HK1152583B - Methods for driving electro-optic displays - Google Patents

Description

Method for driving electro-optic display

The present application relates to:

(a) U.S. Pat. Nos. 6,504,524;

(b) U.S. patent nos. 6,512,354;

(c) U.S. patent nos. 6,531,997;

(d) U.S. patent nos. 6,995,550;

(e) U.S. Pat. Nos. 7,012,600 and 7,312,794 and related patent publication Nos. 2006/0139310 and 2006/0139311;

(f) U.S. patent nos. 7,034,783;

(g) U.S. Pat. Nos. 7,119,772;

(h) U.S. patent nos. 7,193,625;

(i) U.S. Pat. Nos. 7,259,744;

(j) U.S. patent publication nos. 2005/0024353;

(k) U.S. patent publication nos. 2005/0179642;

(l) U.S. patent nos. 7,492,339;

(m) U.S. patent No.7,327,511;

(n) U.S. patent publication No. 2005/0152018;

(o) U.S. patent publication No. 2005/0280626;

(p) U.S. patent publication No. 2006/0038772;

(q) U.S. patent No.7,453,445;

(r) U.S. patent publication No. 2008/0024482;

(s) U.S. patent publication No. 2008/0048969; and

(t) U.S. patent publication No. 2008/0129667.

For convenience, the above-mentioned patents and patent applications are hereinafter referred to collectively as "MEDEOD" (method for driving electro-optic displays) applications.

Technical Field

The present invention relates to a method for driving an electro-optic display, in particular a bistable electro-optic display, and to an apparatus for use in such a method. More particularly, the invention relates to driving methods intended to enable multiple driving schemes to be used simultaneously to update an electro-optic display. The invention is particularly, but not exclusively, intended for use in a particle-based electrophoretic display in which one or more types of electrically charged particles are present in a fluid and move through the fluid under the influence of an electric field to change the appearance of the display (apearance).

Background

Background terminology and the state of the art with respect to electro-optic displays are discussed in detail in U.S. Pat. No.7,012,600, to which the reader is referred for further information. Therefore, the terms and the technical level are briefly summarized as follows.

As applied to materials or displays, the term "electro-optic" is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first display state to its second display state by application of an electric field to the material. Although the optical property is typically a color perceptible to the unaided human eye, it may be other optical properties such as optical transmittance, reflectance, brightness, or, in the case of a display intended for machine reading, a pseudo color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible spectrum.

The term "grey-scale state" is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-to-white transition between the two extreme states. Such as those described in several of the patents and published applications cited below, the extreme states are white and deep blue, so that the intermediate "gray state" is effectively light blue. In fact, as already mentioned, the transition between the two extreme states may not be a color change at all.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume its first or second display state by means of an addressing pulse having a finite duration, that state will continue for a time at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated.

The term "excitation" is used herein in its conventional sense to refer to the integral of voltage with respect to time. However, some bistable electro-optic media are used as charge sensors (transducers), and another definition of excitation, i.e., the integral of current over time (equal to the total charge applied), can be used for such media. Depending on whether the medium is used as a voltage-time excitation sensor or as a charge excitation sensor, a proper definition of excitation should be used.

Much of the discussion below will focus on methods for driving one or more pixels of an electro-optic display to effect a transition from an initial gray level to a final gray level (which may be different from or the same as the initial gray level). The term "waveform" will be used to refer to the entire voltage versus time curve used to effect the transition from one particular initial gray level to a particular final gray level. Typically, such a waveform will include a plurality of waveform elements; wherein the cells are substantially rectangular (i.e., wherein a given cell comprises applying a constant voltage for a certain period of time); the cells may be referred to as "pulses" or "drive pulses". The term "drive scheme" refers to a set of waveforms that is sufficient to achieve all possible transitions between gray levels for a particular display.

Several types of electro-optic displays are known, for example:

(a) rotating two-color component displays (see, e.g., U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071; 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791);

(b) electrochromic displays (see, for example, Nature 1991, 353, 737 to O' Regan, b. et al; Information Display, 18(3), 24 (3.2002), d. Bach, u. et al, adv. mater, 2002, 14(11), 845 to Bach, u. et al, and U.S. patent nos. 6,301,038, 6,870,657, and 6,950,220);

(c) electrowetting displays (see, for example, Hayes, R.A. et al, Nature, 425, 383-385 (9/25 2003), entitled "Video-Speed Electronic Paper Based on electrowetting" and U.S. patent publication No. 2005/0151709);

(d) particle-based electrophoretic displays in which a plurality of electrically charged particles move through a fluid under the influence of an electric field (see U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773 and 6,130,774; U.S. patent application publication No. 2002/0060321; 2002/0090980; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 2004/0075634; 2004/0094422; 2004/0105036; 2005/0062714 and 2005/0270261; and International patent application publication No. WO 00/38000; WO 00/36560; WO 00/67110 and WO 01/07961; and European patent Nos. 1,099,207B 1 and 1,145,072B 1; and other MIT and E Ink patents and applications discussed in the aforementioned U.S. Pat. No.7,012,600).

There are several different variations of electrophoretic media. The electrophoretic medium may use a liquid or gaseous fluid; for gaseous fluids see, for example, Kitamura, T.et al published in IDW Japan, PaperHCS1-1 in 2001 entitled "Electrical inside movement for electronic Paper-like display" and Yamaguchi, Y.et al published in IDW Japan, Paper AMD4-4 entitled "inside display using insulating substrates charged plasma"; U.S. patent publication nos. 2005/0001810; european patent application 1,462,847; 1,482,354, respectively; 1,484,635, respectively; 1,500,971, respectively; 1,501,194, respectively; 1,536,271, respectively; 1,542,067, respectively; 1,577,702, respectively; 1,577,703 and 1,598,694; and international application WO 2004/090626; WO 2004/079442 and WO 2004/001498. The medium may be encapsulated, comprising a plurality of capsules (capsules), each of which itself comprises an internal phase comprising electrophoretically mobile particles suspended in a fluid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymeric binder to form an adhesive layer (coherent layer) located between two electrodes; see the MIT and E Ink patents and applications mentioned above. Alternatively, the walls surrounding the discrete microcapsules in the encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material; see, for example, U.S. patent No.6,866,760. For the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media. Another variation is the so-called "microcell electrophoretic display", in which charged particles and a fluid are retained within a plurality of cavities formed within a carrier medium, typically a polymer film; see, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449.

Encapsulated electrophoretic displays generally do not suffer from the failure modes of aggregation and settling of conventional electrophoretic devices and have additional advantages such as the ability to print or coat the display on a variety of different flexible and rigid substrates. (the use of the term "printing" is intended to include all forms of printing and coating including, but not limited to, pre-coat, such as patch die coating, slot or die coating, slide or cascade coating, curtain coating, roll coating, such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, and other similar techniques.) thus, the resulting display can be flexible. Further, since the display medium can be printed (using various methods), the display itself can be manufactured at low cost.

Although electrophoretic media are typically opaque (e.g., because in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be operated in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light transmissive. See, for example, the aforementioned U.S. patent nos. 6,130,774 and 6,172,798, and U.S. patent No.5,872,552; 6,144,361, respectively; 6,271,823, respectively; 6,225,971, respectively; and 6,184,856. A dielectrophoretic display similar to the electrophoretic display but relying on variations in electric field strength may operate in a similar mode; see U.S. patent No.4,418,346.

The bi-or multi-stable performance of particle-based electrophoretic displays, and the like of other electro-optic displays (for convenience such displays will be referred to hereinafter as "impulse-driven displays") is in sharp contrast to the performance of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bistable or multistable but act as voltage sensors, so that the application of a given electric field to a pixel of such a display produces a particular grey level at that pixel, irrespective of the grey level previously present at that pixel. Furthermore, LC displays are driven in one direction only (from non-transmissive or "dark" to transmissive or "bright"), with a reverse transition from a lighter state to a darker state being achieved by reducing or removing the electric field. Finally, the grey levels of the pixels of an LC display are not sensitive to the polarity of the electric field but only to their intensity, and in practice commercial LC displays usually reverse the polarity of the drive field at frequent intervals for technical reasons. In contrast, a bistable electro-optic display operates approximately as an excitation sensor, so that the final state of a pixel depends not only on the applied electric field and the period of time during which the field is applied, but also on the state of the pixel prior to application of the electric field.

Whether or not the electro-optic medium used is bistable, in order to achieve a high resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One way to achieve this is to provide an array of non-linear elements, such as transistors or diodes, in which each pixel is associated with at least one non-linear element to produce an "active matrix" display. The addressing or pixel electrode addressing one pixel is connected to a suitable voltage source via an associated non-linear element. Typically, when the nonlinear element is a transistor, a pixel electrode is connected to a drain of the transistor, and this structure is also assumed in the following description, although it is arbitrary in nature and the pixel electrode may be connected to a source of the transistor. Typically, in high resolution arrays, pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a given row and a given column. Connecting the sources of all transistors in each column to a single column electrode and the gates of all transistors in each row to a single row electrode; also, the arrangement of source to row and gate to column is conventional, however this connection is arbitrary in nature and can be reversed as desired. Connecting the row electrodes to the row driver substantially ensures that only one row is selected at any given time, i.e. a voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conductive, while a voltage is applied to all other rows to ensure that all transistors in the non-selected rows remain non-conductive. The column electrodes are connected to a column driver which applies selected voltages on the different column electrodes for driving the pixels located in the selected row to their desired optical states. (the foregoing voltages are relative to a common front electrode, which is typically disposed on opposite sides of a non-linear array in an electro-optic medium and extends across the display.) after a pre-selection interval called the "line address time", the selected row is deselected, the next row is selected, and the voltage on the column drivers is changed so that the next line of the display is written. This process is repeated to write the entire display in a row-by-row fashion.

At first glance, the ideal method for addressing such an electro-optic display driven by excitation is the so-called "universal gray scale image stream", in which the controller sets each write of an image such that each pixel transitions directly from its initial gray level to its final gray level. However, there are inevitably some errors in writing the image on the display that is actuated. Some such errors encountered in practice include:

(a) previous state dependencies; for at least some electro-optic media, the actuation required to switch a pixel to a new optical state depends not only on the current and the desired optical state, but also on the previous optical state of the pixel.

(b) Residence time dependence; for at least some electro-optic media, the actuation required to switch a pixel to a new optical state depends on the time that the pixel has spent in its various optical states. Although the precise nature of this dependence is not well understood, the longer the pixel is in its current optical state in general, the more actuation is required.

(c) Temperature dependence; the activation required to switch the pixel to a new optical state is strongly temperature dependent.

(d) Dependence on humidity; for at least some types of electro-optic media, the stimulus required to switch a pixel to a new optical state depends on the ambient humidity.

(e) Mechanical uniformity; the actuation required to switch the pixel to a new optical state can be affected by mechanical changes in the display, such as changes in the thickness of the electro-optic medium or associated lamination adhesive. Other types of mechanical non-uniformities can be caused by unavoidable variations between different manufacturing batches of media, manufacturing tolerances and material variations.

(f) A voltage error; the actual stimulus applied to the pixel will inevitably differ slightly from the theoretically applied stimulus due to the inevitable slight errors in the voltages delivered by the drivers.

A general gray scale image stream suffers from the phenomenon of "error accumulation". For example, suppose that during each transition, the temperature dependence results in an error of 0.2L in the positive direction (where L has the usual CIE definition:

L*＝116(R/R₀)^1/3-16，

wherein R is reflectance and R₀Is a standard reflectance value). After 50 transitions, the error will accumulate to 10L. Perhaps more practically, assume that the average error per transition, expressed in terms of the difference between the theoretical and actual reflectivity of the display, is ± 0.2L. After 100 consecutive transitions, the display of the pixel has an average deviation of 2L from its expected state; such deviations are evident to the average viewer on certain types of images.

This error accumulation phenomenon applies not only to errors due to temperature, but also to all types of errors listed above. As described in the aforementioned U.S. patent No.7,012,600, compensating for such errors is possible, but only to a limited degree of accuracy. For example, temperature errors can be compensated for by using a temperature sensor and a look-up table, but the temperature sensor has limited resolution and the read temperature may differ slightly from the temperature of the electro-optic medium. Similarly, the accuracy of this type of compensation is limited by storing the previous state and using a multi-dimensional transition matrix to compensate for the previous state dependence, but the controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored.

Thus, the generic grayscale image stream requires very precise control of the applied stimulus to give good results, and it has been found empirically that, at the current level of electro-optic display technology, the generic grayscale image stream is not feasible in commercial displays.

In some cases, it may be desirable for a single display to use multiple drive schemes. For example, a display having more than two gray levels may utilize a gray level drive scheme ("GSDS") that enables transitions between all possible gray levels, and a monochrome drive scheme ("MDS") that enables transitions only between two gray levels, the MDS providing faster display rewriting than GSDS. The MDS is used when all pixels that are changed during the rewriting of the display are only transitioning between the two gray levels used by the MDS. For example, the aforementioned U.S. Pat. No.7,119,772 describes a display in the form of an electronic book or similar device capable of displaying grayscale images and also capable of displaying monochrome dialogs that allow a user to enter text related to the displayed images. The fast MDS is used to quickly update the dialog box as the user enters text, thus providing the user with a quick confirmation of the entered text. On the other hand, when changing the entire gray scale image displayed on the display, a slower GSDS is used.

More specifically, the current electrophoretic display has an update time of about 1 second in the grayscale mode and 500 milliseconds in the monochrome mode. Furthermore, many current display controllers may utilize only one update scheme at any given time. As a result, the display is not responsive enough in time to react to user rapid input such as keyboard entry or scrolling of a selection bar. This limits the applicability of the display to interactive applications. It is therefore desirable to provide a driving apparatus and corresponding driving method that provides a combination of driving schemes to allow a portion of the display to be updated by a fast driving scheme while the remainder of the display continues to be updated by a standard gray scale driving scheme.

Disclosure of Invention

One aspect of the present invention relates to data structures, methods and apparatus for driving electro-optic displays that allow for rapid response to user input. The aforementioned MEDEOD applications describe various methods and controllers for driving electro-optic displays. Most of these methods and controllers use a memory with two image buffers, the first image buffer holding the first or initial image (presented on the display at the start of a display switch or rewrite) and the second image buffer holding the final image, which is expected to be presented on the display after the rewrite. The controller compares the initial and final images and, if they are different, applies drive voltages to the individual pixels of the display which cause the optical state of the pixels to change so that at the end of the overwrite (which may also be referred to as an update) the final image is formed on the display.

However, in most of the foregoing methods and controllers, the update operation is "atomic" (atomic), meaning that once the update is started, the memory cannot accept any new image data until the update is complete. This presents difficulties when it is desired to use the display for applications that accept user input, for example via a keyboard or similar data entry means, since the controller does not respond to user input when the update is active. For electrophoretic media, in which the transition between two extreme optical states takes several hundred milliseconds, this unresponsive time varies in the range of about 800 to about 1800 milliseconds, the majority of this time being used for the required update period of the electro-optic material. While the length of the unresponsive period can be reduced by removing some performance artifacts (artifacts) that increase the update time and by improving the response speed of the electro-optic material, such techniques alone are unlikely to reduce the unresponsive time below about 500 milliseconds. This is still too long for the needs of interactive applications, such as electronic dictionaries, for which users desire a fast response to their input. Therefore, there is a need for an image update method and controller with reduced unresponsive periods.

The foregoing 2005/0280626 describes a Driving scheme that utilizes the principle of asynchronous image update (see "Driving an Active Matrix Electrophoretic Display" in the SID argument set 2004 by Zhou et al) to substantially reduce the period of unresponsiveness. Compared to existing methods and controllers, the method described in the paper reduces the non-response time by about 65% using structures already developed for grayscale image displays, while only moderately increasing the complexity and memory requirements of the controller.

More particularly, the foregoing 2005/0280626 describes two methods for updating an electro-optic display having a plurality of pixels, each of which can achieve at least two different gray levels. The first method comprises the following steps:

(a) providing a final data buffer arranged to receive data defining a desired final state for each pixel of the display;

(b) providing an initial data buffer arranged to hold data defining an initial state of each pixel of the display;

(c) providing a target data buffer arranged to hold data defining a target state for each pixel of the display;

(d) determining when the data in the initial and final data buffers is different and updating the value in the target data buffer when such a difference is found by (i) setting the target data buffer to the value when the initial and final data buffers contain the same value for a particular pixel; (ii) setting the target data buffer to the value of the initial data buffer plus the increment when the initial data buffer contains a value greater than the final data buffer for the specific pixel; and (iii) for a particular pixel, when the initial data buffer contains a value less than the final data buffer, setting the target data buffer to the value of the initial data buffer minus the increment;

(e) updating the image on the display using the data in the initial data buffer and the target data buffer as the initial and final state of each pixel, respectively;

(f) copying data from the target data buffer to the initial data buffer after step (e); and

(g) and repeating steps (d) - (f) until the initial and final data buffers contain the same data.

The second method comprises the following steps:

(a) providing a final data buffer arranged to receive data defining a desired final state for each pixel of the display;

(b) providing an initial data buffer arranged to hold data defining an initial state of each pixel of the display;

(c) providing a target data buffer arranged to hold data defining a target state for each pixel of the display;

(d) providing a polarity bit array arranged to hold a polarity bit for each pixel of the display;

(e) determining when the data in the initial and final data buffers differs and updating the values in the polarity bit array and the destination data buffer when such a difference is found by (i) setting the polarity bit of the pixel to a value representing a transition towards the opposite extreme optical state when the value in the initial and final data buffers for a particular pixel differs and the value in the initial data buffer represents the extreme optical state of the pixel; and (ii) when the values in the initial and final data buffers for a particular pixel are different, setting the target data buffer to the value of the initial data buffer plus or minus an increment, depending on the correlation value in the array of polarity bits;

(f) updating the image on the display using the data in the initial data buffer and the target data buffer as the initial and final state of each pixel, respectively;

(g) copying data from the target data buffer to the initial data buffer after step (f); and

(h) and repeating steps (e) - (g) until the initial and final data buffers contain the same data.

Any of the above prior art fails to provide a general solution to the problem of using multiple drive schemes simultaneously on a single display. In the aforementioned U.S. Pat. No.7,119,772, only one of the two drive schemes is applied at any one time; monochrome or similar drive schemes are "local" drive schemes, meaning that only pixels that need to be changed are updated and therefore only operate in a text box or similar selected area. If a portion of the display outside the selected area needs to be changed, the display must switch back to the slower full gray scale drive scheme, making it impossible to quickly update the selected area, where the unselected areas are being changed. Similarly, although the aforementioned 2005/0280626 provides one way to reduce the "wait" time before starting a new update, only a single drive scheme may be used at any one time.

There is a need for a method of driving a bistable electro-optic display that allows multiple drive schemes to be used simultaneously. For example, in the text box/background image example used in the aforementioned U.S. Pat. No.7,119,772, it is often convenient for a user to mark in the text box area with a keyboard or stylus while scrolling through a set of images displayed in the background. Many electro-optic displays also make use of so-called "menu bar operations" in which a set of radio buttons indicate which item on the menu is selected, and in these operations it is important to quickly update the radio button areas so that the user does not accidentally make a wrong selection. It is also highly desirable that the method for driving a bistable electro-optic display allows for the simultaneous use of multiple drive schemes with different update periods (e.g. monochrome drive schemes typically have shorter update periods than grey scale drive schemes) and allows each of the multiple drive schemes to begin overwriting portions of that drive scheme of the display independently of the other drive schemes; the effectiveness of the fast monochrome drive scheme for updating the menu bar is greatly reduced if a new update using the fast monochrome drive scheme can only be started after the slower grayscale drive scheme update of the background region is completed. The present invention provides a data structure, a method for driving a bistable electro-optic display and an electro-optic display that meets these needs.

Accordingly, the present invention provides a data structure for controlling a bistable electro-optic display having a plurality of pixels, the data structure comprising:

a pixel data storage area arranged to hold, for each pixel of the display, data representing an initial state of the pixel, data representing a desired final state of the pixel and a drive scheme index representing a drive scheme applied to the pixel; and

a drive scheme storage area arranged to hold data representing a plurality of drive schemes, the drive scheme storage area holding at least all of the drive schemes denoted by the drive scheme index numbers held in the pixel data holding area.

In a preferred form of this data structure, the drive scheme storage area also holds, for each drive scheme, time data representing a period from the start of a current update effected using the drive scheme.

The invention also provides a method of driving a bistable electro-optic display having a first plurality of pixels, the method comprising:

for each pixel of the display, holding data representing an initial state of the pixel, data representing a desired final state of the pixel and a drive scheme index representing a drive scheme to be applied to the pixel;

storing data representing a plurality of drive schemes, the drive schemes being at least equal in number to different drive scheme index numbers stored for a plurality of pixels of the display; and is

For at least a second plurality of pixels of the display, an output signal is generated representing the stimulus applied to each of the second plurality of pixels, the generation of the output signal for each of the second plurality of pixels being dependent on the initial and final states of the pixel, the drive scheme index number and saved data representing the drive scheme referred to by the drive scheme index number.

In a preferred form of the method, a time value is also saved for each of the saved drive schemes, and the generation of the output signal is dependent on the time value associated with the drive scheme referred to by the drive scheme index.

The invention extends to a bistable electro-optic display having a plurality of pixels and comprising a data structure of the invention, and to such a bistable electro-optic display arranged to carry out the method of the invention.

The display of the present invention may be used in any application where prior art electro-optic displays have been used. Thus, for example, the present displays may be used in electronic book readers, portable notebooks, tablet computers, cellular phones, smart cards, sign boards (signs), watches, shelf labels, and flash memory.

Drawings

FIG. 1 is a diagram of a data structure of the present invention;

FIG. 2 is a schematic diagram of the mode of operation of an electro-optic display utilizing the data structure of FIG. 1.

Detailed Description

As indicated above, the present invention provides data structures and methods for operating a bistable electro-optic display. The data structure and method of operation allow multiple drive schemes to be used simultaneously in a display. In a preferred form of the data structure and method of the present invention, the plurality of drive schemes may be started at different times and thus run independently of each other.

The various drive schemes used in the preferred form of the method may be started at different times, which does not imply that any given drive scheme may be started at any arbitrary time; the manner in which the electro-optic display is driven, the start of the drive scheme will of course be subject to certain limitations. As discussed in the previous MEDEOD applications, most high resolution displays use an active matrix backplane with pixel electrodes arranged in a two-dimensional matrix defined by row and column electrodes. A row driver selects a row of pixel electrodes at a time and applies appropriate voltages to the column electrodes to provide the desired voltages to the electrodes in the selected row. After an appropriate interval, the previously selected row is deselected and the next row is selected so that the entire matrix of pixel electrodes is scanned in a row-by-row manner. Scanning of the entire matrix typically takes about 20 milliseconds.

When selecting a drive scheme for such an active matrix display, to avoid undesirable image artefacts, it is necessary to synchronize the drive scheme using the scanning of the display by dividing each waveform of the drive scheme into frames, each frame representing an integer number (typically one) of scans of the display, the applied voltage for any pixel remaining stable in any one frame. In such active matrix displays, all drive schemes used must use the same frame, and a drive scheme can only start at the start of a new frame, i.e. at a "frame boundary". All waveforms used must also occupy an integer number of frames and all waveforms in a given drive scheme must occupy the same number of frames, but different drive schemes may occupy different numbers of frames. Note that there is no such limitation in so-called "direct drive" displays, where each pixel is provided with a separate conductor, so that the voltage across each pixel can be varied in any way, and no frame is required. When the present data structure and method is used in an active matrix display, it is convenient for the time value saved for each drive scheme to simply represent the number of frames that have passed since the start of the drive scheme, with the overwriting of the relevant area of the display being completed each time the number falls to zero.

The drawing of fig. 1 illustrates the data structure of the present invention, generally designated 100. The data structure 100 includes a pixel data storage area, generally indicated at 102, and a drive scheme storage area, generally indicated at 104. The pixel data storage area 102 is divided into an initial state storage area 106, a final state storage area 108, and a drive scheme selector area 110. Each of the three regions 106, 108 and 110 is arranged to hold one integer per pixel of the display. The initial data storage area 106 holds an initial gray level for each pixel, and the final state storage area 108 holds a desired final gray level for each pixel. The drive scheme selector area 110 holds for each pixel an integer indicating which of a plurality of possible drive schemes is being used for the associated pixel. As shown in fig. 1, the drive scheme selector area 110 is saving a value of "1" for all pixels in a single rectangle 112, a value of "2" for each pixel in each of three small rectangles 114 (intended to be used as radio buttons) and a value of "3" for all other pixels.

It will be apparent to those of ordinary skill in the computer arts that although regions 106, 108 and 110 are shown in FIG. 1 as occupying separate areas of memory, in practice this is not the most convenient way. For example, for the data associated with each pixel, it is more convenient to group them together into a single long "word". For example, if each pixel is associated with a four bit word in region 106, a four bit word in region 108, and a four bit word in region 110, it is most convenient to save the data as a string of twelve bits, one for each pixel, with the first four bits defining the initial gray level, the middle four bits defining the final gray level, and the last four bits defining the drive scheme. It will also be apparent to one of ordinary skill in the art that regions 106, 108 and 110 need not be the same size; for example, if the display is a 64 gray scale (six bit) display, which can only use four synchronous drive schemes, regions 106 and 108 will hold six bits per pixel, while region 110 need only hold two bits per pixel.

Furthermore, although region 110 is shown in FIG. 1 to hold the value of the drive scheme selector for each pixel of the display, this is not strictly necessary. The invention may be modified such that each saved value in the area 110 may determine a drive scheme to apply to a group of adjacent pixels (e.g., a 2 x 2 or 3 x 3 group of pixels). In fact, the drive scheme may be selected based on a "super pixel" of pixels larger than the controlled gray level. However, this approach is not suggested since the size of the storage space required for the region 110 is generally not a major issue, and is useful because the ability to control the drive scheme used on a per pixel basis allows different regions to use different drive schemes to have entirely arbitrary shapes. For example, when a display with, for example, VGA resolution (640 x 480) is used to display a menu system, and a single menu item is selected by clicking on a radio button, this ability to control the drive scheme used on a per pixel basis allows the use of radio buttons of the type commonly used in personal computer programs, each button displaying a permanent ring and the selected button display having a solid black circle in its ring, rather than using a simple rectangular area as a radio button.

The data in regions 108 and 110 is written directly by host 116 via data lines 118 and 120, respectively. The manner in which data is written to the area 106 is described in detail below.

The drive scheme storage area 104 shown in fig. 1 includes a set of rows, each row including a look-up table (labeled LUT1, LUT2, etc.) and time integers (labeled T1, T2, etc.). The time integer represents the number of frames that have passed since the start of the associated drive scheme. It will be appreciated that different look-up tables may have different sizes; for example, if the display is a 16 gray scale (4 bit) display, a full gray scale look-up table would require 256 entries (16 initial states x 16 final states), while a look-up table for a monochrome area of the display would require only 4 entries.

As mentioned above, figure 1 is highly schematic and figure 2 provides a somewhat realistic, but still schematic, view showing how a bistable electro-optic display is driven in practice. As shown in fig. 1, the system shown in fig. 2 is controlled by a host 116, which host 116 feeds drive scheme selection data to the drive scheme selector region 110 via data lines 120. However, in the system shown in FIG. 2, the host 116 feeds image data representing a new image to be displayed on the display to the image buffer 222 via the data line 118. Image data is asynchronously copied from the image buffer to the final state storage area 108 via data lines 124.

The data in regions 106, 108 and 110 are asynchronously copied into the update buffer 226, whereby the data is copied into the two shadow data storage regions labeled 106 ', 108 ', 110 ' and 106 ", 108", 110 ", respectively. Data is copied from storage area 108 "to storage area 106 at appropriate time intervals to provide the initial gray scale data as described above.

The shadow data storage areas 106 ', 108 ', 110 ' are used to calculate output signals in the method of the present invention. As described in the aforementioned MEDEOD application, the look-up table basically comprises a two-dimensional matrix, one axis of the matrix representing the initial state of the pixel and the other axis representing the desired final state of the pixel. Each entry in the look-up table defines the waveform required to effect a transition from the initial state to the final state and typically comprises a string of integers representing the voltages applied to the pixel electrodes during the frame string. For each successive pixel, the display controller (not explicitly shown in fig. 2) reads the drive scheme selector number from region 110 ', determines the associated look-up table, and then reads the associated entry from the selected look-up table using the initial and final state data from regions 106 ' and 108 ', respectively. The display controller also compares its internal clock (not shown) to the time integer associated with the selected look-up table to determine which integer in the selected look-up table entry is associated with the current frame and outputs the associated integer on output signal line 230.

The selection of the plurality of regions to which the plurality of different drive schemes are applied is controlled by the host system 116. The selection of such multiple regions may be predetermined or controlled by an operator. For example, if a database program provides a dialog box for text entry, the size specification and placement (placement) of the dialog box is typically predetermined by the database program. Similarly, in the e-book reader menu system, the positions of radio buttons, text, and the like are predetermined. On the other hand, the display may serve as an output device for an image editing program, and such programs typically allow a user to select ("lasso") an arbitrarily shaped area that is desired to be processed.

It will be apparent that many changes can be made to the data structures and methods of the present invention. Such data structures and methods may include any of the optional features of the driving schemes proposed in the aforementioned MEDEOD applications. For example, various MEDEOD applications describe the use of multiple look-up tables to account for the sensitivity of the electro-optic medium to various factors, such as gray scale level before initial state, temperature, humidity, and operational lifetime of the electro-optic medium. Such multiple look-up tables may also be used in the present invention. It will be appreciated that providing multiple sets of look-up tables to allow for adjustment of multiple different environmental parameters and multiple driving schemes used in the present invention may result in the need to save an extremely large amount of data. In a system with limited capacity RAM, it may be desirable to keep the look-up table in non-volatile memory (e.g., a hard disk or ROM chip) and only move the specific look-up table needed at any given time to ROM.

From the foregoing, it will be seen that the present invention may provide an improved user experience by making image update operations appear faster, because the present invention has the ability to provide partial update operations that achieve overlap of different image regions. The present invention also allows electrophoretic and other electro-optical displays to be used in applications requiring final user interface operations, such as mouse, stylus tracking, or menu bar operations.

Claims (14)

1. A bistable electro-optic display having:

a plurality of pixels;

a pixel data storage area arranged to hold, for each pixel of the display, data representing an initial state of the pixel, data representing a desired final state of the pixel and a drive scheme index representing a drive scheme applied to the pixel; and

a drive scheme storage area arranged to hold data representing a plurality of drive schemes, the drive scheme storage area holding at least all of the drive schemes referred to by the drive scheme index numbers held in the pixel data holding area.

2. A bistable electro-optic display according to claim 1, of the active matrix type wherein the pixels are arranged in a two-dimensional matrix defined by row and column electrodes, one row of pixel electrodes at a time being selected by a row driver and appropriate voltages being applied to the column electrodes to provide the desired voltages to the electrodes in the selected row, and after an appropriate interval the previously selected row being deselected and the next row being selected such that the entire matrix of pixel electrodes is scanned in a row-by-row manner during a frame interval, wherein the drive scheme timing data is arranged such that each drive starts at the start of a frame.

3. A display according to claim 2, wherein the time value saved for each drive scheme represents the number of frames that have passed since the start of the drive scheme.

4. A method for driving a bistable electro-optic display having a first plurality of pixels, the method comprising:

for each pixel of the display, holding data representing an initial state of the pixel, data representing a desired final state of the pixel and a drive scheme index representing a drive scheme to be applied to the pixel;

storing data representing a plurality of drive schemes, the drive schemes being at least equal in number to the different drive scheme index numbers stored for respective pixels of the display; and is

For at least a second plurality of pixels of the display, an output signal is generated representing the stimulus applied to each of the second plurality of pixels, the generation of the output signal for each of the second plurality of pixels being dependent on the initial and final states of the pixel, the drive scheme index number and saved data representing the drive scheme referred to by the drive scheme index number.

5. The method of claim 4, further comprising saving a time value for each of the saved drive schemes, and wherein the generation of the output signal is further dependent on the time value associated with the drive scheme referred to by the drive scheme index.

6. The method of claim 5, wherein the time value saved for each drive scheme represents a number of frames that have passed since the start of the drive scheme.

7. A bistable electro-optic display having a plurality of pixels and arranged to perform the method of any of claims 4 to 6.

8. A bistable electro-optic display according to claim 7, of the active matrix type wherein the pixels are arranged in a two-dimensional matrix defined by row and column electrodes, one row of pixel electrodes at a time being selected by a row driver and appropriate voltages being applied to the column electrodes to provide the desired voltages to the electrodes in the selected row, and after an appropriate interval the previously selected row is deselected and the next row is selected such that the entire matrix of pixel electrodes is scanned in a row-by-row manner during a frame interval, wherein the drive scheme time data is saved and arranged such that each drive starts at the start of the frame.

9. An electronic book reader, portable notebook, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash memory comprising a display according to claim 1 or 7.

10. A display according to claim 1 or 7 wherein the electro-optic material of the bistable electro-optic display comprises a rotating bichromal element or electrochromic material.

11. A display according to claim 1 or 7 wherein the electro-optic material of the bistable electro-optic display comprises an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and movable through the fluid under the influence of an electric field.

12. The display of claim 11, wherein the charged particles and the fluid are confined in a plurality of capsules or microcells.

13. An electro-optic display according to claim 11 wherein the electrically charged particles and the fluid are present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material.

14. The display of claim 11, wherein the fluid is gaseous.