframe to address all display elements and set them to a reset voltage and a second subframe to address all display elements again and set them to the voltage for the required display effect; the first and second subframes are relatively close together because of the short duration of the reset pulse. - When the data input in the display apparatus requires large and frequent changes of the display effect of a DC-driven display element, frames of short duration may be required to avoid blurring. The application of a reset pulse with the extra addressing act combined with the short period frame puts a high demand on the
display column driver 35, giving rise to a high power consumption. - Embodiments of the method described below make a selection between a DC driving scheme and an AC driving scheme. However, in comparative examples not part of the invention, more drive schemes may be added out of which a selection can be made. Some drive schemes that can be applied simultaneously may be selected together.
- In an AC driving scheme the voltages applied to the electrodes of a display element and indicative of a display effect of the display element include at least two voltages having different polarities. Such a scheme may change the polarity of the voltage applied to the electrodes at regular intervals. Since the movement of the first fluid within the electrowetting cell is faster for AC driving than for DC driving, the display element will show a better response to data representing large and frequent changes of the display effect when using AC driving compared with using DC driving.
- The reversal of polarity reduces backflow in the electrowetting cell. Hence, reset pulses may be omitted when using this method of driving and the extra addressing act need not be applied. The reduced demand on the
display column driver 35 allows the use of shorter frames for improved switching of images with motion. - The above method that can select between DC driving and AC driving may consume less power than a method using AC driving. Although AC driving may properly display both large and frequent changes and small and slow changes of the display effect, AC driving may consume more energy than DC driving. The above method uses AC driving for proper response to large and frequent changes and DC driving for low power consumption during display of small and slow changes.
- An AC driving scheme can be implemented as shown in
frames Figure 5 , where the polarity of the voltage applied changes for each subsequent frame. Another embodiment of AC driving is shown inframes 6 to 8 ofFigure 5 . In this embodiment a frame is divided in two subframes, indicated by suffixes a and b; the same voltage level is applied to the electrowetting cell in both subframes, however the polarity changes between the subframes. The display effect will be substantially the same in both subframes, because the display effect is hardly affected by the polarity of the applied voltage. In further embodiments the number of subframes per frame can be increased to 3, 4 or more. -
Frames frame 9 by applying the same voltage, indicative of a display effect, having the same polarity in bothsubframes frame 9 andframe 10 the display control switches from DC driving to AC driving. Insubframe 10a a voltage level having a certain polarity is applied to the display element; insubframe 10b the same voltage level is applied having the opposite polarity. The change of polarity may also occur on the transition from one frame to the next; for example, the polarity may change at the transition fromsubframe 9b to 10a, making the voltage insubframe 10a have a negative polarity. - The subframes may be used for displaying interframe data. When the change in data is large between subsequent frames, the display controller can form intermediate data or interframe data by interpolating between subsequent frames. The interframe data can be set on the display elements during a subframe. In such an embodiment, the subframes within a frame need not have the same level of applied voltage anymore.
- The AC driving scheme can be implemented in the direct drive method as shown in
Figure 2 by causing thedisplay driver 207 to change the polarity of the voltage on thesignal lines 218 when moving to the next frame or subframe. The implementation in the active matrix method can be made by changing the polarity of the column signals VI and Vl+1 in frame p+1 ofFigure 4 . As a result, the voltage applied to the display elements M to M+4 will change polarity between frame p and p+1. In another embodiment each frame can be divided into two or more subframes, each subframe applying the same voltage level and a polarity inverted with respect to that of the previous subframe. - When the driving of the display device is to be switched from AC to DC driving, the voltages are no longer applied with alternating polarity but the voltages are applied with the same polarity. When switching from DC driving to AC driving, the voltages will no longer be applied with the same polarity but at least two voltages are applied with changing polarities.
- In examples, the selection of a driving scheme for the display element is made in dependence on a characteristic of the data representing the display effect for display by the display element.
- The characteristic of the data used for the selection of the driving scheme may be the frame rate of driving a display element or the rate at which new data for display on a display element is input on the input line of the display apparatus. The frame rate is indicative of a rate of consecutively addressing a display element to change a display effect of the display element.
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Figure 6 shows schematically an example of a so-called DC-AC driving method. Once the frame rate of the data representing the display effect to be displayed has been determined, the DC driving scheme will be used when the frame rate is smaller than a predefined frame rate and the AC driving scheme will be used when the frame rate is larger than or equal to the predefined frame rate. The predefined frame rate may for example be 20 Hz. The selection of the driving scheme may be made in thedisplay controller - The characteristic of the data used for the selection of the driving scheme may be a difference value, representing a change in display effect between subsequent display effects of a display element. In an implementation using that characteristic, the display controller receives data representing a first display effect and data representing a second display effect for display by a display element. The display controller compares data representing the first display effect and data representing the second display effect and determines a difference value, which is indicative of a change of the display effect of the display element. The selection of the driving method may be based on this difference value. For example, when the changes in display effect are large, the AC driving scheme may be selected and when the changes in display effect are small, the DC driving scheme may be selected.
- When driving a display device having a plurality of display elements for displaying images in frames, the change in display effect can be determined for all display elements between two subsequent frames. Two frames may be considered subsequent if one frame follows the other frame and any number of frames, including zero, may be between them. In contrast, two frames may be considered consecutive if one frame follows the other frame immediately, without other frames between them. Data of a first frame representing display effects is stored in the
memory 38 of thedisplay controller 33. This data is compared with data of a second, subsequent frame representing display effects, in theprocessor 37 of the display controller. The difference between the display effects in the two frames can be used as the characteristic used for selecting AC or DC driving. In an example, the selection should be carried out such that AC driving is not used when only a few display elements show a large difference in display effect when most of the display elements change display effect by a small amount. - The difference value indicative of a change in display effects between two frames can be determined in various ways.
- The difference value may be determined on the basis of the magnitude of the changes in the display effect between the first and second frame or on relative changes in the display effect between the first and second frame. For example, the display state may be any one of 256 display effects in the form of grey levels, for example display effect intensity levels, numbered from 1 to 256, 1 being a closed state and 256 being a fully open state of the display element. The
processor 37 may calculate the magnitude of the change in display effect between two frames for all display elements in a frame. A predefined difference value can be that the display effect changes by more than a predefined change for a predefined number of display elements of the display device. A change from DC to AC driving may be made if the difference value is larger than the predefined difference value, for example 65% of the display elements of the display device change grey level by 60 or more between two frames. A change from AC to DC driving may be made if the difference value is less than the predefined difference value for two frames, or, in this example, the number of display element that change grey level by 60 or more is less than half of the display elements. - The quantity for selection may also be the number of display elements of which the grey state changes by more than a certain relative amount, for example 40%. A predefined difference value can be that the display effect changes by more than 40% for half of the display elements. If the difference value between the frames is that for example 60% of the display elements change their display effect by more than 40%, a change may be made by choosing an AC driving scheme.
- Another method of determining the difference value is by comparing the display effects a first frame and a second frame for display elements of the display device and arranging the differences between the display effect of a display element in the first frame and in the second frame in a histogram.
Figure 7 shows a histogram of the magnitude of the changes in display effect between two frames. The parameter ni along the vertical axis is the number of display elements of which the display effect changes in a certain range i. The horizontal axis shows five ranges of increasing change of display effect. The actual histogram of changes may be compared with a predefined histogram for determining a difference value to base the selection of driving scheme on. The histogram may also be used to derive statistical parameters, such as average and spread, on which the selection may be based. - In comparative examples useful for understanding the claimed concepts, the difference value indicative of a change in display effects between two frames may also be expressed in the form of a motion estimate, such as a motion vector. A motion vector represents a change of position of an object in an image between subsequent frames of the image. A motion estimate can be determined by a method such as the block-matching algorithm, phase correlation and frequency domain methods, pixel recursive algorithms and optical flow; these techniques are known. Such a method usually forms a field of motion vectors within the image. The difference value between frames can be expressed as for example the length of the largest motion vector or the average length of the motion vectors in the image.
- When the difference value in terms of a motion estimate is below a certain predefined value, the DC driving method may be selected; when the difference value is larger than the predefined value, the AC driving method may be selected.
- The above described embodiments of the first class may apply the same driving scheme to all display elements of the display device. It is also possible to drive different parts of the display device with different driving schemes. For example, if part of the image displayed is static and part is dynamic, the dynamic part can be driven using the AC driving scheme and the static part using the DC driving scheme. In a display device of the active matrix type, the dynamic part of the image corresponds to certain rows and certain columns. When these rows are selected, the voltage applied to these columns should apply an alternating voltage, for example as shown in
frames 6 to 8 ofFigure 5 . The display elements located in the static part of the image are driven as shown inframes Figure 5 . - When driving an active matrix display device such as shown in
Figure 3 , thedisplay column driver 35 provides voltages on eachcolumn signal line 44.Figure 4 shows by way of example the varying voltages VI and Vl+1 forcolumns driver stage 43 for that column must output a high-frequency signal having a high voltage. This may occur when a checkerboard pattern or a pattern with dark and light squares or squares with different colours, is displayed. It is desirable to reduce the relatively high power consumption. - In accordance with examples to be described, an electrowetting display device may have a plurality of display elements arranged in an active matrix having rows and columns, a specific display element being addressed by applying a voltage to the display elements along the column of the specific display element and selecting the row of the specific display element. The method of driving the electrowetting display device may comprise: determining a first group of rows for which voltages to be applied to display elements in a predefined column or group of columns are within a first range, an extent of the first range being smaller than an extent of a range over which the voltages are controllable; and selecting the rows in the first group consecutively.
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Figure 8 shows schematically stages of a so-called row-interleave driving method.Stages column driver 35 is reduced. -
Figure 9a shows a diagram of voltages versus time within one frame for an embodiment of the row-interleaved driving method. The first stage of the method requires determining a first group of rows where the voltages to be applied in a column are within a first, predefined range. In the example ofFigure 8a , display elements m, m+2 and m+4 in column ā require a voltage falling within afirst range 80. The extent of the first range is smaller than the extent of the voltage range from Vmin till Vmax over which the applied voltage can be controlled. Common values for Vmin and Vmax are 0 and 30 V, respectively. The first range in this example extends from 0 to 5 V and is at the end of the voltage range; that is, an end of the first range coincides with an end of the voltage range. The first range may also extend from 0 to 15 V. However, the first range may be located anywhere in the voltage range from Vmin till Vmax. The display elements m, m+2 and m+4 are situated on rows k, k+2 and k+4 (seeFigure 3 ). These rows therefore belong to the first group. - In the following stage of the method the rows of the first group are selected consecutively. This means that rows not belonging to the group are selected before and/or after selection of the rows of the first group.
Figure 8a shows the selection of the rows of the first group by the consecutive pulses for rows k, k+2 and k+4. In this example the other rows, i.e. k+1 and k+3, are selected after the last row (k+4) of the first group has been selected. The voltage on thecolumn signal line 44 of column ā varies within the bounds of the first range during selection of the rows in the first group and the power consumption of thecolumn driver 35 will be relatively low. The method is also called the row-interleaved driving method, where the interleaved refers to the changes in timing of selecting the rows. -
Figure 9b shows the voltage on thecolumn signal line 44 of column ā if the row-interleaved driving method is not used and the rows are selected in the order in which they are arranged in the matrix. The same voltages are applied to the five display elements m to m+4 inFigure 9a and 9b . It is apparent that the method ofFigure 9b requires substantially more large changes in voltage than the method ofFigure 9a . Hence, the power consumption of thecolumn driver 35 is smaller for the method ofFigure 9a than for the method ofFigure 9b . -
Figure 9c shows another embodiment of the row-interleaved driving method, similar to the embodiment ofFigure 9a , but wherein a second, predefined range 81 is used. The second range is at the upper end of the voltage range over which the applied voltage is controllable; however, it may be located anywhere in the voltage range from Vmin till Vmax. The first stage of the method requires determining a first group of rows where the voltages to be applied in a column are within the first range and determining a second group of rows where the voltages to be applied in that column are within the second range. In the example ofFigure 8c , display elements m and m+2 incolumn 1 require a voltage falling within thefirst range 80 and display elements m+1 and m+3 incolumn 1 require a voltage falling within the second range 81. The extent of the second range is smaller than the extent of the voltage range from Vmin till Vmax. The second range in this example extends from 25 to 30 V. Hence, rows k and k+2 belong to the first group and rows k+1 and k+3 belong to the second group. - In the example of
Figure 9c , the rows k and k+2 of the first group are selected first, next a row k+4 not belonging to either the first or second group is selected and subsequently the rows k+3 and k+1 are selected. The rows within each group are selected consecutively. The order of the selection within a group may be changed and the order of the groups and other rows may also be changed. The rows may be selected in order of increasing or decreasing applied voltage. The rows may also be selected in the order in which they are arranged in the matrix. The voltage Vā applied to thecolumn signal line 44 as shown inFigure 9c has fewer large changes than for a non-row-interleaved method. The stages of determining the rows belonging to the second group and selecting these rows are shown inFigure 8 asstages - For a display device having a plurality of columns of display elements, the method of determining which rows fall in a first group may be applied for a certain column. The method may also be amended by determining the average of the voltages to be applied to the display elements in each row. The display elements over which the averaging in a row is made, are arranged in a group of columns. This group may include all columns of the display device or a selection of the columns. The rows having an average of the voltages within a first range belong to the first group. Other methods of determining the rows belonging to a group are possible.
- Since the data in different frames is often different, the stage of determining the rows of any group may be repeated for each frame.
In a colour display device the display elements may be divided into sub-display elements, each sub-display element designed for displaying a particular colour, for example red, green, blue and white (RGBW). The sub-display elements of a display element may be divided over different rows. Two exemplary layouts are shown inFigures 10a and 10b. Figure 10a shows twodisplay elements 90, each having four sub-display elements distributed over two rows and two columns. Thedisplay element 91 inFigure 10b has three sub-display elements distributed over three adjacent rows. If the display device having the layout ofFigure 10a displays a uniform magenta colour, e.g. as a background, the R and B sub-display elements will be switched on and the G and W sub-display elements switched off. The voltage for columns ā and ā+1 will be castellated when the rows are scanned in the order in which they are arranged in the matrix. The row-interleaved driving method will provide a substantially smoother voltage and, hence, lower power consumption of thedisplay column driver 35. - An electrowetting display device can be driven by an analog driving scheme. In an analog driving scheme a voltage is applied to a display element that is indicative of the display effect. Examples of analog driving are AC driving and DC driving. When the data input in the display apparatus requires large and frequent changes of the display effect of an analog-driven display element, the quality of an image displayed is reduced. It is desirable to improve the quality of the images displayed. The embodiments described below improve the quality of the images displayed.
- A pulse width modulation (pwm) scheme may use a first voltage during a first period and a second voltage during a second period for driving the display element. The first period may be before the second period and the second period may be before the first period. The first voltage is higher than the second voltage. The durations of the first period and the second period determine the display effect observed. The first voltage may be equal to Vmax and the second voltage equal to Vmin, where Vmax and Vmin are a maximum and a minimum voltage of a voltage range over which the applied voltage can be controlled. Since the movement of the first fluid within the electrowetting cell is faster when driving with large changes in applied voltage, the display element has an improved response to data representing large and frequent changes of the display effect when using pulse width modulation driving compared with using analog driving.
- Since analog driving requires less power than pwm driving, a method using an analog driving scheme or a pwm driving scheme depending on the input data requires less power than driving with a pwm driving scheme for all input data.
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Figure 11 shows a diagram of the analog-pwm driving method. It shows the voltage Ve applied to the electrodes of a display element as a function of time for three frames f1 ... f3 Each frame has three subframes, which are labelled fla, f1b, f1c for frame f1. The durations of the three subframes are in the ratio 1:2:4. The display elements of the display device are addressed three times during each frame, once for each subframe. - Frame f1 shows an analog driving scheme, in which the voltage Ve is applied to the display element for the duration of the frame. At the start of each subframe in frame f1 the same voltage is applied to the display element. This is an example of DC driving, where all voltages applied to the display element indicative of a display effect have a same polarity. In another example the subframes within a frame have equal length, which may be used for an analog driving scheme and a pwm driving scheme. In an analog driving scheme, a different applied voltage causes a different configuration of the fluids in the display element and, hence, a different display effect. The number of different display effects or grey levels that can be obtained by analog driving depends on the number of voltage levels that can be output by the drivers. A driver may for example be able to
output 64 voltage levels, corresponding to 6 bit depth. - Instead of using DC driving, AC driving may also be used, alternating the polarity of the applied voltage, for example between frames or between subframes.
- Frames f2 and f3 show a pulse width modulation (pwm) driving scheme. In the first and third subframe f2a, f2c of f2 the applied voltage is low, and the first fluid in the display element will be covering the entire display area of the display element and the display element will be in an off or closed state. In the second subframe f2b of f2 the applied voltage is high, causing the first fluid to contract and showing an open state of the display element. The voltages applied in the pwm scheme are not indicative of the display effect. The display effect depends on the period the first voltage is applied and the period the second voltage is applied.
- The display effect in frame f2 is closed during 5/7 of the frame period and open during 2/7 of the frame period. Since the duration of the frame is relatively short, an eye of an observer will average the impression of both states. Hence, frame f2 will show grey level of 3 on the scale of 1 (closed) to 8 (open). Frame f3 shows a grey level of 6. The three subframes can display 8 grey levels, or a 3 bit depth.
- The voltage changes in Ve of
Figure 11 can be implemented in a display apparatus of the direct-drive type by programming thedisplay controller 206 and thedisplay driver 207 inFigure 2 such that the required voltages are set on thesignal lines 5, which are connected to the electrodes of the display elements, the timing corresponding to the subframes in each frame. - The implementation in a display apparatus of the active matrix type provides for addressing of the display elements during the subframes in each frame. The choice of analog or pwm driving can be realized by an appropriate choice of voltages on the column signal lines for the subframes.
- The change from analog driving to pulse-width modulation driving and back is made in dependence on a characteristic of the data representing a display effect, input into the display apparatus. The characteristic of the data is similar to the characteristic of embodiments in the first class described above. The characteristic of the data may for example be a frame rate or a difference value. The characteristic may be obtained in the same manner as described above for embodiments of the first class and have similar selection criteria. The determination of the characteristic for a display device having a plurality of display elements may be performed as described above for the first class.
- An electrowetting display device can be driven by a DC driving scheme. When the data input in the display apparatus requires large and frequent changes of the display effect of display element, the quality of an image displayed is reduced. It is desirable to improve the quality of the images displayed. The embodiments described below improve the quality of the images displayed.
- In a method of driving an electrowetting display device has at least one display element for displaying a display effect during a display period. The method may comprise for example receiving data representing a first display effect for display by the at least one display element; and receiving data representing a second display effect for display by the at least one display element subsequent to display of the first display effect. Data representing the first display effect and data representing the second display effect may be compared to determine a difference value indicative of a change of the display effect. A driving scheme for the at least one display element can be selected in dependence on the difference value. The selection can be made from at least a first driving scheme and a second driving scheme. In the first driving scheme a voltage indicative of a display effect is applied to the at least one display element for a first number of times during the display period. In the second driving scheme a voltage indicative of a display effect is applied to the at least one display element for a second number of times during the display period. The second number is different from the first number. The first number may be larger than the second number and the second number may be larger than the first number. The method applies a number of times the voltage indicative of the display effect to the display element or to the electrodes of the display element, where the value of the number depends on the difference value. The number may have any integer value larger than zero.
- The method can be configured such that an increasing step in display effect causes a larger number of applications of a voltage to the display element, so the required charge and voltage are attained on the electrodes of the display element.
- It has been observed that large and frequent changes of the display effect of display element may require several applications of a voltage for charging the display element up to the level appropriate for the display effect to be displayed. In the above embodiment, the number of applications of the voltage has been made dependent on the difference value, indicative of a change of the display effect.
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Figure 12 shows a diagram of the multiple data write driving method. The voltage Ve applied to the electrodes of a display element is shown as a function of time for six frames f10 ... f15. A display effect of the display device is displayed during adisplay period 100, which in this example is the same as the duration or length of a frame. - The change in voltage between frames f10 and f11 is indicative of a display effect of the display element. Hence, the change in applied voltage between frames f10 and f11 is indicative of a change in display effect. The magnitude of the change in display effect can be taken as the difference value between frames f10 and f11. In the example of
Figure 1 , the difference value between frames f10 and f11 is smaller than a predefined value and, hence, during frame f11 the display element is driven using the first driving scheme with the first number equal to one. - In the transition from frame f11 to f12 the change in display effect has a different sign than in the transition from frame f10 to f11. These changes can be treated similarly if the difference value is the magnitude of the change of the display effect, making the difference value independent of the sign of the change. Alternatively, the sign can be taken into account, for example by taking into account only positive changes of the display effect.
- The transition from frame f12 to f13 shows a relatively large change in display effect. The difference value is now larger than the predefined value and the second driving scheme is selected for frame f13. The value of the second number in this example is two, causing two applications of the voltage during the display period. The
first application 101 is at the start of the display period, thesecond application 102 is after the first application but within the display period. The second and further applications of a voltage are effective when they are grouped near the start of the display period, as shown inFigure 12 . - The still larger change in display state at the transition from frame f14 to frame f15 causes three
applications - The method can be implemented in a display apparatus of the active matrix type by using subframes for each second or further application of the voltage. For example, in frame f13 the display elements of the display device are addressed and the appropriate voltages applied to the column signal lines. Shortly after this addressing action, a subframe causes as next addressing action, applying the same voltages to the column signal lines.
- The change from applying the voltage to a display element once or more times in a display period is made in dependence on a difference value, characteristic of the data representing a display effect and which is input into the display apparatus. The difference value is similar to the difference value of embodiments in the first class described above. The difference value may for example be a change of the display effect or a number of display elements of the display device that have a change in display state larger than a predefined change. The predefined change is for example 60 on a grey scale from 1 to 256.
- The difference value may be obtained in the same manner as described above for embodiments of the first class and have similar selection criteria. The determination of the characteristic for a display device having a plurality of display elements may be performed as described above for the first class.
- The above embodiments are to be understood as illustrative examples. Further variations not part of the invention may be envisaged. For example, the variations may include a driving scheme having a higher frame rate, which may be selected, either separately or in combination with one of the above-mentioned driving schemes.
- It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments without departing from the scope of the accompanying claims. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the accompanying claims.
Claims (15)
- A method of driving an electrowetting display device having a plurality of display elements for displaying a respective image in each of a plurality of consecutive frames, the method comprising:receiving grey level data to be displayed by at least one display element of the plurality of display elements in a first frame of the plurality of consecutive frames, the first frame consecutive to a previous frame;receiving previous grey level data displayed by said at least one display element in the previous frame;comparing the grey level data and the previous grey level data to determine a difference value indicative of a change of grey level between the previous frame and the first frame;a) in dependence on the difference value being smaller than a predefined difference value:selecting a DC driving scheme in which all voltages applied to electrodes of a display element which are indicative of a grey level to be displayed have the same polarity;orb) in dependence on the difference value being larger than or equal to the predefined difference value:selecting an AC driving scheme in which voltages applied to electrodes of a display element which are indicative of a grey level to be displayed include at least two voltages having different polarities; anddriving said at least one display element in the plurality of consecutive frames using the DC driving scheme or the AC driving scheme selected respectively in a) or b).
- A method according to claim 1, in which the difference value represents a magnitude of the change of the grey level of the at least one display element.
- A method according to claim 1, in which the difference value represents a number of display elements for which the change of grey level is larger than a predefined change.
- A method according to claim 1, comprising arranging the change of grey level for each display element of the at least one display element in a histogram and deriving the difference value from the histogram.
- A method according to claim 1, comprising applying a reset pulse to the at least one display element during DC driving to reduce backflow in the at least one display element.
- A method according to claim 1, comprising changing, in the AC driving scheme, a polarity of a voltage applied to the at least one display element between successive frames of the plurality of consecutive frames.
- A method of driving an electrowetting display device having a plurality of display elements for displaying a respective image in each of a plurality of consecutive frames, the method comprising:receiving, during the plurality of consecutive frames, data for display by the plurality of display elements as an input to an input line of the electro wetting display device;determining a frame rate indicative of a rate at which the data for display by the plurality of display elements is input to the input line of the electrowetting display device;a) in dependence on the frame rate being smaller than a predefined frame rate:selecting a DC driving scheme in which all voltages applied to electrodes of a display element which are indicative of a grey level to be displayed have the same polarity;orb) in dependence on the frame rate being larger than or equal to the predefined frame rate:selecting an AC driving scheme in which voltages applied to electrodes of a display element which are indicative of a grey level to be displayed include at least two voltages having different polarities; anddriving the plurality of display elements in the plurality of consecutive frames using the DC driving scheme or the AC driving scheme selected respectively in a) or b).
- A method according to claim 7, in which driving the display element using the DC driving scheme comprises applying a reset pulse to the plurality of display elements to reduce backflow in the plurality of display elements.
- A method according to claim 7, comprising changing, in the AC driving scheme, a polarity of voltages applied to the plurality of display elements between successive frames of the plurality of consecutive frames.
- A display controller for an electrowetting display device, the display controller comprising at least one processor, and at least one memory including computer program instructions,
the electrowetting display device including a plurality of display elements, each for displaying a grey level, respectively;
the at least one memory and the computer program instructions being configured to, with the at least one processor, cause the display controller to perform the method according to claim 1 or claim 7. - A display controller according to claim 10, wherein the at least one memory and the computer program instructions are configured to, with the at least one processor, cause the display controller to perform the method according to claim 1, in which the difference value represents a magnitude of the change of the grey level of the at least one display element.
- A display controller according to claim 10, wherein the at least one memory and the computer program instructions are configured to, with the at least one processor, cause the display controller to perform the method according to claim 1, in which the difference value represents a number of display elements for which the change of grey level is larger than a predefined change.
- A display controller according to claim 10, wherein the at least one memory and the computer program instructions are configured to, with the at least one processor, cause the display controller to perform the method according to claim 1, the method comprising arranging the change of grey level for each display element of the at least one display element in a histogram and deriving the difference value from the histogram.
- A display controller according to claim 10, configured to apply a reset pulse to the at least one display element during the DC driving scheme to reduce backflow in the at least one display element.
- A display controller according to claim 10, configured to change, in the AC driving scheme, a polarity of a voltage applied to the at least one display element between successive frames of the plurality of consecutive frames.
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US14/137,091 US9318060B2 (en) | 2013-12-20 | 2013-12-20 | Display driving method |
US14/137,017 US9293089B2 (en) | 2013-12-20 | 2013-12-20 | Display driving method |
US14/136,980 US9299295B2 (en) | 2013-12-20 | 2013-12-20 | Display driving method |
US14/137,071 US9430972B2 (en) | 2013-12-20 | 2013-12-20 | Electrowetting display device driving method |
PCT/EP2014/078920 WO2015092050A1 (en) | 2013-12-20 | 2014-12-19 | Display driving method |
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EP3084750B1 true EP3084750B1 (en) | 2019-02-20 |
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GB2533951A (en) * | 2015-01-08 | 2016-07-13 | Sharp Kk | Active matrix device and method of driving |
US10255835B2 (en) * | 2015-09-28 | 2019-04-09 | Amazon Technologies, Inc. | Luminance and reducing power consumption in electrowetting displays |
US10133057B1 (en) | 2015-12-21 | 2018-11-20 | Amazon Technologies, Inc. | Electrowetting element with different dielectric layers |
CN108648701B (en) * | 2018-05-25 | 2020-08-11 | ē¦å·å¤§å¦ | Method and device for reducing ink backflow of electrowetting electronic paper |
CN111402819B (en) * | 2020-04-26 | 2021-09-21 | åååøčå¤§å¦ | Electrowetting display driving system, method and apparatus |
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JP4608546B2 (en) * | 2005-07-14 | 2011-01-12 | ć·ć£ć¼ćę Ŗå¼ä¼ē¤¾ | Display element and electric device using the same |
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JP2009003420A (en) * | 2007-05-21 | 2009-01-08 | Victor Co Of Japan Ltd | Video signal display apparatus and method for reproducing video signal |
TW201106077A (en) * | 2009-08-05 | 2011-02-16 | Ind Tech Res Inst | Electrowetting display device and driving method for display device |
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US8653832B2 (en) * | 2010-07-06 | 2014-02-18 | Sharp Kabushiki Kaisha | Array element circuit and active matrix device |
US8173000B1 (en) * | 2011-01-18 | 2012-05-08 | Sharp Kabushiki Kaisha | Active matrix device and method of driving the same |
WO2013024754A1 (en) * | 2011-08-12 | 2013-02-21 | ć·ć£ć¼ćę Ŗå¼ä¼ē¤¾ | Display device |
GB201121928D0 (en) * | 2011-12-20 | 2012-02-01 | Samsung Lcd Nl R & D Ct Bv | Driving of electrowetting display device |
JP6235196B2 (en) * | 2012-05-31 | 2017-11-22 | ć¤ć¼ ć¤ć³ćÆ ć³ć¼ćć¬ć¤ć·ć§ć³ | Display medium drive device, drive program, and display device |
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