JP5701104B2 - Driving method of bistable liquid crystal display device - Google Patents

Description

  The present invention relates to a driving method of a bistable liquid crystal display device, and more particularly, to a liquid crystal display device using chiral nematic liquid crystal driven by simple matrix using bistable switching.

  Currently, twisted nematic (TN) type liquid crystal, super twisted nematic (STN) type liquid crystal, ECB type liquid crystal that electrically controls birefringence, vertical alignment nematic (VA) type liquid crystal, and the like are used for liquid crystal display devices. These liquid crystals are classified into monostable types, and the liquid crystal molecules have a single texture when no electric field is applied to the liquid crystal layer. When an electric field is applied to the liquid crystal layer, the texture of the liquid crystal molecules is continuously deformed, and the optical characteristics change with the deformation of the texture. When the electric field disappears in the liquid crystal layer, it returns to a single texture (monostable texture) again. These liquid crystal display devices have no memory effect. Therefore, it is necessary to drive continuously by static drive, multiplex drive, or active matrix drive. On the other hand, a bistable or multi-stable liquid crystal display device has a memory effect. These liquid crystal display devices can form different textures in the cell even when no electric field is applied to the liquid crystal layer. The texture can be changed by applying a specific electric field to the liquid crystal layer. Such a display device, once the texture is determined, maintains the texture state due to its bistability even when the electric field is lost. In the bistable liquid crystal display device having such a memory effect, the image refresh rate is greatly reduced and the power consumption is reduced.

  In recent years, a bistable liquid crystal display device using a bistable liquid crystal panel in which chiral nematic liquid crystal is sealed between a pair of substrates provided with an electrode and an alignment film has been proposed (see, for example, Patent Document 1). FIG. 5 is a schematic diagram for explaining the behavior of liquid crystal molecules. As shown in the drawing, an electrode 33 is formed on each of the first substrate 31 and the second substrate 32, and a first alignment film 34 and a second alignment film 35 are further provided thereon. The liquid crystal 6 is sealed in the gap between the first substrate 31 and the second substrate 32. At the electrode interface of such a bistable liquid crystal panel, the liquid crystal molecules are oriented in a desired direction by the alignment film layer. Here, the first alignment film 34 has stronger anchoring force than the second alignment film 35. For this reason, the anchoring force of the liquid crystal molecules of the liquid crystal layer 6 is strong and has a slight angle at the interface of the first alignment film 34, and the anchoring force is weak and the molecules are almost horizontal at the interface of the second alignment film 35. At the liquid crystal interface, molecular anchoring exhibits monostability, but at the level of the liquid crystal layer 6 there are two stable states when no electric field is applied. One is a uniform or slightly twisted U texture (uniform-texture) 7, and the other is a twisted T texture (twisted-texture). The twists of the two textures differ by about 180 ° and cannot coexist individually. The spontaneous pitch p0 of the nematic liquid crystal molecules is at most about 4 times the cell thickness d so that the energy of the uniform U texture and the twisted T texture are essentially the same. Selected. Therefore, when no electric field is applied, there is no other state having a lower energy level, and the state is stable in either the U texture 7 or the T texture 8 (bistable).

Regardless of the texture, by destroying anchoring, the texture changes from one texture to the other. Conversion from U texture to T texture (or T texture to U texture) occurs by applying a strong electric field from the outside and destroying the anchoring force on the alignment film surface. When a specific electric field is applied in both the U texture and the T texture, the liquid crystal layer 6 becomes a homeotropic texture. Hereinafter, it is abbreviated as H texture. The electric field threshold value for setting the liquid crystal layer to H texture 9 is defined as Vc (anchoring breakdown threshold value). The anchoring breakdown threshold Vc is determined by the polar-angle anchoring energy Az inherently possessed by the liquid crystal material and the alignment film material. When an electric field exceeding the anchoring destruction threshold is applied, the anchoring is destroyed. In addition, this electric field promotes rearrangement of liquid crystal molecules in the vicinity of the surface of the alignment film. It is necessary to apply this electric field for a sufficient time until the H texture 9 is finally formed. The minimum time depends on the magnitude (amplitude) of the applied electric field and the physical characteristics of the liquid crystal and alignment film. When the electric field is applied for 1 to 3 milliseconds or more, the anchoring breakdown threshold Vc can be defined as the following Equation 1.
(Formula 1) Vc = d × Az / (K33 × ε0 × Δε) ^1/2
Here, d is the thickness of the liquid crystal cell, Az is the polar angle anchoring energy, K33 is the elastic torsion coefficient of the liquid crystal, Δε is the dielectric anisotropy of the liquid crystal, and ε0 is the dielectric constant of the liquid crystal in vacuum.

  When this anchoring breakdown threshold Vc is applied, the angle between the orientation and the perpendicular of the liquid crystal molecules at the alignment film interface becomes sufficiently small (for example, 0.5 ° or less), and the liquid crystal molecules at the alignment film interface The influencing stress is sufficiently low. In this state, the liquid crystal molecules in the vicinity of the surface of the alignment film are in an unstable equilibrium state when the electric field is turned off, and return to the original direction or rotate in the opposite direction and twist by 180 ° different from the original texture. Triggered on one of the new textures. Which texture ultimately results depends on the shape of the applied electrical signal, specifically how this electric field (voltage) is returned to zero.

  When the electric field is lowered step by step, the flow (hydrodynamic force of the liquid crystal molecules) is minimized, and the liquid crystal molecules near the surface of the alignment layer with strong anchoring slowly settle down to the equilibrium state. The liquid crystal molecules are rotated in the same direction by the elastic coupling. This movement spreads to the weak anchoring side substrate and rotates one after another quickly in the same direction, and a uniform state (U texture 7) is formed in the cell.

  On the other hand, when the electric field is suddenly dropped, for example, when switching is performed in a short time, a strong flow (hydrodynamic force of liquid crystal molecules) is induced near the surface of the alignment film with strong anchoring, and this is applied to the entire liquid crystal layer. Spread and instantaneously reach the surface of the alignment film with weak anchoring. On the surface of the weak anchoring alignment film, the hydrodynamic force becomes stronger and a twisted T texture 8 is induced. In this way, by controlling the electric field applied to each pixel, it is possible to change from one texture to the other.

  The electric field that changes the texture will be described with reference to FIG. FIG. 6 is a schematic diagram showing a voltage waveform applied to the liquid crystal layer. FIGS. 6A and 6B are waveforms when changing from the U texture to the T texture. In FIG. 6A, a voltage pulse P1 larger than the anchoring destruction threshold is applied in the first period τ1. The breakdown threshold voltage is about 2 to 10 volts. The period τ1, which is the pulse width of the waveform, is 2 to 3 milliseconds. In this first period τ1, the texture changes from U texture to H texture. The voltage is then quickly reduced in 2-3 microseconds, or at most 20-30 microseconds. That is, the voltage pulse P2 lower than the voltage pulse P1 by ΔV is applied in the second period τ2. By quickly reducing this voltage difference ΔV, a sufficiently strong hydrodynamic effect is induced in the liquid crystal layer, resulting in a T texture. The second period τ2 required for the T texture generation is about one-tenth of the first period τ1, even at a maximum of 500 microseconds even in the case of a long pulse, and it is necessary to reduce the voltage within this period. The potential difference ΔV is determined by the difference between the voltage pulses P1 and P2 applied during the periods τ1 and τ2. In the experiment, when the voltage pulse P1 in the first period τ1 is 24V, the voltage pulse P2 in the second period τ2 is applied to about 8V or less. Next, 0 V is applied in the third period τ3. When the continuous operation in the period τ1 to τ2 to τ3 is performed, the texture changes from the U texture to the T texture via the H texture. FIG. 6B shows a case where the potential difference between the second period τ2 and the third period τ3 is larger than the potential difference between the first period τ1 and the second period τ2. At this time, the texture changes from U texture to H texture in the first period τ1, changes from U texture to H texture in the first period τ1, and changes from H texture to T texture in the third period τ3. Specifically, the same effect can be obtained by setting P2 to about 10V or more.

  FIGS. 6C and 6D show waveforms when changing from the T texture to the U texture. Similarly to the above, in the first period τ1, a voltage pulse P1 larger than the anchoring destruction threshold is applied. In this first period τ1, the texture changes from U texture to H texture. Next, it is gently lowered in the second period τ2. FIG. 6C shows a case where the voltage is lowered in a slope shape during the second period τ2, and FIG. 6D shows a case where the voltage is dropped in a step shape. It is ideal to lower the voltage in a slope shape, but usually the voltage is lowered stepwise. Specifically, when the voltage pulse P1 applied in the first period τ1 is 16V, the voltage pulse P2 of 8V to 12V is applied in the second period τ2, and 0V is applied in the third period τ3. When the continuous operation in the period τ1 to τ2 to τ3 is performed, the texture changes from the T texture to the U texture through the T texture. Thus, which texture is settled depends on what voltage is applied in the second period τ2.

  Next, a method for driving a dot matrix type bistable liquid crystal display panel will be described. FIG. 7 shows signal waveforms applied to the bistable liquid crystal display panel. 7A shows the common signal (COM) applied to the common electrode, FIG. 7B shows the segment signal (SEG) applied to the segment electrode, and FIG. 7C shows the common-segment voltage, A waveform (COM-SEG) applied to a liquid crystal layer sandwiched between a common electrode and a segment electrode is shown. In the figure, the waveform on the left is when the pixel where the common electrode and the segment electrode intersect is a U texture, and the waveform on the right is when the pixel is a T texture. FIG. 7D schematically shows the relationship between the applied waveform and the state of the liquid crystal molecules.

  First, the left waveform will be described. When the common signal shown in FIG. 7A and the segment signal shown in FIG. 7B are applied, the voltage waveform shown in FIG. 7C is applied to the pixel (liquid crystal layer). Since + V and −V applied to the liquid crystal layer are equal to or greater than the anchoring breakdown threshold Vc, the texture is H texture during the period in which −V is applied to the liquid crystal layer, regardless of the texture of the liquid crystal layer before signal application. . A weak liquid crystal follow occurs due to the waveform that becomes 0V in the next step shape, and becomes U texture when it reaches the 0V potential. On the other hand, the waveform on the right side will be described. Whatever the texture of the liquid crystal layer before the signal is applied, it becomes the H texture during the period in which −V is applied to the liquid crystal layer, as in the case of the left waveform. In the next period, the potential quickly reaches 0 V with a certain potential difference. At this time, the texture changes from the H texture to the T texture with a large liquid crystal flow. As can be seen from the figure, the common signal is the same but the segment signal is different in both the left and right U textures and the T texture. The LCD panel displays the entire screen by scanning one line's black and white (texture state) according to the common of one line supplied with the selection signal and the signal state of all segments, and sequentially scanning all commons. (Monochrome) is determined. At the moment of scanning, only one common of the entire screen is supplied with a selection signal. The voltage waveform of the non-selection signal is supplied to the remaining majority of commons. At this time, since the waveform applied to the liquid crystal layer is not a waveform that causes anchoring destruction, the texture determined by the selection signal is maintained. This series of operations is performed by the control circuit (MPU) controlling the COM driver and the SEG driver. As described above, after all the commons are scanned and determined to be displayed on the bistable liquid crystal display panel, the display image is held even if the common and the segment are not applied. That is, even if the power is turned off after writing the image, the image of the bistable liquid crystal display panel is maintained. Therefore, power is required at the time of rewriting, but after that, display is possible even without power (see, for example, Patent Document 2).

Japanese Patent No. 3293138 Japanese Patent Laid-Open No. 2004-4552

  As described above, conventionally, after a voltage pulse larger than the anchoring breakdown threshold Vc is applied in the first period τ1, a potential for making a desired texture is applied to the segment electrode in the second period τ2, and then the third period A voltage of 0 V was applied in the period τ3. Therefore, the behavior of the liquid crystal first becomes an H texture by a potential equal to or higher than the anchoring destruction threshold Vc, and changes from this H texture to a T texture or a U texture according to the display content of the pixel. Therefore, a portion where the T and U textures are adjacent is generated. Hydrodynamic interference occurs between liquid crystal molecules adjacent to the T and U textures, and the T and U textures are mixed in one pixel. This occurs particularly frequently at the edge of the display pixel and the edge of the display screen. For this reason, display unevenness in which the display state becomes thin or dark within the surface occurs. In particular, when there are many checkerboard displays (checker patterns) or black displays that are inverted in units of one dot, display unevenness is conspicuous. Therefore, an object of the present invention is to realize a bistable liquid crystal display device in which display unevenness does not occur in a plane.

  Therefore, in the driving method of the bistable liquid crystal display device of the present invention, in order to make the liquid crystal layer of all the pixels T texture, an initialization period in which a reset pulse having a voltage equal to or higher than the anchoring breakdown value is applied, and U texture A first selection period in which a voltage equal to or higher than the anchoring breakdown value is applied to the liquid crystal layer of the pixel to be changed, or a voltage equal to or lower than the anchoring breakdown value is applied to the liquid crystal layer of the pixel that maintains the T texture; In order to prevent the liquid crystal molecules from causing a flow, a second selection period in which a voltage equal to or lower than the anchoring breakdown value is applied to the liquid crystal layer and a third selection period in which 0 V is applied to the liquid crystal layers of all pixels are sequentially provided. It was. With such a driving method, the liquid crystal layer of all the pixels is rewritten to the T texture in the initialization period, and it is determined whether to change to the U texture or to maintain the T texture in the first selection period and the second selection period. Furthermore, the U texture or the T texture is maintained by applying a voltage of 0 V in the third selection period. Therefore, when obtaining the T texture, a voltage higher than the anchoring breakdown value is not applied during the selection period from the first to the third, so that the liquid crystal does not flow.

  Further, the third selection period is longer than the sum of the first selection period and the second selection period. In addition, a period in which 0 V is applied to the liquid crystal layers of all the pixels is provided between the initialization period and the first selection period.

  Further, in order to make the waveform applied to the liquid crystal layer alternating, the first selection period and the second selection period are repeated, and then the third selection period is started. In the second selection period, a waveform in which the potential changes stepwise may be applied.

  Further, based on temperature information from the temperature sensor, at least one of a voltage value or application period applied in the first selection period, a voltage value or application period applied in the second selection period, and a third selection period is determined. It was decided.

  According to the driving method of the present invention, a liquid crystal flow does not occur even when a pixel is set to a T texture, so that a display in which the T texture and the U texture are not mixed at the edge of the display pixel or the edge of the display screen is possible. Therefore, uniform display without unevenness is possible. In addition, even in matrix driving, display rewriting can be performed without interfering between pixels. Furthermore, it has become possible to display the edges of display pixels clearly even in icon, character display and segment display.

FIG. 4 is a schematic diagram illustrating a driving waveform of Example 1. FIG. 6 is a schematic diagram illustrating a driving waveform of Example 2. FIG. 10 is a schematic diagram illustrating a driving waveform of Example 3. It is a circuit block diagram of a bistable liquid crystal display device. It is a schematic diagram explaining the texture state of a liquid crystal layer. It is a schematic diagram explaining the conventional applied voltage. It is a schematic diagram explaining the conventional drive waveform.

  In the driving method of the bistable liquid crystal display device of the present invention, first, an initialization reset pulse is applied to all display pixels to make the liquid crystal layer have a T texture. Next, a voltage pulse higher than the anchoring destruction threshold is applied to the pixel to be changed to U texture (this voltage application period is set as the first selection period), and is lower than the anchoring destruction threshold Vc before dropping to 0V. A voltage is applied (this voltage application period is a second selection period). When such a waveform is applied to the liquid crystal layer, the texture changes from T texture to H texture in the first selection period, and changes to U texture when the second selection period ends. Immediately after the end of the second selection period, a voltage of 0 V is applied for a predetermined period (this 0 V application period is set as the third selection period), and the U texture is maintained. Thus, the texture state of the liquid crystal can be changed in the first to third selection periods. On the other hand, a voltage lower than the anchoring destruction threshold Vc is applied to the pixel for which the T texture is to be maintained during the first selection period. Further, a voltage lower than the anchoring breakdown threshold Vc is also applied in the second selection period. When such a waveform is applied to the liquid crystal layer, the T texture formed by the reset pulse does not change to another texture in the first selection period or the second selection period, and the T texture is maintained. Further, a line signal pause period (third selection period) is provided immediately after the second selection period, and a voltage of 0 V is applied during the line signal pause period. This maintains the T texture.

  With such a driving method, it is possible to mix U texture pixels and T texture pixels in the display screen. At this time, the T texture state is changed from the H texture to the T texture. Therefore, the T texture can be obtained without causing the liquid crystal molecules to flow. Therefore, the U texture is not affected by the flow.

In addition, the common lines are selected and driven line by line. It is necessary to always apply a voltage lower than the anchoring breakdown threshold Vc to the unselected common line.
The line signal pause period may be set to be longer than the sum of the first selection period and the second selection period.

  Next, a method for driving the bistable liquid crystal panel of this embodiment will be described. FIG. 1 shows driving waveforms applied to a dot matrix liquid crystal panel using bistable chiral nematic liquid crystal. Fig. 1 (a) shows the signal applied to the segment line (SEG1), Fig. 1 (b) shows the signal applied to the common line (COM1), and Fig. 1 (c) shows the signal applied to the intersection of COM1 and SEG1. 1 (d) shows a signal applied to COM2, and FIG. 1 (e) shows a waveform applied to the intersection of COM2 and SEG1.

  In this embodiment, the pixel to which the waveform shown in FIG. 1C is applied is a T texture, and the pixel to which the waveform shown in FIG. 1E is applied is a U texture. In either case, the reset pulse 1 is first applied to change the entire surface of the liquid crystal panel to T texture. In actual operation, the segment reset signal S1 shown in FIG. 1A is supplied to the segment line, and the common reset signal C1 shown in FIG. 1B is supplied to the common line. For example, V0 = 30 and V5 = GND (0V). A waveform obtained by synthesizing these signals becomes a reset waveform 1 having an amplitude twice as large as V0 with 0V as the center. With the reset waveform 1, a rectangular wave with a negative level of −4 is applied after a rectangular wave with a positive level of +4. Therefore, no direct current is applied to the liquid crystal, and the charge component does not stay in the liquid crystal layer. Here, the anchoring breakdown value + vc is between the potential level +4 and the potential level +3.

  Next, in the time interval D after the application of the reset pulse 1, both the common signal and the segment signal are changed to V5. Therefore, 0V is applied to the pixel as shown in FIG. This is a pause period waveform 3 applied during the line signal pause period. This rest period waveform 3 is necessary to stabilize this state after the entire surface is changed to the T texture.

  Hereinafter, waveforms applied to the pixels that maintain the T texture will be described with reference to FIGS. A specific example of a signal supplied to the SEG 1 will be described with reference to FIG. After the segment reset signal S1, the S2 signal and the S4 signal are alternately supplied. The S2 signal cooperates with the selected common signal to maintain the T texture, and the S4 signal cooperates with the selected common signal to change to the U texture. The S2 signal corresponds to the time intervals E and F, the voltage value is V34, the S4 signal corresponds to the time intervals H and I, and the voltage value is V0. The voltage value of the time intervals G and J following the S2 signal and the S4 signal is V5. A specific example of a signal supplied to the COM 1 will be described with reference to FIG. The C2 signal is supplied after the common reset signal C1, and then the C4 signal is supplied. A voltage value V5 is taken between the signals and corresponds to a pause period waveform. Since the common line is driven line-sequentially, the C2 signal is supplied during the selection period of the common line. That is, the time interval E, F is a selection period for COM1, and after this time, the C4 signal is supplied until all the common lines have been written with images. Therefore, the waveform shown in FIG. 1C is applied to the liquid crystal layer of the pixel where COM1 and SEG1 intersect. In order to maintain the T texture, the waveform shown in FIG. 1C is configured with a potential equal to or lower than the anchoring breakdown voltage Vc after the reset pulse 1. In the time intervals E and F, the first selection waveform 2 shown in FIG. 1C is applied to the liquid crystal layer of this pixel. Here, in the first selection waveform 2, the voltage level of the first selection period 2 a is “−2”, and the voltage level of the second selection period 2 b is “+1”. The first selection waveform 2 must be configured with a voltage level that is equal to or lower than the anchoring breakdown threshold Vc. Specifically, a voltage of about -4V and a waveform width of about 1 millisecond are set in the first selection period 2a, and a voltage of about +2 to + 4V and a waveform width of about 50 to 500 microseconds are set in the second selection period 2b. did.

  Since the optimum applied voltage and time vary depending on the environmental temperature, it is set in consideration of this. Then, the line signal pause period immediately follows the second selection period 2b. At this time interval G, a voltage of level 0V, that is, the pause period waveform 3 is obtained. When such a first selection waveform 2 and a rest period waveform 3 are applied to the liquid crystal layer, the T texture is maintained and no liquid crystal flow occurs. At this time, the time during which the rest period waveform 3 is applied last is longer than the sum of the first selection period 2a and the second selection period 2b. Specifically, it is set to 2 milliseconds or more.

  After the line signal pause period indicated by the time interval G in FIG. 1, the C4 signal (unselected common waveform) shown in FIG. 1B and the liquid crystal layer of this pixel are shown in FIG. 1B. The combined waveform of the S4 signal or S2 signal is applied alternately. That is, the non-selection period waveform 4 shown in FIG. 1C is continuously applied until image data is written to all the common lines. Here, the non-selection period waveform 4 is a rectangular wave having a voltage level of −2 or +2. A voltage of 0 V, that is, a rest period waveform 3 is applied between the non-selection period waveforms 4 of different voltage levels. In this way, after making the entire surface T textured with the initial reset pulse 1, the rest period waveform 3, the first selection waveform 2, the rest period waveform 3, and then the non-selection period waveform 4 and the rest period waveform 3 are the remaining lines. By repeating several times, the T texture is maintained.

  Hereinafter, the waveform applied to the pixel to be converted from the T texture to the U texture will be described. Here, it is assumed that the liquid crystal layer of the pixel where COM2 and SEG1 intersect is converted into a U texture. FIG. 1D shows a signal supplied to COM2, and FIG. 1E shows a waveform applied to the liquid crystal layer of the pixel where COM2 and SEG1 intersect. As shown in FIG. 1D, the COM4 is supplied with the C4 signal after the common reset signal C1, and then the C2 signal is supplied to the COM2. Since the common line is driven line-sequentially, the selection periods of COM1 and COM2 are shifted. That is, in COM2, the time intervals H and I are the selection period, and after this time, the C4 signal is supplied until the writing of images to all the common lines is completed.

  The state of the liquid crystal layer of the pixel where COM2 and SEG1 intersect will be described with reference to FIG. First, when the reset pulse 1 is applied at the time intervals B and C, the liquid crystal layer has a T texture. Then, the second selection waveform 5 is applied at the time intervals H and I which are the selection period of COM2. The second selection waveform 5 needs to include a voltage level that is equal to or higher than the anchoring breakdown threshold Vc. The second selection waveform 5 is a combined waveform of the segment signal S4 shown in FIG. 1 (a) and the common selection signal C2 shown in FIG. 1 (d). The second selection waveform 5 has a voltage level of “−4” in the first selection period 5a and is a voltage level equal to or higher than the anchoring destruction threshold Vc. The voltage level in the second selection period 5b is “−3”, which is a voltage level equal to or lower than the anchoring breakdown threshold Vc. Immediately after the second selection period 5b is a line signal pause period, and during this time interval J, a voltage of level 0 V (that is, the pause period waveform 3) is applied. When such a second selection waveform 5 is applied to the liquid crystal layer of the T texture, the anchoring is destroyed in the first selection period 5a and the H texture is changed. Then, when the second selection period 5b ends, the texture changes to a U texture. This is because both the difference in voltage applied in the first selection period 5a and the second selection period 5b and the difference in voltage applied in the second selection period 5b and the line signal pause period are from the anchoring breakdown threshold Vc. Because it is low. As described above, the liquid crystal layer of the T texture is changed to the U texture through the H texture by the second selection waveform 5. The U texture is maintained by the rest period waveform 3. Further, after that, the non-selection period waveform 4 and the rest period waveform 3 are applied, but the U texture is maintained because a voltage equal to or lower than the anchoring destruction threshold value Vc is set.

  Specifically, the voltage applied in the first selection period 5a is set to about -13V to -20V, and the application time is set to about 1 millisecond. In the second selection period 5b, a voltage of about −8 to −13 V and a time of about 50 to 500 microseconds are set. The line signal pause period G is set to be longer than the sum of the first selection period 5a and the second selection period 5b, specifically, 2 milliseconds or more. The time intervals E and H, F and I, and G and J have the same time width. After the time interval J, the non-selection period waveform 4 and the rest period waveform 3 are repeated for the remaining number of lines. Further, as shown in FIGS. 1C and 1E, the non-selection period waveform 4 appears alternately on the + side and the − side, so that the liquid crystal layer is not affected by the DC component.

  Here, the configuration of the bistable liquid crystal panel will be described. In the bistable liquid crystal panel, polarizing plates having an absorption axis at an angle of about 45 degrees from the orientation angle are arranged in parallel (Nicol) on both sides of the liquid crystal layer. When the liquid crystal layer has a T texture, the transmittance is high, and white display is obtained when observed in the reflection mode. This is because the T texture does not generate much phase difference in linearly polarized light and passes through the polarizing plate as it is. On the other hand, when the liquid crystal layer has a U texture, the liquid crystal molecules are aligned at almost 0 degrees and a phase difference of Δnd occurs. For example, when the product of the refractive index Δn of the liquid crystal and the liquid crystal layer thickness d is set to the wavelength λ / 2, the linearly polarized light has a phase difference of 180 degrees and the direction is rotated by 90 degrees. Therefore, when the parallel Nicols are arranged, the transmittance is low, so that black is displayed in the reflection mode. That is, display is performed using a mode in which white is used for the T texture and black is used for the U texture. When the polarizing plates are arranged in crossed Nicols, the T texture is displayed in black and the U texture is displayed in white, which can be reversed compared to parallel Nicols. Hereinafter, the case where the polarizing plates are arranged in parallel Nicols will be described.

  FIG. 3 schematically shows a circuit configuration of the bistable liquid crystal display device. The bistable liquid crystal panel 30 includes a common driver 41 that drives a horizontal common line, a segment driver 42 that drives a vertical segment line, and a power supply circuit 43 that generates drive potentials (V0, V12, V34, V5, and VCX). They are driven by a control circuit (MPU) 44 that controls the common driver 41, the segment driver 42, and the power supply circuit 43. Control signals supplied from the control circuit 44 to the common driver 41 and the segment driver 42 are the same as those in a normal STN drive circuit. That is, the initialization signal (RESETX), the signal for determining the scan timing (C-data), the write clock (CL), the alternating signal (FRCOM) and the display erase signal (DispOffx) are input to the common driver 41. The segment driver 42 receives an initialization signal (RESETX), a display image data signal (S-data), a write clock (XCK), an alternating signal (FRSEG), and a display erase signal (DispOffx). A temperature sensor 45 is connected to a control circuit (MPU) 44. It is possible to incorporate the power supply circuit 43 into the common driver 41 and further incorporate the segment driver 42 into one IC. First, the MPU 44 reads the environmental temperature value from the temperature sensor 45, acquires each voltage information from the temperature table stored in the MPU, sets the value according to the table to the power source 43, and then sets each potential (V0, V12, A sequence for outputting Vcx, V34, V5) is performed. At the same time, the width information of each time interval is acquired from the temperature table, and the pulse width is determined for each sequential output. Based on these pieces of information, the liquid crystal panel is driven with a pulse width suitable for the temperature.

  As described above, when a U texture (black display) is desired, a ± 4 level higher than the anchoring destruction threshold Vc is applied during the first selection period, and when a T texture (white display) is desired to be maintained during the first selection period. The ± 2 level below the anchoring destruction threshold Vc is applied. In either case, a voltage level equal to or lower than the anchoring breakdown threshold Vc is applied in the second selection period. In the third selection period immediately after the second selection period, a voltage level of 0 V (that is, the rest period waveform 3) is applied. Furthermore, even during the non-selection period, a voltage level (± 2 level) that is equal to or lower than the anchoring breakdown threshold Vc is applied across the line signal pause period. As described above, a pixel to which a voltage level equal to or higher than the anchoring destruction threshold Vc is applied in the first selection period is changed from the T texture (white display) to the U texture (black display). A pixel to which a voltage level equal to or lower than Vc is applied continues to maintain the T texture (white display). The waveform information is obtained by reading environmental temperature data with a temperature sensor in the same manner as the reset pulse, and acquiring voltage and width information of the waveform corresponding to the temperature from a preset temperature table and reflecting it in the waveform. Since this driving does not obtain the T texture by changing from the H texture, the liquid crystal flow does not occur. Therefore, interference between pixels does not occur. Therefore, rewriting is performed smoothly and display interference does not occur between pixels. Since the waveform applied during the non-selection period does not reach the anchoring destruction threshold, the texture does not change. The bistable liquid crystal panel 30 determines the display of the entire screen by sequentially scanning the common for one screen. After the image is written on the bistable liquid crystal panel 30 as described above, the written display image is held even if the common voltage and the segment voltage are set to GND and the bistable liquid crystal panel 30 is not applied. That is, after the image is written, the display image on the bistable liquid crystal panel 30 is maintained even if the power is turned off. Therefore, power is required at the time of writing, but display is possible without power thereafter.

  Next, the driving method of this embodiment will be described with reference to FIG. FIG. 2 is a schematic diagram showing a driving waveform applied to a dot matrix liquid crystal panel using a bistable chiral nematic liquid crystal. The details of the first selection period and the second selection period constituting the first selection waveform and the second selection waveform are different from those of the first embodiment. That is, in this embodiment, the first selection period 12a and the second selection period 12b appear twice in one selection waveform. Others are the same as those in the first embodiment, and therefore, redundant description is omitted as appropriate.

  2 (a) shows the signal applied to the segment line (SEG1), FIG. 2 (b) shows the signal applied to the common line (COM1), and FIG. 2 (c) shows the signal applied to the intersection of COM1 and SEG1. 2 (d) shows a signal applied to COM2, and FIG. 2 (e) shows a waveform applied to the intersection of COM2 and SEG1. Also in this embodiment, a pixel to which the waveform shown in FIG. 2C is applied becomes a T texture, and a pixel to which the waveform shown in FIG. 2E is applied becomes a U texture.

  First, a waveform applied to a pixel (intersection of COM1 and SEG1) that maintains the T texture will be described with reference to FIG. First, a reset pulse 1 that is a composite wave of the segment reset signal S1 and the common reset signal C1 is applied to the pixel. Since the reset pulse 1 has a voltage equal to or higher than the anchoring breakdown value Vc, the liquid crystal layer has a T texture. A line signal pause period is provided immediately after the initialization period in which the reset pulse 1 is applied. A pause period waveform 3 (a potential of 0 V) is applied during the line signal pause period. This rest period waveform 3 is necessary to stabilize the entire surface after changing to the T texture.

  Next, a first selection waveform 12 that is a combined wave of the S12 signal and the C12 signal is applied to the pixel. The C12 signal is a signal applied during the selection period of the common line. In order to maintain the T texture, the first selection waveform 12 is configured with a potential equal to or lower than the anchoring breakdown voltage Vc. In this embodiment, the waveform is level +2 at time interval E, level -1 at time interval F, level -2 at time interval G, and level +1 at time interval H. A line signal pause period is provided after the first selection waveform 12, and a voltage of 0 V (pause period waveform 3) is applied during this period. Thus, the potential level below the anchoring destruction threshold Vc must be set within the time interval E to H. The T texture is maintained by such a waveform. Therefore, no flow occurs in the liquid crystal layer. Specifically, in the first selection period 12a, a voltage of about ± 4V is set to about 1 millisecond, and the potential level ± 1 of the second selection period 12b is from about ± 2 to ± 4V and 50. A waveform width of about 500 microseconds is set. Here, the first selection period 12a is the time intervals E and G, and the second selection period 12b is the time intervals F and H. Therefore, in the first selection waveform 12, it can be considered that the set of the first selection period 12a and the second selection period 12b is repeated twice. Here, the line signal pause period is longer than the time between the time intervals E to H, specifically, preferably 2 milliseconds or longer.

  In order to obtain such a first selection waveform 12, signals supplied to SEG1 and COM1 will be described below. As shown in FIG. 2A, the segment voltage waveform of the S12 signal is a waveform having V12 at the first time intervals E and F and V34 at the time intervals G and H. In the time interval I immediately after the S12 signal, V5 is applied. The time intervals E and F and the time intervals G and H are inverted by the FRSEG signal. Also, as shown in FIG. 2B, the common voltage waveform of the C12 signal is a waveform that becomes V0 at the first time interval E, Vcx at the time interval F, V5 at the time interval G, and Vcx at the time interval H. . In the time interval I immediately after the C12 signal, V5 is applied. The time intervals E and F and the time intervals G and H are inverted by the FRCOM signal. Here, the anchoring breakdown value vc is between the potential levels ± 4 and ± 3 and between V12 and V34. For example, V0 = 30V and V5 = GND (0V).

  Next, a waveform applied to a pixel (intersection of COM2 and SEG1) that is converted into a U texture and maintained will be described with reference to FIG. The waveform shown here is a composite waveform of the SEG1 signal in FIG. 2A and the COM2 signal in FIG. As in FIG. 2C, first, a reset pulse 1 that is a combined wave of the segment reset signal S1 and the common reset signal C1 is applied to the pixel. Since the reset pulse 1 has a voltage equal to or higher than the anchoring breakdown value Vc, the liquid crystal layer has a T texture. Immediately after the reset pulse 1 is applied, the rest period waveform 3 is applied. The rest period waveform 3 in this time interval D is necessary to stabilize all pixels after changing to T texture. In COM2, the non-selection period waveform 14 is applied because it is a non-selection period immediately after the line signal pause period (corresponding to the time interval D).

  The non-selection period waveform 14 must be composed of a voltage equal to or lower than the anchoring breakdown threshold Vc. Then, a second selection waveform 15 that is a combined wave of the S14 signal and the C12 signal is applied to the pixel. The C12 signal is a signal applied during the selection period of the common line. Similarly to the first selection waveform 12, the second selection waveform 15 can be regarded as a set of the first selection period 15a and the second selection period 15b being repeated twice. In the first selection period 15a, a voltage equal to or higher than the anchoring breakdown threshold Vc is applied. In the first selection period 15a, the liquid crystal layer changes from T texture to H texture. Then, after applying a voltage equal to or lower than the anchoring breakdown threshold Vc in the second selection period 15b, the rest period waveform 3 which is a voltage of 0 V is applied. Thereby, the liquid crystal layer is changed from the H texture to the U texture, and the U texture is maintained. By such driving, a U texture can be obtained without generating a liquid crystal flow.

  In the present embodiment, the non-selection period waveform 14 has a potential level of −2 at time intervals E and F, and a potential level of +2 at time intervals G and H. Specifically, a voltage of about ± 4V to ± 8V is set. The second selection waveform 15 has a potential level +4 at time interval J (first selection period 15a), a potential level +3 at time interval K (second selection period 15b), and a subsequent time interval L (first selection period 15a). The potential level is -4, and the time interval M (second selection period 15b) is the potential level -3. Specifically, the potential level ± 4 in the first selection period 15a is set to about ± 13V to ± 20V and about 1 millisecond, and the potential level ± 3 in the second selection period 15b is about ± 8V to ± 13V. It was set to about 50 to 500 microseconds. Time data acquired from the temperature table is used in the first selection period and the second selection period. Further, the line signal pause period is longer than the time from the time intervals E to H, specifically, preferably 2 milliseconds or longer. Further, the width of the non-selection period and the line pause period immediately thereafter is the same as that of the selection period and the line pause period immediately thereafter. The time intervals E and J, F and K, G and L, H and M, and I and N have the same time width.

  As apparent from the above-described waveform, when changing to the U texture, a potential level ± 4 equal to or higher than the anchoring destruction threshold Vc is applied during the first selection period, and when maintaining the T texture, the anchor is selected during the first selection period. A potential level of ± 2 below the ring breakdown threshold Vc is applied. In any texture, a potential level ± 2 that is equal to or lower than the anchoring destruction threshold Vc is applied in the second selection period, and a rest period waveform is applied in the subsequent line signal rest period. With this waveform, T texture and U texture can exist in the screen without causing liquid crystal flow.

  The first selection periods 12a and 15a and the second selection periods 12b and 15b are repeated with the polarity reversed. Further, the non-selection period waveform 14 also appears alternately on the + side and the − side as in the first embodiment. In this way, since the waveform applied to the liquid crystal layer is AC, the liquid crystal layer is not affected by the DC component.

  Next, the driving method of this embodiment will be described with reference to FIG. FIG. 3 is a schematic diagram showing drive waveforms applied to the dot matrix liquid crystal panel. The second embodiment is different from the second embodiment in that the second selection period of the first selection waveform and the second selection waveform is subdivided. The other parts are the same as those in the second or first embodiment, and a duplicate description will be omitted as appropriate.

  3A shows a signal applied to the segment line (SEG1), FIG. 3B shows a signal applied to the common line (COM1), and FIG. 3C shows a signal applied to the intersection of COM1 and SEG1. 3D shows a signal applied to COM2, and FIG. 3E shows a waveform applied to the intersection of COM2 and SEG1. Also in this embodiment, a pixel to which the waveform shown in FIG. 3C is applied becomes a T texture, and a pixel to which the waveform shown in FIG. 3E is applied becomes a U texture. In any case, first, a reset pulse 1 which is a composite wave of the segment reset signal S1 and the common reset signal C1 is applied to the pixel. Since the reset pulse 1 has a voltage equal to or higher than the anchoring breakdown value Vc, the liquid crystal layer has a T texture. A line signal pause period is provided immediately after the initialization period in which the reset pulse 1 is applied. A pause period waveform 3 (a potential of 0 V) is applied during the line signal pause period. This rest period waveform 3 stabilizes the T texture changed by the reset pulse 1. Immediately after this, COM1 becomes a selection period, and COM2 becomes a non-selection period.

  During the selection period, the first selection waveform 22 shown in FIG. 3C is applied to COM1. The first selection waveform 22 is a composite wave of the S22 signal and the C22 signal. The C22 signal is a signal supplied during the selection period of the common line. Since the first selection waveform 22 is configured with a potential equal to or lower than the anchoring breakdown voltage Vc, the T texture is maintained. Similarly to the second embodiment, in the first selection waveform 22, it can be considered that the set of the first selection period 22a and the second selection period 22b is repeated twice. Although the first selection period 22a is composed of one potential (one pulse), in the present embodiment, the second selection period 22b is composed of three potentials (three periods). A specific potential is shown in FIG. After the first selection waveform 22, the non-selection period waveform 24 is applied through the rest period waveform 3. Since the non-selection period waveform 24 is also configured with a potential equal to or lower than the anchoring breakdown voltage Vc, the T texture is continuously maintained.

  As illustrated in FIG. 3E, the non-selection period waveform 24 is applied to the COM 2 before the second selection waveform 25. The non-selection period waveform 24 is a composite wave of the S22 signal and the C24 signal. Since the non-selection period waveform 24 is composed of a potential equal to or lower than the anchoring breakdown voltage Vc, the T texture is maintained. The second selection waveform 25 is a composite wave of the S24 signal and the C22 signal. Similar to the second embodiment, in the second selection waveform 25, it can be considered that the set of the first selection period 25a and the second selection period 25b is repeated twice. Although the first selection period 25a is composed of one potential (one pulse), in the present embodiment, the second selection period 25b is composed of three potentials (three periods). The second selection waveform 25 applies a voltage equal to or higher than the anchoring breakdown voltage Vc in the first selection period 25a. Therefore, the texture changes from T texture to H texture. Next, a line signal pause period is reached after a second selection period 25b in which a potential that decreases stepwise is applied. That is, the first selection period 25a is driven so that the potential gradually decreases to 0V. Therefore, the liquid crystal layer changes from H texture to U texture. A specific potential is shown in FIG.

  As shown in FIG. 3, when changing to U texture, a voltage level ± 5 that is equal to or higher than the anchoring destruction threshold is applied in the first selection period 25a. When maintaining the T texture, a voltage level ± 2 level below the anchoring destruction threshold is applied in the first selection period 25a. In any case, in the second selection period 25b following the first selection period 25a, a voltage level equal to or lower than the anchoring destruction threshold is applied so that the texture does not change. A line signal suspension period is provided after the second selection period 25b.

  If the voltage is lowered stepwise in the second selection period 25b as in this embodiment, the T texture is likely to change to the U texture. The voltage drop may be performed in a slope shape.

  The present invention is not limited to the above-described embodiments, and various modifications and improvements can be made without departing from the scope of the present invention. The same effect can be obtained with waveforms other than those shown in the embodiments. For example, by controlling the pulse width, the same effect can be obtained even if the waveform does not decrease the voltage stepwise. This is closely related to the anchoring breakdown voltage value and the pulse width. In particular, in the case of a low temperature, the voltage can be considerably lowered by increasing the pulse width. In this case, the same effect can be obtained without using a high voltage driver.

  When driving as in the above-described embodiments, liquid crystal follow-up does not occur, so that it becomes easy to form a U texture, and the yield of the LCD can be improved. In matrix driving, display can be rewritten without interfering with pixels. In addition, the icon, character display and segment display (number 8 and alphabetic alphanumeric characters) can be clearly displayed up to the edge of the display pixel.

1 Reset pulse 2, 12, 22 First selection waveform 3 Rest period waveform 4, 14, 24 Non-selection period waveform 5, 15, 25 Second selection waveform 6 Liquid crystal layer 7 U texture 8 T texture 9 H texture 30 Bistable liquid crystal Panel 31 First substrate 32 Second substrate 33 Electrode 34 Strong anchoring alignment film 35 Weak anchoring alignment film 41 Common driver 42 Segment driver 43 Power supply circuit 44 Control unit 45 Temperature sensor 2a, 5a First selection period 2b, 5b Second selection period

Claims (7)

An initialization period in which a reset pulse having a voltage equal to or higher than the anchoring breakdown value is applied to make the liquid crystal layer of all pixels T texture,
A first selection period in which a voltage equal to or higher than the anchoring breakdown value is applied to the liquid crystal layer of the pixel to be changed to the U texture, and a voltage equal to or lower than the anchoring breakdown value is applied to the liquid crystal layer of the pixel maintaining the T texture;
A second selection period in which a voltage equal to or lower than the anchoring breakdown value is applied to the liquid crystal layer so as not to cause a flow of liquid crystal molecules in the liquid crystal layer;
A driving method for a bistable liquid crystal display device, comprising: a third selection period in which 0 V is applied to the liquid crystal layers of all the pixels in order.   The method of driving a bistable liquid crystal display device according to claim 1, wherein the third selection period is longer than the sum of the first selection period and the second selection period.   3. The driving method of a bistable liquid crystal display device according to claim 1, further comprising a period in which 0 V is applied to the liquid crystal layers of all the pixels between the initialization period and the first selection period.   4. The method according to claim 1, wherein the first selection period and the second selection period are repeated before the waveform applied to the liquid crystal layer is changed to the third selection period. A driving method of the bistable liquid crystal display device according to the item.   5. The driving method of a bistable liquid crystal display device according to claim 1, wherein a waveform whose potential changes stepwise is applied in the second selection period.   The bistable according to any one of claims 1 to 5, wherein a voltage value or an application period applied in the first selection period or the second selection period is determined based on temperature information. Type liquid crystal display device driving method.   The method of driving a bistable liquid crystal display device according to claim 6, wherein the third selection period is determined based on the temperature information.

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