WO2002058042A2

WO2002058042A2 - Method for operating a bistable display

Info

Publication number: WO2002058042A2
Application number: PCT/DE2002/000071
Authority: WO
Inventors: Klaus Budde; Markus Hermsen
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-01-19
Filing date: 2002-01-11
Publication date: 2002-07-25
Also published as: DE10102361A1; EP1352383A2; WO2002058042A3; EP1352383B1

Abstract

The invention relates to a method for operating a bistable display, whose individual cells are transferred from a first state to a second state by applying a write voltage and can be stabilized in the second state by maintaining the write voltage over a prescribed period of time whereby causing the cells to remain in the second state without the write voltage. The cells can be operated according to choice in an unstable mode, in which only one write voltage is applied in such a manner that the cells are unstably held in the second state and transfer back to the first state when the write voltage is removed.

Description

description

Method for operating a bistable display

The invention relates to a method for operating a bistable display. In addition, the invention relates to a corresponding control device for controlling a bistable display and a device with a bistable display and such a control device.

So-called “bistable displays” are displays whose individual cells which form the image pixels are brought from a first to a second state by applying a writing voltage and are thus stabilized in the second state by maintaining the writing voltage over a certain switching time that the respective cells remain in the second state even without the write voltage. This means that the individual cells are "frozen", so to speak. So far, only a few prototypes of such bistable displays have been available. These are various displays with liquid crystal cells made of cholesteric, ferroelectric, nematic or antiferroelectric materials. In principle, these bistable displays work in a similar way to previously known LCDs. The different states of the cells, which are associated with a different optical behavior, for example in which the cells are transmissive or blocked, depend on the orientation of the crystals in the cell. By applying the write voltage, the crystals in the respective cell are gradually aligned in a preferred direction. This alignment takes time. After the crystals are largely completely aligned, a stable state is reached, so that the cell remains in this state even after the writing voltage has been removed. If, on the other hand, the write voltage is removed before the crystals are sufficiently fully aligned, they tilt back into the initial state, that means that the respective cell does not stabilize in the desired second state. The time required to align the crystals to such an extent that the stable state is reached, the so-called “switchover time”, depends on the level of the writing voltage as well as on the material and the structure of the respective cells.

However, the bistable displays currently available have two major disadvantages compared to the conventional known displays.

A disadvantage is that all image information has to be transferred to the display in a single operation step. Such displays can therefore only be used to display static information, since it is not possible to edit the content shown, which is necessary, for example, for entering and changing a text. Furthermore, it is not possible for a user to use a cursor or the like. highlighted area on the display. Likewise, it is not possible to use such displays as a “touchscreen” for entering sketches or the like, as with a palm top, since the points to which a stylus points cannot be shown. Overall, this means that the groove zen of such displays is restricted to a few areas, for example electronic books or magazines, and the associated advantage of energy saving cannot be used in most applications.

A second disadvantage is that if new information is to be displayed, the old content must first be completely deleted. An erase pulse is applied to the cells. This is usually a short-term, higher counter-voltage, which ensures that the aligned crystals tilt back into the original basic state. Unfortunately, this deletion process is a visible process. During the deletion After the display and the subsequent writing of new information, a black area, which is characteristic of the deletion process, runs over the display. Since the deletion is usually carried out line by line, this is usually a black bar that runs from top to bottom on the display.

It is an object of the present invention to provide a way to make bistable displays usable for a wide range of applications without the disadvantages mentioned above.

This object is achieved by a method according to claim 1 and a control device according to claim 11.

According to the method according to the invention, a write voltage is applied to at least some of the cells in an unstable mode only in such a way that the respective cell is essentially kept unstable in the second state and changes back to the first state after removal of the write voltage. These are expediently the cells which are intended to represent information which can be changed quickly, and in extreme cases this part of the display which is operated in the unstable mode can also comprise only a single cell. In principle, there are various options for operating in unstable mode. It is essential that the respective information is clearly visible to the user on the display, but on the other hand it prevents the respective cell from getting into the stable state, i.e. that the crystals of the cell are sufficiently fully aligned. This means that the alignment of the crystals must be interrupted or disrupted in good time.

In this case, the respective cell is preferably controlled in such a way that the write voltage is maintained continuously for a maximum of a period of time that is shorter than the switchover time of the cell. So that the image continues for the user remains visible, the write voltage must be applied repeatedly, i.e. the write voltage is not applied over a longer period in which the cell is brought into the stable state, but only for a short time, but with a correspondingly higher frequency. Since the switchover time also depends on the voltage, the level of the write voltage can also be varied. By adjusting the height, duration and frequency of the write voltage, an optimum can be sought which, on the one hand, guarantees the best possible legibility of the information for the user even in the unstable display area, but on the other hand keeps the power consumption as low as possible and provides sufficient security ensures that the respective cells do not accidentally get into the stable state. It should also be taken into account here that the cells which are to be operated in the stable mode, ie which are to be frozen in the second state, are also usefully operated with the lowest possible voltage. As a result, the possible variations with regard to the write duration will in practice often be greater than with regard to the level of the write voltage.

A corresponding control according to the invention must, in addition to the usual means for operating the cells of the display in stable mode, additional means for alternative operation of the

Have cells in unstable mode. These means for alternative operation of the cells in the unstable mode preferably comprise a device for setting parameters of the write voltage. These parameters can be, for example, the level of the write voltage. However, this is preferably the length of the period of time over which the write voltage at the respective cell is maintained at a maximum uninterrupted, or the frequency with which frequency the write voltage is applied to the cell in an unstable mode, the information make visible without reaching the stable state. The device for parameter setting or the other means for alternative operation of the cells can be implemented in hardware or software, for example within a microcontroller, preferably directly in the display controller. In the simplest case, it is an additional time control that regulates how long and at what intervals a specific, fixedly specified write voltage is applied to a specific cell. The production of such a special control, which allows the alternative operation of a bistable display in stable mode and in unstable mode, consequently requires only little additional effort compared to the production of a conventional control device for a bistable display, which operates the display only in stable mode.

In the simplest case, it is sufficient if it is determined beforehand that the write voltage in the unstable mode is maintained uninterrupted for a maximum of a previously precisely defined write time, then interrupted and then re-applied, and vice versa, the write voltage for operation in stable mode is at least one previously defined writing time is maintained.

In the bistable displays known to date, however, this switching time is strongly temperature-dependent. In ferroelectric cells, the switchover time is approx. 1000 μs at approx. 0 ° C and below 50 μs at 60 ° C. At room temperature, the switchover time is approx. 200 μs. So that a bistable display can still be operated reliably in both modes even with large temperature fluctuations, the maximum uninterrupted write time in the unstable mode should always be less than 50 μs and stable in the stable state when setting fixed write times for the stable and unstable state over 1000 μs. In both cases, this leads to unnecessary power consumption. This is obvious in the stable state, because, for example, at room temperature write over a duration of 200 μs would suffice and thus voltage is applied to the cells completely unnecessarily for almost 800 μs. In the unstable state, the relatively short, uninterrupted writing time means that an unnecessarily high frequency has to be used in order to make the respective information visible to the user in the unstable state. Due to the short writing times in which the crystals are aligned, a reduced contrast and thus poorer legibility can be expected.

It is therefore optimal to adapt the write times in the stable and in the unstable mode in such a way that in the stable mode the write time is only slightly above the respective switchover time and in the unstable mode only slightly below the switchover time. On the other hand, in the stable mode the previously defined write time must surely be above the switchover time, in the unstable state it must surely be below the switchover time.

The control device therefore preferably comprises a device for determining an ambient temperature of the respective cell or the display, as well as means for determining the parameters of the write voltage as a function of this ambient temperature, that is to say the respective

Write times or frequencies in the unstable mode and possibly also in the stable mode, and possibly also the amount of the write voltage or other parameters, are selected as a function of the ambient temperature.

In addition, the control device preferably has means for selecting whether a cell or a region of neighboring cells, in extreme cases even the entire display, is operated in the stable or in the unstable mode. This selection can be made explicitly by the user, for example by using a cursor or other control commands via the usual user interface of the respective device Area of the display selected. However, the selection can also be made automatically, for example depending on a specific behavior of the user.

In a particularly preferred exemplary embodiment, the cell or the area of cells of the display which is to be unlocked and / or operated in the unstable mode is based on a position and / or a position sequence and / or a movement of a display symbol, for example a cursor or an arrow selected on the display. An example of this is a method in which it is automatically recognized when the user moves the cursor against the writing direction into a higher line when writing to a display. As soon as the cursor remains on an upper line for a longer period of time, it is assumed that the user wants to insert something here, and the relevant line is unlocked and at least temporarily operated in unstable mode.

Preferably, not only a single cell, but an entire area of neighboring cells of the display is operated in the unstable mode. In this area, constantly changing information, for example the time, can then be displayed, the rest of the display showing information which is static in relation thereto, for example a newspaper or book page. In terms of control technology, it is particularly advantageous if this area operated in the unstable mode comprises at least an entire row and / or an entire column of the display, so that rows or columns can be set whether the display is operated in the stable or in the unstable mode becomes.

There are various options for using such a display for writing and editing. On the one hand, the display can be operated completely in unstable mode during a write process. After the writing process is complete, the display is then completely or partially in the

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This is advantageous in that the entire display is deleted once, as with a conventional rewriting, without great circuitry outlay, without having to note in an additional memory which cells have previously been operated in a stable or unstable state.

The method according to the invention makes it possible to use bistable displays in a practical manner similar to conventional LCDs. If the times and frequencies with which the

Write voltage is applied to the cells in the unstable state, suitably adapted to the respective cell material and the ambient temperature, it can be assumed that there is no visibly weaker contrast and thus a loss of quality in the display.

With long-term use, such displays consume considerably less energy than conventional, reflective LCDs. Only when the display is operated in an unstable state, for example during operations such as text editions, clock updates or the like, does it use slightly more energy than a conventional LCD due to the somewhat more complicated control method.

The invention is explained in more detail below with reference to the attached figure using exemplary embodiments.

The single figure shows a schematic representation of a bistable display with an area operated in the unstable mode and an area operated in the stable mode, and with a control device according to the invention.

The structure shown in the figure is a bistable display 1, the upper area 2, consisting of several lines, is operated in the unstable mode. The entire area 3 of the display 1 underneath is in the operated stable mode. The text shown there is displayed statically. In contrast to this, the information displayed there is constantly changed in the unstable area 2. This is indicated in the figure by the display of the current time.

This display 1 is operated by means of a control device 4, which contains a display controller 5, which controls the individual cells of the display 1. This is a display controller 5, which is constructed similarly to a conventional display controller for a bistable display 1. However, an essential modification to such a conventional display controller is that the display controller 5 used for the control device according to the invention has options which it is possible to vary the write time in which a write voltage is continuously present at a specific point and the repetition frequency. This parameter controls whether a cell is operated in the unstable mode as in area 2 or in the stable mode as in area 3.

In addition, the control device 4 has a temperature measuring device 7 with which the ambient temperature of the display 1 or of the individual cells is determined. This value is used by the display controller 5 to set the optimum write time and repetition frequencies, so that the longest possible write time and low frequency are selected in the unstable mode without the respective cell being stably frozen in the second state and on the other hand during operation In the stable mode, the shortest possible write time is selected so that the respective cell is brought into the stable state safely and does not accidentally return to its initial state after the write voltage has been removed.

In addition, the control device 4 has a memory 6 in which the information shown in the display 1 is stored. be stored in parallel to prevent the information being lost during the necessary deletion process when unlocking cells or areas of cells that are in stable mode but should be operated in unstable mode. This can be the memory 6 shown in the figure. However, the memory can also be part of the display controller 5, ie it does not have to be a separate memory. This memory can also be part of a general memory of the device which has the display according to the invention. This is inexpensive insofar as a special separate memory can be dispensed with and the display controller itself only has to have a normal graphical “frame buffer” in a size corresponding to the screen.

The operating mode of the display is explained in more detail below using two example situations.

The first example is a text-only entry, for example the entry of a daily schedule, a calendar, an email, a text document or an electronic worksheet, for example for spreadsheets etc.

As a rule, it is only necessary to keep a small number of lines in an unstable mode. For example, the unstable mode could be limited to only one line. This means that if the cursor is moved to the next line after entering a line, the cells of the previous line are only continuously subjected to the write voltage for a longer period of time, as a result of which the content of this line is "frozen" on the display Modus is particularly suitable for text input devices that are supposed to have a low energy consumption. To show clocks on the display, the area can even be applied to a few cells or pi in unstable mode. xel or segments, which are responsible for the representation of the seconds, are limited. All other segments can be operated in a stable state.

To unlock the written text, a special user-controllable function is required. This function can be implemented, for example, in software within the display controller. Since the control device 4 for the display 1 - in the present exemplary embodiment via a bus 8 - is connected to the other components in the device, this function can also be implemented in these other components of the device. A CPU of the device is particularly suitable for this. If the user wants to unlock a single line in such a text-based representation, for example to make corrections there, the system can automatically recognize this by moving the cursor upward in the opposite direction to the writing direction. As soon as the cursor stops in a certain row, the respective row can then be unlocked by applying a delete pulse to the cells of this row. The information in this line, which is stored in parallel in the memory 6, is then rewritten, this rewriting initially taking place in the unstable mode.

In an alternative exemplary embodiment, as soon as the user moves the cursor upward in the opposite direction to the writing direction, the entire screen is first deleted and the writing continues in the unstable mode. As soon as it is certain that the user has selected a certain line, for example in the line in which the cursor remains while other actions, for example a text entry, are being carried out on the screen, all other lines are returned to the brought stable mode. Alternatively, only the lines above the selected line can be brought into stable mode immediately, whereby the lines below the line continue operated in unstable mode. This makes sense because it is not unlikely that the user will make a correction in the text over several lines from the relevant line. As soon as the user leaves the special line, it is automatically "frozen" again. This can be controlled, for example, by automatically detecting when the user moves the cursor down line by line.

The second exemplary embodiment is a graphic-oriented input, for example using a bistable display as a so-called “touchscreen”, in which inputs can be made simultaneously by touching the display with a finger or with special input aids. Examples of this are painting programs, navigation system maps, so-called imaging programs or use as a viewfinder for cameras etc.

In these examples, the entire screen must be operated in unstable mode until work on the image has ended. This depends on the individual user himself and he must therefore be given the opportunity to freeze the entire image content or to leave it in an unstable state. This can be done, for example, by entering an explicit lock command.

A simple display controller suitable for such an application is relatively similar to a conventional display controller for conventional LCDs, but it only has to have the additional function of stabilizing the information on the screen by applying a writing voltage over a longer writing time. This can be done, for example, by a relatively slow scan over the area to be operated in stable mode.

As in the aforementioned embodiment of text-based input, the user can unlock the display here, where either the entire display is unlocked at the request of the user or only these areas are unlocked, for example by entering or otherwise marking certain areas. When unlocking - as in previous bistable display applications - the user first sees a black bar that moves across the display, with the same image as before on the black bar. This time, however, the image is actively displayed from the memory at a high frequency so that the display does not switch to stable mode in these areas. The user then has full access to change the content of these areas again.

Since the previous image content is lost when deleted, all images should also be stored in parallel in a memory of the control device. This can be any type of memory, for example a RAM, a DRAM, a flash EEPROM or the like. act. With this graphically oriented use, it makes sense, due to the logical connection between the image content of the stored information, to place this memory within the display controller or to use the graphical “frame buffer” there.

The invention enables a significant expansion of

Application area of bistable displays. The invention can be used in particular in mobile radio devices, PDAs (Personal Digital Assistants) or organizers with large-area displays for text input, table calculations, database functions, maps for GPS navigation systems, daily plans, calendars etc. In all of the cases mentioned, there is an opportunity to continue to display all of the information while the device itself is switched off. This reduces the energy consumption to the moments when the information is updated, i.e. the device can be switched off or in an energy-saving standby mode for a considerable part of the time.

Claims

claims

1. Method for operating a bistable display (1), the individual cells of which can be brought from a first state into a second state by applying a write voltage and can be stabilized in the second state by maintaining the write voltage over a certain switching time so that the cells can also remain after the removal of the write voltage in the second state, characterized in that at least to a part of the cells in an unstable mode, only a write voltage is applied such that the respective cell is kept unstable essentially in the second state and after removing the write voltage go back to the first state.

2. The method according to claim 1, characterized in that the write voltage is maintained uninterrupted for a maximum of a period of time that is shorter than the changeover time.

3. The method according to claim 1 or 2, characterized in that the write voltage is maintained over a predetermined write time.

4. The method according to claim 1 or 2, characterized in that an ambient temperature of the cell is determined and the write voltage and / or the period over which the write voltage is maintained on the cell is selected depending on the ambient temperature.

5. The method according to any one of claims 1 to 4, characterized in that an area (2) of adjacent cells of the display (1) is operated in the unstable mode.

6. The method according to claim 5, characterized in that the area (2) comprises an entire row and / or column of the display (1).

7. The method according to any one of claims 1 to 5, characterized in that a cell which is stable in the second state and which is to be operated in the unstable mode is first applied to unlock a cell to clear the cell from the second Bring condition.

8. The method according to claim 7, characterized in that for unlocking a cell or a region (2) of cells of the display (1) first at least to all the cells of the display (1), which are stable in the second state, a deletion pulse is applied and then the relevant cells of the display (1), which are not to be operated in the unstable mode, are brought stably back into the second state.

9. The method according to any one of claims 1 to 8, characterized in that the display (1) is operated completely in an unstable mode during a writing process and the display (1) is brought into a stable mode in whole or in regions after the writing process has ended which the cells in question are stabilized in the second state.

10. The method according to any one of claims 1 to 9, characterized in that the cell or the area of cells of the display, which is to be unlocked and / or operated in unstable mode, based on a position and / or position sequence and / or movement of a display symbol is selected on the display.

11. Control device (4) for controlling a bistable display (1) with means for operating cells of the display in a stable mode in which the relevant cells are brought into a second state by applying a write voltage and stabilized in the second state by maintaining the write voltage for a certain switching time, so that the cells remain in the state even after the write voltage has been removed second state remain, characterized by

Means for alternative operation of the cells in an unstable mode in which such a cell is connected to the cell in question

Write voltage is applied that the cell is kept unstable essentially in the second state and changes back to the first state after removal of the write voltage.

12. Control device according to claim 11, characterized in that the means for alternative operation of the cells in an unstable mode comprise a device for setting parameters of the write voltage.

13. Control device according to claim 12, characterized in that the parameters comprise the length of the period over which the write voltage is maintained at the respective cell as a maximum.

14. Control device according to claim 11 or 12, characterized by a device (7) for determining an ambient temperature of the respective cell and means for determining the parameters of the write voltage as a function of the ambient temperature.

15. Control device according to one of claims 11 to 14, characterized by means for selecting whether a cell and / or a region of neighboring cells is operated in stable mode or in unstable mode.

16. Control device according to one of claims 11 to 15, characterized by means for unlocking a cell or a region of cells of the display, which is operated in the stable state, so that it is operated in the unstable mode after unlocking.

17. Control device according to one of claims 11 to 16, characterized by a memory (6) in which the information shown in the bistable display (1) is stored in parallel with the display (1).

18. Device with a bistable display and a control device according to one of claims 11 to 17.