JP2575196B2 - Driving method of display device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device using a ferroelectric liquid crystal, and more particularly to a display device suitable for gradation display in which flicker is not noticeable.

[Conventional technology]

2. Description of the Related Art Conventionally, a liquid crystal display element in which a scanning electrode group and a signal electrode group are configured in a matrix, a liquid crystal compound is filled between the electrodes to form a large number of pixels, and an image or information is displayed is well known. . As a driving method of the display element, a time division drive in which an address signal is sequentially and selectively applied to the scanning electrode group and a predetermined information signal is selectively applied to the signal electrode group in parallel in synchronization with the address signal is used. Has been adopted.

Most of these have been put to practical use, for example, in "Applied Physics Letters".
Letters ", 1971, 18 (4), pages 127-128, co-authored by M. Schadt and W. Helfrich," Voltage Day Pendant Optical.
"Activity of a Twisted Nematic Liquid Crystal" (“Voltage Dependent Op
tical Activity of a Twisted Nematic Liquid Crysta
l ") was a TN (Twisted Nematic) type liquid crystal.

Recently, the use of a liquid crystal device having bistability as an improved type of a conventional liquid crystal device has been disclosed by both Clark and Lagerwall in JP-A-56-107216 and U.S. Pat. It has been proposed in written documents. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or an H phase (SmH ^* ) is generally used. In these states, the first liquid crystal responds to an applied electric field. Has the property of taking one of the optically stable state and the second optically stable state and maintaining the state when no electric field is applied, that is, it has bistability, and has a quick response to a change in the electric field. Thus, wide use in fields such as high-speed and storage type display devices is expected.

However, the above-described ferroelectric liquid crystal element has a problem that flicker occurs during multiplexing driving. In particular, according to European Patent Publication No. 149899, an AC voltage in which the phase of the scan selection signal is reversed for each writing frame is applied, and white (cross nicol is turned to a bright state in a certain frame) as shown in FIG. A multiplexing driving method is disclosed in which selective writing is performed in an arrangement) and black is selected (arranged so that the crossed Nicols are in a dark state) in a subsequent frame. Also, in addition to the driving method described above,
The driving methods disclosed in U.S. Pat. No. 4,548,476 and U.S. Pat. No. 4,655,561 are known.

In this driving method, at the time of black selective writing after white selective writing, white pixels selectively written in the previous frame are half-selected, and an effective voltage smaller than the writing voltage is applied. Therefore, in this multiplexing driving method, at the time of selective writing of black, the half-selection voltage is uniformly applied to the selected pixel with white, which is the background of the black character, for a half frame period (one screen scanning time, which is one frame scanning time). The optical characteristics of a white selected pixel to which a half-selection voltage has been applied and a half-selection voltage has been applied change every half-frame period. For this reason, in the case of a display in which black characters are written on a white background, the number of pixels that have selected white is overwhelmingly greater than the pixels that have selected black, and the white background appears to flicker. Also, in contrast to the above-described display in which black characters are written on a white background, the occurrence of flicker is also observed in the case of a black character and a white character display. When the normal frame frequency is set to 30 Hz, the above-mentioned half-selection voltage is applied at 15 Hz, which is a half frame frequency, so that it is perceived as flicker by an observer, and display quality is significantly impaired.

In particular, the ferroelectric liquid crystal requires a longer drive pulse (scan selection period) in low-temperature driving compared to, for example, high-temperature scanning driving at a 15 Hz frame frequency. Scan driving at a low frame frequency was required. Therefore, when driving at low temperatures,
Flicker caused by scanning drive at a low frame frequency has occurred.

[Problems to be Solved by the Invention] Flicker can be eliminated to some extent by configuring the scanning period of one screen by three or more vertical scanning periods and increasing the frequency of one vertical scanning. It is difficult to eliminate flickering and image flow (which can be regarded as another type of flicker) caused by the fact that adjacent scanning lines are always selected during the period. The purpose of the present invention is
It is an object of the present invention to provide a display device driving method capable of preventing flicker due to low frame frequency scanning and also preventing the above-mentioned flicker.

[Means for Solving the Problems] Means for solving the above-mentioned technical problems and achieving the above object is a driving method of a display device having a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes. ,
A screen scanning period for displaying one screen is composed of a plurality of vertical scanning periods for selecting at least every third scanning electrode, and at least one set of two continuous vertical scanning periods is selected from the plurality of vertical scanning periods. In the scanning period, two non-adjacent scan electrodes are sequentially selected, and the selected scan electrodes are
After supplying a first voltage for erasing a pixel on the selected scan electrode, supplying a second voltage for inverting or holding the erased pixel in cooperation with an information signal. This is a driving method of the display device, which is a feature.

Further, it has a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes, and in a method of driving a display device in which information electrodes having different electrode widths are opposed to one scanning electrode, A screen scanning period for displaying one screen is composed of a plurality of vertical scanning periods for selecting at least every third scanning electrode, and at least one set of two continuous vertical scanning periods is selected from the plurality of vertical scanning periods. In the scanning period, two scanning electrodes that are not adjacent to each other are sequentially selected,
After a first voltage for erasing a pixel on the selected scan electrode is supplied to the selected scan electrode, a second voltage for inverting or holding the erased pixel in cooperation with the information signal is supplied. A method for driving a display device, characterized by supplying the following voltages.

Further, in a driving method of a display device having a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes, a plurality of screen scanning periods for displaying one screen are scanned at least every four lines. A plurality of vertical scanning periods for selecting electrodes, and among all of the plurality of vertical scanning periods, two consecutive non-adjacent scanning electrodes are sequentially selected in all two consecutive vertical scanning periods.
After supplying a first voltage for erasing a pixel on the selected scan electrode, supplying a second voltage for inverting or holding the erased pixel in cooperation with an information signal. This is a driving method of the display device, which is a feature.

(Detailed description of embodiments of the invention)

An embodiment of the present invention will be described using a ferroelectric liquid crystal (hereinafter, FLC).

FIG. 1 shows a first embodiment of the present invention (FIG. 2 shows A in FIG. 1).
-A 'sectional view), the upper electrode groups 11A and 11B (hereinafter referred to as information electrode group) and the lower electrode group 12 (hereinafter referred to as scan electrode group C) are configured so as to be in matrix with each other. And a structure in which the FLC material 15 is sandwiched between them. Also, as shown in FIG.
Are C ₀ , C ₁ , C ₂ …, and the information electrode groups are A (A ₁ , A ₂ , A ₃ …) and B
(B ₁ , B ₂ , B ₃ , B ₄ ...), And a region E (electrode line width A> B) surrounded by a dotted line in the drawing of one pixel, that is, for example, a scanning electrode C ₂ and an information electrode A composed of regions E ₂ and B ₂ are Obaratsupu. At this time, the electrode line width is A> B. Each of the scanning electrode groups C and the information electrode groups A and B are connected to a power supply unit (not shown) via a switch.
It is connected to a controller circuit (not shown) that controls its ON / OFF. With this configuration, under the control of the controller circuit, for example, the gray scale expression at the pixel E is performed as follows. When the common electrode C ₂ is being scanned, white (hereinafter W) is A _2, when each of the B ₂ was applied a signal composed is W, black gray 1 (hereinafter Gray1) to A ₂ W, the B _2,
(Hereinafter referred to as B), the gray 2 (hereinafter referred to as Gray)
2) When B signal is applied to A ₂ and W signal is applied to B ₂ , Black is when B signal is applied to A ₂ and B ₂ respectively, and FIG. 3 shows the above W, Gray 1, Gray ₂ and B signals. Is shown in the halftone expression.

With such a simple configuration, it is possible to express a quaternary gray scale on a binary FLC.

In a preferred embodiment of the present invention, a plurality of intersections constituting one pixel E are formed with different intersection areas, respectively.
In particular, this different intersection area is 2 for the minimum intersection area 1:
4: 8: 16:...: 2 ⁿ (n = the number of intersections in one pixel E) is preferable.

In the present invention, the scanning electrode is divided into two, and when the electrode line width C = B, eight gradation levels are possible, and when C ≠ D, 16 gradation levels are possible.

In addition, when the division is performed only on the information electrode side, if the electrode line width is A = B and the color filters are provided so as to have a complementary color relationship with A and B, four colors can be displayed. For example, by arranging complementary color relations of [A = yellow; B = blue], [A = magenta; B = green] or [A = cyan; B = red], the colors of white, black, A and B Color 4
Color display becomes possible.

The polarizers 16A and 16B shown in FIG. 2 are arranged so that their polarization axes cross each other, and the crossed polarization axes are set so that a dark state is formed at the erasing phase in the driving example described below. Is good.

The matrix electrode shown in FIG. 1 is driven by the driving method described below.

FIG. 4 shows a driving waveform used in the present invention. FIG. 4A shows a scanning selection signal, a scanning non-selection signal, a white information signal, and a black information signal, respectively. When a white information signal is applied from the information electrode to a pixel on the scan electrode to which the scan selection signal is applied, the pixel is erased to a dark (black) state at phase T ₁ (V ₂ , phase t ₁ ) voltage V ₃ + V ₂ is applied in t ₂ is erased) a black state, is written voltage V ₁ + V ₃ in the subsequent phase t ₃ is applied to the state of the light (white). On the other hand, the pixels on the same scanning electrode, the black information signal is applied from the information electrode, V ₂ at the pixel is erased to the black state in phase T ₁ (phase t _1, a phase t ₂ -V ₃ + V _second voltage is applied erased) a black state, followed by a voltage V ₃ -V ₁ in phase t ₃ is applied, prior to the black state is written is held in a black state.

In this embodiment, the above-described scanning selection signal is applied to the scanning electrodes at intervals of three or more lines. FIG. 4B shows an example in which a scan selection signal is applied by skipping every third scan electrode for reference in understanding the present invention.

FIG. 4C shows a voltage waveform applied to the ferroelectric liquid crystal pixel, and shows an example of a driving waveform which causes the display state shown in FIG. In FIG. 5, a black dot indicates a black writing state, and a black circle indicates a white writing state. According to FIG. 5, the scanning electrode S ₁
Crossing area between to S ₉ and the data electrodes I ₁ (pixel area) scanning electrodes
It is set to twice the intersection area between the S ₁ to S ₉ and data electrodes I _2, pixels P ₁ to P ₉ are formed. As described above, in the pixel P ₁ to P _4, the area ratio of the black state and white state, being different 4
The gradation is expressed.

An embodiment of the present invention is shown in FIGS. 4 (D) and (E). In the driving example shown in FIG. 4 (D), a scan selection signal is applied to every other scan electrode every six scans, and the scan selection signal is divided into a first field, a second field,..., A seventh field (the number of field scans is denoted by F). 1) (F + 1), 5 (F + 5), 3 (F + 3), and 7 (F +
7), 2 (F + 2), 6 (F + 6) and 4
As the (F + 4) th is applied, the scanning signal application order is not in the order of each row corresponding to the field order. That is, according to the driving example shown in FIG. 4 (D), the scanning selection signal is applied to the non-adjacent scanning electrodes in seven consecutive fields constituting one screen scanning (one frame scanning). FIG. 4E shows another specific example (selection of skipping every third line). The driving examples in FIGS. 4D and 4E are more effective in suppressing flicker than in the case of the scanning signal application method shown in FIG. 4B.

FIG. 6 shows a driving waveform different from that of the present invention, in which every four scanning electrodes are selected.

FIGS. 6 (A-1) and (A-2) show the scanning selection signal and the scanning non-selection signal.
(B-4) represents an information signal synchronized with the scanning selection signal. In the driving examples of FIGS. 6A and 6B, every third scanning line is scanned during one vertical scanning period (one field), and the next scanning line after the scanning line scanned during the previous vertical scanning period. Are sequentially scanned in the next vertical scanning period. Therefore 1
The number of scanning lines scanned during one vertical scanning period is 1/1 of the total number of scanning lines.
4, which means that all the scanning lines are scanned in four vertical scans.

Table 1 shows that when a white pixel is formed in the driving of FIG. 6, the white selection voltage Sw applied to the pixel and the half-selection voltage H at that time are applied in the fields F ₁ , F ₂ , F ₃ , F ₄ ,. The timing is shown.

According to Table 1, when the (8M-7) fields F ₁ , F _9, ..., The white selection voltage is applied to the pixels (white selection pixels) on the (8M−7) th scanning lines S ₁ , S ₈ ,. is applied, half-selection voltage is pressurized in (8N-3) th scan line S _5, S ₁₃ ... on the pixels (white selection pixels), (8N-6), (8N-5), (8N -4), (8N-
2), (8N-1) , 8N th scan line _{S 2, S 3, S 4} , S 6, S 7, S 8 ...
The top and pixels are not scanned. Therefore, in the driving method shown in FIG.
Since only the total number of scanning lines of the 1/4 of the scan line at the vertical scanning period (one field) is not scanned, field field (reciprocal of one vertical scanning period) frequency f ₁₀ of the drive method at the timing of Table 1 Frequency f ₁ (F ₁₀ = 2f ₁ ).

In the driving method shown in FIG. 6, the number of pixels to which the half-selection voltage is applied in one field period is 1/2 of that in the driving example shown in Table 1, so that flicker is further prevented.

FIGS. 7A and 7B show another driving example of the present invention. That is, in the driving example shown in FIG. 6, the scanning lines scanned at the (8M-7) field are the (8N-7) and (8N-3) th scanning lines, and the scanning is performed at the (8N-6) field. The scanning lines were the (8N-6) and (8N-2) th scans. That is, a scanning method is performed in which a scanning line next to the scanning line scanned in the previous field is scanned in the next field, and in the next field, the next scanning line is sequentially scanned. According to such a scanning method, as is clear from Table 1, the timing at which the selection voltage is applied moves sequentially for each field, and when there is a contrast difference between the selection and the half selection, the selection voltage is applied to the scanning line. The above-mentioned contrast difference occurs at the timing of the movement, and this causes a line flow to sequentially move on the screen, which significantly lowers the display quality.

Table 2 shows that the white selection voltage Sw applied to the pixel in the driving of FIG. 7 and the half selection voltage H at that time are the fields F ₁ , F ₂ , F ₃ , F
₄ represents the timing of application.

The driving examples shown in FIGS. 7A and 7B are conceived to solve the above-described problem that occurs at the timing when the selection voltage is applied to the scanning lines.

That is, as apparent from Table 2, the timing at which the selection voltage is applied is sequentially prevented from moving in the same direction as much as possible, and flicker is effectively prevented without deteriorating the display quality.

Further, another reference example is shown in FIG. According to FIG. 11, the scanning selection signal is applied to the non-adjacent scanning electrodes in two consecutive fields of four consecutive fields constituting one screen scan (one frame scan). That is, FIGS. 11 (A-1) and (A-2) show that the scanning selection signal is applied in the order shown in FIGS. 6 (A-1) and (A-2).
5 shows a driving waveform different from that of FIG. Fig. 11 (B-1)
(B-4) are information signals used at this time. The driving example shown in FIG. 11 is more effective in further suppressing flicker than the driving example shown in FIG.

FIG. 8 is a configuration diagram showing an example of the display device of the present invention.
A display panel 801 is composed of a scanning electrode 802, a signal electrode 803, and a ferroelectric liquid crystal filled between the scanning electrode 802 and the signal electrode 803.
At the intersection of the matrix composed of 802 and the signal electrode 803, the orientation of the ferroelectric liquid crystal is controlled by the electric field generated by the voltage applied to the electrode.

Reference numeral 804 denotes a signal electrode drive circuit, which is a video data shift register that stores serial video data from the information signal line 806.
8041, line memory 8042 for storing parallel video data from video data shift register 8041, line memory
A signal electrode driver 8043 for applying a voltage to the signal electrode 803 in accordance with the video data stored in the 8042, and further, the voltages V _D , O and −V _D applied to the signal electrode 803 are switched to a switching control line 811.
And an information-side power switch 8044 that switches according to a signal from

Reference numeral 805 denotes a scan electrode drive circuit, and a scan address data line 807
, A scan electrode driver 8052 for receiving a signal from the decoder 8051 and applying a voltage to the scan electrode 802 in response to a signal from the decoder 8051, and further scanning. The voltage V _S applied to the electrode 802,
O, having a scanning side power switch 8053 for switching the signal of the -V _S from the switching control line 811.

A CPU 808 receives the clock pulse from the oscillator 809, controls the image memory 810, and controls signal transfer to the information signal line 806, scan address data line 807, and switching control line 811.

FIG. 9 schematically illustrates an example of a ferroelectric liquid crystal cell. 91a and 91b are substrates (glass plates) coated with a transparent electrode such as In ₂ O ₃ , SnO ₂ or ITO (indium-tein-oxide), between which the liquid crystal molecule layer 9 is perpendicular to the glass surface The liquid crystal of the SmC ^* phase, which is oriented likewise, is sealed. A line 93 shown by a thick line represents liquid crystal molecules,
The liquid crystal molecule 93 has a dipole moment (P⊥) 94 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 91a and 91b, the liquid crystal molecules 9
The orientation of the liquid crystal molecules 93 can be changed so that the helical structure is unwound and the dipole moment (P⊥) 94 is all directed to the direction of the electric field. The liquid crystal molecules 93 have a slender shape and exhibit refractive index anisotropy in the major axis direction and the minor axis direction. It can be easily understood that the liquid crystal optical modulation element changes its optical characteristics depending on the polarity of the applied voltage. Further, when the thickness of the liquid crystal cell is sufficiently reduced (for example, 1 μm), the helical structure of the liquid crystal molecules is released even when no electric field is applied as shown in FIG.
The dipole moment Pa or Pb takes either the upward (104a) or downward (104b) state. When an electric field Ea or Eb having a different polarity or more than a certain threshold is applied to such a cell for a predetermined time as shown in FIG. 10, the dipole moment becomes upward 104a or downward 104b with respect to the electric field vector of the electric field Ea or Eb. The orientation is changed, and the liquid crystal molecules are accordingly oriented in either the first stable state 103a or the second stable state 103b.

There are two advantages of using such a ferroelectric liquid crystal as the optical modulation element. First, the response speed is extremely fast, and second, the orientation of the liquid crystal molecules has a bistable state. The second point will be described with reference to, for example, FIG. 10. When an electric field Ea is applied, the liquid crystal molecules are oriented to a first stable state 103a. This state is stable even when the electric field is turned off. When an electric field Eb in the opposite direction is applied, the liquid crystal molecules are in the second stable state.
Orientation to 3b changes the orientation of the molecule, but it remains in this state even after the electric field is turned off. As long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, generally 0.5 μm to 20 μm, particularly 1 μm to
5μ is suitable.

[Effects of the Invention] According to the present invention, when a ferroelectric liquid crystal device in which one frame scanning time is long (for example, a low frame frequency such as 2 to 10 Hz) is applied to display, flickering based on low frame frequency scanning is reduced. The generation can be suppressed, and a gradation display based on the flicker suppression effect can be realized.

[Brief description of the drawings]

FIG. 1 is a plan view of a matrix electrode used in the present invention. FIG. 2 is a sectional view taken along the line AA 'of the FLC element used in the present invention. FIG. 3 is an explanatory diagram schematically showing a halftone. 4 (A) to 4 (E) are driving waveform diagrams used in the present invention. FIG. 5 is a schematic diagram showing a display state of the matrix electrode structure. FIG. 6 (A-1), (A-2), (B-
1), (B-2), (B-3), (B-4) and FIG. 7 (A-1), (A-2), (B-1), (B-2),
(B-3) and (B-4) are other drive waveform diagrams used as reference examples. FIG. 8 is a block diagram of the liquid crystal device of the present invention. 9 and 10 are perspective views of the ferroelectric liquid crystal cell used in the present invention. FIG. 11 (A-1), (A-
2), (B-1), (B-2), (B-3), (B-
4) is another drive waveform diagram used as a reference example.

Continuing on the front page (72) Inventor Akira Tsuboyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Osamu Taniguchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Yoshihiro Onitsuka 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-62-221281 (JP, A) JP-A-61-272724 (JP, A) JP-A-61-144698 (JP, A)

Claims (13)

(57) [Claims] 1. A method for driving a display device having a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes, wherein a plurality of screen scanning periods for displaying one screen are provided at least every three lines. A vertical scanning period for selecting a scanning electrode, and in at least one set of two consecutive vertical scanning periods of the plurality of vertical scanning periods, two scanning electrodes that are not adjacent to each other are sequentially selected, and the selected scanning is performed. After a first voltage for erasing a pixel on the selected scanning electrode is supplied to the electrode, a second voltage for inverting or holding the erased pixel in cooperation with an information signal is supplied. A method for driving a display device. 2. The apparatus according to claim 1, wherein the first voltage and the second voltage have different polarities with respect to a voltage applied to a non-selected scan electrode. The driving method of the display device. 3. The method according to claim 1, wherein the first voltage and the second voltage have different polarities and different pulse widths with respect to a voltage applied to a non-selected scan electrode. The method for driving a display device according to claim 1. 4. The method of driving a display device according to claim 1, wherein the display device is a ferroelectric liquid crystal display device in which a ferroelectric liquid crystal is disposed between a pair of substrates. 5. A method for driving a display device comprising a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes, wherein information electrodes having different electrode widths are opposed to one scanning electrode. The screen scanning period for displaying one screen is constituted by a plurality of vertical scanning periods for selecting at least every third scanning electrode, and at least one set of two consecutive scanning electrodes is selected from the plurality of vertical scanning periods. In the vertical scanning period, two scan electrodes that are not adjacent to each other are sequentially selected, and a first voltage for erasing pixels on the selected scan electrode is supplied to the selected scan electrode. A method for driving a display device, comprising: supplying a second voltage for inverting or holding an erased pixel in cooperation. 6. The apparatus according to claim 5, wherein the first voltage and the second voltage have different polarities with respect to a voltage applied to an unselected scan electrode. The driving method of the display device. 7. The method according to claim 1, wherein the first voltage and the second voltage have different polarities and have different pulse widths with respect to a voltage applied to a non-selected scan electrode. The method of driving a display device according to claim 5. 8. The method according to claim 5, wherein the display device is a ferroelectric liquid crystal display device in which a ferroelectric liquid crystal is disposed between a pair of substrates. 9. The method according to claim 5, wherein, of the two information electrodes having different electrode widths, the electrode width of one information electrode is a power of two with respect to the electrode width of the other information electrode. Display device driving method. 10. A method of driving a display device having a matrix electrode composed of a plurality of scanning electrodes and a plurality of information electrodes, wherein a plurality of screen scanning periods for displaying one screen are provided at least every four lines. A vertical scanning period for selecting a scanning electrode; of all the plurality of vertical scanning periods, two scanning electrodes that are not adjacent to each other are sequentially selected in all two consecutive vertical scanning periods; Supplying a first voltage for erasing a pixel on the selected scan electrode, and then supplying a second voltage for inverting or holding the erased pixel in cooperation with an information signal. A method for driving a display device, comprising: 11. The method according to claim 11, wherein the first voltage and the second voltage have different polarities and have different pulse widths with respect to a voltage applied to a non-selected scan electrode. 11. The method for driving a display device according to claim 10, wherein 12. The method according to claim 10, wherein the display device is a ferroelectric liquid crystal display device in which a ferroelectric liquid crystal is disposed between a pair of substrates. 13. The display device according to claim 1, wherein two information electrodes having mutually different electrode widths are arranged to face one scanning electrode, and one of the two information electrodes has one information electrode. 11. The driving method for a display device according to claim 10, wherein the electrode width of the other information electrode with respect to the electrode width is in a power of two relationship.