JP3356430B2 - Multiple addressing of ferroelectric liquid crystal displays - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ferroelectric liquid crystal display (ferro-elect).
ricliquid crystal display). Such displays use graded chiral smectic C, I or F liquid crystal materials.

Liquid crystal devices typically have a thin layer of liquid crystal material contained between two glass slides. Light transmissive electrodes are formed on the inner surfaces of both slides. When a voltage is applied to these electrodes, the resulting electric field changes the molecular alignment of the liquid crystal molecules. This change in molecular alignment is easily observable and forms the basis for various types of liquid crystal display devices.

In a ferroelectric liquid crystal device, liquid crystal molecules switch between two different alignment directions depending on the polarity of the applied electric field. These devices have some degree of bistability and therefore tend to stay in one of the two switching states until they are switched to the other. This allows for multiple addressing of very large displays.

One common multiplex display has display elements (ie, pixels) arranged in an x, y matrix format, for example, for alphanumeric display. This matrix format is
Form electrodes as a series of column electrodes on one slide,
And by forming the electrodes as a series of row electrodes on the other slide. The intersection between each column and row forms an addressable display element or pixel. Other matrix styles are known, such as polar coordinates (r-θ) and seven bar numeric displays.

There are many different multiple addressing schemes. A common feature is that a voltage called a "strobe voltage" is applied to each row or column in turn. At the same time that a strobe pulse is applied to each row, an appropriate voltage called the "data voltage" is applied to all column electrodes. The difference between the various addressing schemes is found in the shape of the strobe voltage waveform and the data voltage waveform.

European Patent Application No. 0,306,203 describes one of the multiple addressing schemes for ferroelectric liquid crystal displays. In this patent application, the strobe pulse is a unipolar pulse of alternating polarity and the two data waveforms are square waves of opposite signs. The strobe pulse width is 1/2 of the data waveform period. The combination of the strobe and the appropriate one of the data waveforms results in switching of the liquid crystal material.

Other addressing schemes are GB 2,146,473-A; GB-2,17
3,336-A; GB-2,173,337-A; GB-2,173,629-A; WO 89/0
5025; Harada et al. 1985 SID Digest Paper 8.4 pp 131-1
34; Lagerwall et al. 1985 IEEE, IDRC pp 213-221; Proc 1988
IEEE, IDRC p 98-101 "Fast Addressing for Ferroelectric Liquid Crystal Display Panels
Ferro Electric LC Display Panels) ", P Maltese et al.

The liquid crystal material is coupled to the data waveform by two strobe pulses of opposite signs to each other.
It is possible to switch between the two states. Instead, a blanking pulse is used to switch the liquid crystal material to one state, and a single strobe pulse is appropriate to selectively switch the pixel back to the other state. It may be used together with. In order to maintain a net zero DC value, the sign of the blanking pulse and the sign of the strobe pulse are alternated periodically.

Such blanking pulses generally have a greater amplitude and applied length than the strobe pulse, and therefore switch their liquid crystal material regardless of which of the two data waveforms is applied to any one intersection. A blanking pulse may be applied line by line prior to the strobe, the entire display may be blanked at once, or a group of lines may be blanked simultaneously.

One known blanking scheme is equal to the "voltage (V) time (t) product" Vt of the strobe pulse, but the polarity is reversed.
Use blanking pulse of Vt product. This blanking pulse is
It has half the amplitude of the strobe pulse and twice the application time of the strobe pulse. These values ensure that the blanking and strobe pulses have a net zero DC value without periodic reversal of polarity. Experimental use has proven that this scheme has poor performance.

Another known scheme for blanking pulses is described in EP 0,378,2
It is described in 93. This is because a conventional DC-balanced strobe pulse (equal period / reverse polarity) and a conventional DC-balanced blanking pulse (such as a blanking pulse whose width can be several times the width of the strobe pulse). Period / reverse polarity). Such a scheme has a net zero DC value without the alternating polarity of the blanking and strobe pulse waveforms.

The DC balance feature is particularly important in projection displays because periodic reversal of polarity is not possible when it is necessary to switch the gap between pixels to one optical state.

One of the problems with existing displays is the time required to address complex displays. Driving a complex display at video frame rates requires that the display be addressed at high speed. Because the column waveform is at a high frequency in response to fast addressing,
The contrast ratio can also be improved by addressing at high speed. However, simply increasing the addressing speed will not always result in proper switching. It is an object of the present invention to reduce the time required to address a matrix display and improve the contrast of the display.

In accordance with the present invention, a method for multiple addressing a ferroelectric liquid crystal matrix display formed by the intersection of a first set of electrodes and a second set of electrodes comprises adding a strobe waveform of positive and negative pulses. Or by adding a blanking pulse followed by a strobe pulse with a periodic polarity reversal to maintain a net zero DC value. Individually, and one data waveform is the inverse of the other data waveform, and the period (2ts) of those data waveforms is twice the period (ts) of a single strobe pulse Alternately applying one of two data waveforms of positive and negative values to each of the electrodes of the second set of electrodes in synchronization with a strobe waveform; And the sign of the appropriate magnitude and overall net zero DC to change the intersection to the required display state only once per display address period. Each intersection is characterized by a pulse being a value, which is addressed.

The strobe waveform initially has a zero over the first period (ts) followed by a period greater than ts, for example (1.
5, 2.0, 2.5, 3.0 or more) × ts non-zero voltage (primary) pulses. The strobe waveform may have a non-zero voltage within the first ts period of the same polarity or opposite polarity as the rest of the strobe. This first voltage pulse is of variable amplitude to provide temperature compensation. The strobe waveform may be followed by a non-zero voltage over a time period of opposite polarity (eg, greater than, equal to, or less than ts) than the main voltage pulse.

The liquid crystal material can be switched between two states of the liquid crystal material by the coexistence of a strobe pulse and an appropriate data waveform. Instead, the liquid crystal material is switched to one of the two states by a blanking pulse, followed by the coexistence of a strobe pulse and the appropriate data waveform to cause the selected pixel to switch to the other state. The state may be switched back.

The blanking pulse may consist of two parts, the first of which
The polarity of the portion is opposite to the polarity of the second portion. The two parts of the blanking pulse are made to have a voltage-time product Vt that is combined with a single strobe Vt product to give a net zero DC value.

Extending the time length of the strobe pulse is the first
Means overlapping addressing on successive electrodes of the set of electrodes. Such overlap effectively increases the width of the switching pulse without adversely affecting other waveforms, thus addressing the entire display while maintaining a good contrast ratio between the pixels in the two different switching states. Reduce the total time required to specify.

A smaller pre-pulse having the same sign as the sign of the associated strobe pulse or the opposite sign.
se) may precede each strobe pulse. This prepalace can be used to change the switching characteristics of the liquid crystal material. This pre-pulse may be used as part of the temperature compensation. In this case, the temperature of the liquid crystal material is sensed and the amplitude of the pre-pulse is adjusted appropriately.

Immediately after each strobe pulse, a pulse of the opposite sign may follow.

According to the present invention, a multi-addressed liquid crystal display is a liquid crystal cell formed by a layer of liquid crystal material contained between two cell walls, wherein said liquid crystal material has a negative dielectric anisotropy. A liquid crystal material, wherein said two cell walls have electrodes formed as a first set of electrodes on one cell wall and a second set of electrodes on the other cell wall; Are arranged collectively to form a matrix of addressable intersections,
And a liquid crystal cell in which at least one of the cell walls is surface-treated so as to provide a surface alignment for liquid crystal molecules along a single direction; and each of the electrodes of the first set of electrodes. A driver for sequentially applying a strobe waveform, a driver for applying a data waveform to the second set of electrodes, and a strobe waveform and two data waveforms for applying to the driver. And a means for controlling the order of the data waveforms so as to obtain the required display pattern, the two sets of data having equal amplitude and equal frequency but opposite signs. A data waveform generator for generating a waveform, wherein each of the data waveforms includes a DC pulse of an alternating code, and a strobe pulse having a duration longer than 1/2 of the data waveform period. And each of said strobe pulses extends during the addressing period of the next electrode.

Simple analysis of liquid crystal switching characteristics (Liquid Crysta
1, 1989, vol. 6, No. 3, pp. 341-347) give the following equation for the electric field in which the response time-voltage switching characteristics of the liquid crystal material show a minimum response time in that electric field: In the above formula, Emin is the electric field in which the response time-voltage switching characteristic of the liquid crystal material exhibits the minimum response time in the electric field, ε ₀ is the permittivity of free space, and Δε is the (negative)
Is the dielectric anisotropy, θ is the taper angle of the liquid crystal material, and Ps is the natural polarization.

This simple analysis is only valid for certain materials, and the values of Ps and Δε can be adjusted to obtain the required operating voltage. Recent research (Fine Chemicals
for the Electronics Industry II, Chemical Applicat
EP Raynes, "The Physics of Displays for th 1990s"
e 1990's) ", pp 130-146; Jones, Raynes and Towle.
r, "The Importance of Dielectric Biaxiality for Ferr
o electric Liquid Crystal Devices) ", 3rd Internat
ional Conference on Ferro electric Liquid Crystal
s, Univ of Boulder Colerado USA 24-28 June 1991) shows that dielectric biaxiality is important for the existence of a minimum in response time-voltage characteristics. The data relating to FIGS. 16 to 20 described below were obtained by experiments.

In the following, the invention will be described purely by way of example with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a time multiplex addressing x, y matrix.

FIG. 2 is an enlarged cross-section of a portion of the display of FIG.

FIG. 3 is a "log time" versus "log voltage" graph showing the switching characteristics of smectic material for two different forms of addressing waveforms.

4 to 8 show various usable strobe waveforms and data waveforms.

FIG. 9 is a waveform diagram having a strobe modified from the strobe waveform of FIG.

FIG. 10 is a diagram showing a blanking pulse waveform, a strobe waveform, and a data waveform.

FIG. 11 is a diagram showing a strobe waveform, a data waveform, and an addressing waveform used in a conventional display.

FIGS. 12a and 12b show waveforms for addressing the 4 × 4 display element display shown in FIG.

Figure 13 shows some intersections switched to the ON state and OFF
It is a figure which shows the 4x4 display element arrangement | sequence which shows the remaining intersection which remains in a state.

14 and 15 are graphs of "contrast ratio" versus "applied voltage pulse width" for two different materials.

FIGS. 16-20 are graphs of "log time" versus "log applied voltage" showing the switching characteristics of one material for different applied waveforms.

21 and 22 are diagrams showing various blanking pulse waveforms, strobe waveforms, and data waveforms.

FIG. 23 and FIG. 24 are diagrams showing a row waveform and a column waveform in the case of the display of the related art.

FIG. 25 and FIG. 26 are diagrams showing a row waveform and a column waveform in the case of the modification of FIG.

The display 1 shown in FIGS. 1 and 2 comprises two glass walls 2, 3 separated by a spacer ring 4 and / or dispersive spacers by about 1-6 μm.

Transparent tin oxide electrode structures 5, 6 are formed on the inside surfaces of both glass walls. These electrodes are shown as rows and columns forming an X, Y matrix, but may have other shapes. For example, the shape may be a radial or curved shape for an r, θ display, or a segment shape for a digital seven bar display.

A liquid crystal material layer 7 is accommodated between the walls 2, 3 and the spacer ring 4.

Polarizers 8, 9 are arranged at the front and back of the cell 1.
Row driver 10 and column driver 11 apply a voltage signal to cell 1. To power row driver 10 and column driver 11,
Three sets of waveforms are generated. Strobe waveform generator
12 supplies the column waveform and the data waveform generator 13 is the column driver
Supply the ON waveform and OFF waveform to 11. The overall control of timing and display format is controlled by control logic unit 14. The temperature of the liquid crystal layer 7 is measured by a thermocouple 15 whose output is sent to the strobe waveform generator 12.
The output of the thermocouple 15 may be fed directly into the strobe waveform generator 12, or a proportional element, such as a program ROM chip, for changing a portion of the strobe pulse and / or data waveform.
nt) 16 to the strobe waveform generator 12.

Prior to assembly, for example, a thin layer of polyimide or polyamide is applied and dried, cured if necessary,
The cell walls are surface-treated in a known manner by buffing with a cloth (eg rayon) in one direction R1, R2. Alternatively, a thin layer of, for example, silicon monoxide may be deposited at an oblique angle. Such treatment results in surface alignment of the liquid crystal molecules. The alignment / polishing directions R1, R2 may be parallel or non-parallel. When an appropriate one-way voltage is applied, the long axis of the molecule aligns in one of two directions D1, D2, depending on the polarity of the voltage. Ideally, the angle between D1 and D2 is about 45 °. In the absence of an applied electric field, the molecule takes an intermediate alignment direction between directions R1, R2 and directions D1, D2.

The device can operate in a transmission mode or a reflection mode. In the transmissive mode, light passing through the device (eg, from a Tansten bulb) is selectively transmitted or blocked to form the required display. In the reflection mode, a mirror is placed behind the second polarizer 9 to reflect the external light back through the cell 1 and the two polarizers. By making the mirror partially reflective, the device can be used in both transmissive and reflective modes.

A polychromatic dye may be added to material 7. In this case, only one polarizer is needed and its layer thickness is between 4 and
It may be 10 μm.

A suitable liquid crystal material has a Ps of 5 nC / cm 2 at 30 ° C., a dielectric anisotropy of about −2.0, and a phase order of Sc 59 ° C. Sa 79 ° C. N98 ° C.
ck catalog reference number SCE 8 (available from Merck Ltd, Poole, England) with 5% racemic dapant in host
A mixture A containing 3% chiral dopant and 3% chiral dopant, and a mixture B containing 9.5% racemic dopant and 3.5% chiral dopant in the host.

The symbol "*" represents chirality, and without this symbol the material is racemic.

Both mixture A and mixture B have a Ps of 7 nC / cm @ 2 at 30 DEG C. and a dielectric anisotropy of about -2.3.

Mixture A has a phase order of Sc 100 ° C. Sa 111 ° C. N 136 ° C.

Mixture B has a phase order of Sc 87 ° C. Sa 118 ° C. N 132 ° C.

The liquid crystal material at the intersection of the row and column electrodes is switched by applying an addressing voltage. This addressing voltage is obtained by a combination of applying a strobe waveform Vs to a row electrode and applying a data waveform Vd to a row electrode.

 That is, Vr = Vs-Vd, where Vr = the instantaneous value of the addressing waveform, Vs = the instantaneous value of the strobe waveform, and Vd = the instantaneous value of the data waveform.

Chiral graded smectic materials switch based on the product of voltage and time. This feature is illustrated in FIG. The voltage-time product above the curve in this figure will switch the liquid crystal material. Below that curve is the non-switching region. Note that this switching characteristic is independent of the sign of the voltage, that is, the liquid crystal material switches for both positive and negative voltages of a given amplitude. The switching direction of the liquid crystal material is determined by the polarity of the voltage.

Two curves are shown in FIG. 3 because their switching characteristics are determined by the form of the addressing voltage pulse combination. Immediately before the addressing voltage, the upper curve is obtained when a small pre-pulse of the opposite sign precedes the voltage, ie, a small negative pulse is followed by a larger positive pulse. This material performs the same operation by applying a small positive pulse followed by a larger negative pulse. This upper curve generally indicates a turn, or minimum response time, at one voltage. This is not as given by Equation 1 because its switching characteristics are modified by the prepare. This small pre-pulse may be referred to as a “leading pulse (Lp)” and the larger addressing pulse may be referred to as a “post-pulse (Tp)”. This upper curve applies for the case of a negative ratio Lp / Tp.

Immediately before the addressing voltage, a lower curve is obtained when a small pre-pulse of the same sign as that voltage precedes, ie, a small positive pulse is followed by a larger positive pulse. This lower curve is also true for a small negative pulse followed immediately by a large negative pulse. This lower curve has a positive ratio Lp / Tp. This lower curve has a different shape than the shape of the upper curve. This curve may not have the minimum of the voltage-time curve for some materials.

The difference in shape between these two curves allows the device to be operated without ambiguity over a very wide range of time values. This is obtained by operating the device in the area between these two curves (eg, indicated by hatching).
The intersections that need to be switched are addressed by an addressing voltage having a configuration in which the lower curve applies and the voltage and pulse width are located above the lower curve. Intersections that do not need to be switched receive an addressing voltage having a configuration in which the upper curve applies and the voltage and pulse width are located below the upper curve, or receive only the data waveform voltage. This is described in more detail below.

FIG. 4 shows a strobe waveform, a data waveform, and an addressing waveform of one embodiment of the present invention. The strobe waveform is initially zero over time period ts, followed by 2
+3 over × ts. This waveform is applied sequentially to each row, ie, in one time frame period. The second part of the strobe waveform is
Zero over time 1.times.ts followed by -3 over time 2.times.ts. Similarly, this waveform is applied sequentially to each row in one time frame period. Addressing the entire display takes two frame time periods. The values +3 and -3 are units of voltage given for illustration, and the actual values for a particular material are shown below.

The data waveform can be either data ON and data OFF, or D
Arbitrarily defined as 1 and D2. The data ON initially has a value of +1 over the first time period ts and a subsequent value of -1 over the time period ts. This is repeated, that is, data ON is an alternating signal having an amplitude of 1 and a period of 2 ts. Data OFF is similar, but has an initial value of -1 followed by +1. That is, data OFF is the reverse of data ON. The first portion of the data waveform (e.g., value +1 over time period ts in the case of data ON) coexists with the first portion of the strobe waveform (i.e., zero over time period ts). ing.

The addressing waveform is the sum of strobe and data. The combination of positive strobe pulse and data ON is-
1, 4, 2, 1, -1, 1, etc. A value "4" immediately preceded by a value "-1" ensures that the switching characteristics of the material are determined by the upper curve of FIG. The combinations of the negative strobe pulse and the data ON are -1, -2, -4, 1, -1, 1, and the like. The combination of smaller pulses with the same sign as the larger (-4) pulse ensures that the switching characteristics of the material are determined by the lower curve in FIG.
Similarly, a combination of a positive strobe pulse and data OFF gives 1, 2, 4, -1, 1 and the like, and a combination of a negative strobe pulse and data OFF gives 1, -4, -2, -1, 1,. −
Give 1 mag.

When not receiving a strobe pulse, each row is grounded, ie, receives zero voltage. Each row constantly receives either data ON or data OFF. The effect is that when not addressed, all intersections will receive an alternating signal caused by the data waveform. This provides an AC bias at each intersection and helps the liquid crystal material to maintain its switched state.
A larger amount of AC bias is in Proc 4th IDRC 1984, p
The known AC stabilization described in pages 217-220 leads to improved contrast.

An additional AC bias may be provided directly to the rows that do not receive the strobe pulse, for example from a 50 KHz bias source. AC bias to contrast ratio (both magnitude and width) for material SCE8 and mixture A
14 and 15 are shown in FIG. These figures are
Figure 4 shows the measured intrinsic contrast ratio (CR) as a function of the AC frequency when the cell is switched between its two bistable states and measured at various AC bias levels.

The alternating strobe waveform is shown in FIGS. In FIG. 5, the strobe waveform is initially zero over 1 × ts, followed by 3 over 3 × ts, and
Subsequently, the reverse waveform is shown. In FIG. 6, the strobe waveform is initially zero over 1 × ts, followed by 3 over 4 × ts, and vice versa. In FIG. 7, the strobe waveform is initially zero over 1 × ts, followed by 2
It is 3 over xts, followed by -1 over 1xts. This is followed by the opposite waveform.

FIG. 8 is a modification of FIG. 4, which uses a non-zero prepare for the strobe waveform. As shown, the first part of the strobe is between -1 and 1, not the zero value of FIG. The rest of the strobe is the same as in FIG. 4, i.e., amplitude 3 over 2 * ts. Therefore, the resulting addressing waveforms are the first and second
Is the first pulse between -2 and -1 over both fieids. The effect of this pre-pulse is to change the position of the switching curve (FIG. 3, etc.). Changing the value of this pre-pulse is described below.
Change the shape and vertical position of the switching curve as described with reference to 16 and FIG. Table 8 below shows how the switching time varies with temperature. Such changes can be reduced by changing the pre-pulse amplitude.

FIG. 9 shows a modification of FIG. In this variation, the strobe waveform has zero over the first ts, followed by 1.
3 over 5ts. Any value greater than ts is about 5ts
The 1.5ts above is only an example, since it can be used up to.

FIG. 10 shows a single blanking pulse of amplitude 4 applied over 4 ts. This pulse switches all intersections to one switching state. Thereafter, a strobe pulse is used to switch the selected intersection to the other switching state. The sign of the blanking pulse and the sign of the strobe pulse are periodically inverted to maintain an overall net zero DC voltage. The use of a blanking pulse and a single strobe pulse can be applied to all of the schemes in FIGS. An advantage of the blank ratio / strobe system is that the entire display can be addressed within a single field time period.

For comparison, FIG. 11 shows a strobe waveform, a data waveform, and an addressing waveform in the case of the conventional display method (single pulse addressing method).

Figures 21 and 22 illustrate the addressing scheme of the present invention using blanking and single strobe pulses to provide a net zero DC value.

In FIG. 21, the blanking pulse includes two parts:
Includes reverse sign prepalace and major blanking pulses. The function of this prepulse is to provide zero DC balance.
The prepulse has a value of 3 for 4 ts, followed immediately by -3 for 6 ts. The strobe pulse is
Initially zero for 1 ts and 3 immediately following 2 ts. This strobe is the same as the strobe of FIG. The data waveforms D1 and D2 are also the same as in FIG. The combination of the blanking pulse and D1 or D2 results in a large negative V that switches all pixels in the addressed row to the OFF state.
Indicates the t product. The strobe pulse combined with D2 is
Turn on the selected pixel as described above with reference to FIG.
Switch to state.

FIG. 22 is similar to FIG. 21, but has a different form of blanking pulse. This blanking pulse has a pre-pulse of amplitude 3 over 4 ts followed immediately by -4.5 over 4 ts. The strobe pulse has an amplitude of 3 over 2 ts, as in FIG. The combination of the blanking pulse and D1 or D2 is shown to provide a large negative Vt product that switches all of the addressed rows to the OFF state. Here, similarly, the selected pixel is switched to the ON state by the strobe pulse and D2.

The blanking pulses of FIGS. 21 and 22 have an amplitude and / or Vt product determined to give a net zero DC value.
It is added together with a strobe pulse of another shape shown in FIG. In the case of the example of FIG. 8 where the first time slot of the strobe varies, for example, with temperature, the amplitude of the pre-pulse and / or the main blanking pulse is also adjusted to maintain a net zero value.

This blanking pulse can precede the strobe pulse by a variable amount, but there is an optimal position in terms of response time, contrast and visible flicker in the display. This is typically the case with a blanking pulse starting six lines ahead of the strobe pulse, but depends on the parameters of the liquid crystal material and the details of the multiplexing scheme.

12a and 12b show the information as shown in FIG.
3 shows a waveform relating to addressing of a × 4 matrix array. The solid black circle is arbitrarily shown as the ON electrode intersection (ie, the display element), and the unmarked intersection is OFF. The addressing method is the addressing method used in FIG.

A positive (leading) strobe pulse is sequentially applied to each of rows 1-4. This forms the first field.
After the last row has been addressed by this leading strobe pulse, a negative (succeeding) strobe pulse is sequentially applied to each of rows 1-4 to form a second field. Note that there is overlap between these rows. For example, the third ts cycle for row 1 occurs simultaneously with the first ts cycle for row 2. 5 and 6
This overlap is even more pronounced for displays using the strobe waveform shown in FIG.

Since each intersection point in column 1 is always ON, the data waveform "DATA ON" applied to column 1 remains unchanged. Similarly, in the case of column 2, the data waveform is "data OFF" and remains unchanged because each intersection of column 2 is OFF. Row 3
In the case of, the data waveform is data OFF when rows 1 and 2 are addressed, changes to data ON when row 3 is addressed, and turns OFF when row 4 is addressed. Is returned to. This means that for one field time period, which is the time required for the positive strobe pulse to address all rows, columns 3
Receive data OFF for × ts and data O for 2 × ts
N means that data OFF is received for 2 × ts. Similarly, in the case of column 4, the data waveform is data OFF for 2 × ts, data ON for 2 × ts, and data OFF for 2 × ts. And the data is ON for 2 × ts. This is repeated over a further field period when a negative strobe pulse is applied. Two field periods are required to provide one frame period and complete the addressing of the display. The above waveform is repeated until a new display pattern is needed.

The resulting addressing waveform is shown in FIG. 12b. At the intersection of row 1 and column 1 (R1, C1), the switching of the material follows the upper curve of FIG. 3, and the time and applied voltage level are located below the switching curve. Therefore, the material is first
Are not switched during the field period of Instead, the lower voltage /
Due to the time requirement, the material switches during the second field period in which the material switches. A similar inference is
The material is assigned to the intersection (R1, C2) where it switches during the first field period.

In the case of the intersection (R3, C3), the material is not the first because the time / voltage applied during the first field period does not reach the higher values required by the upper curve of FIG. It switches between two field periods. Intersection (R4, C
4) switches at the end of the second field period when a negative strobe pulse is being applied.

The shape of the waveform applied to column 4 introduces difficulty. Because of the ON-OFF-ON-OFF pattern of the display, its data waveform has, for example, twice the period of the data waveform of column 1. This can result in lower contrast, as shown in FIGS. 14 and 15, where longer pulse widths (low frequencies) give very low contrast ratios. In addition to this, the amplitude of the large addressing pulse in the first field that does not cause a switch will be
In contrast to the smaller amplitude switching pulses in the field. Therefore, a large difference between the two switching curves shown in FIG. 3, for example, is necessary for reliable switching.

The contrast ratio (CR) curves of FIG. 14 (mixture A) and FIG. 15 (mixture SCE8) show the inherent contrast of the device when switching between its two bistable positions in the presence of an AC bias. Clearly, operation along a short pulse width plateau is desirable for both proper and uniform contrast. Because the multiplexed AC bias from the column waveforms will carry variable frequency components determined by the pixel pattern, the contrast in the display may change. this is,
When all pixels are in one state (the highest frequency component) due to two factors in the column waveform frequency
And the case of the alternate pixels in the opposite state (lowest frequency component) is particularly remarkable. Two such cases are illustrated by FIGS. 12 and 13 for columns 1 and 4.

FIG. 16 shows a log-time / log-voltage graph showing switching characteristics for a material SCE8 in the form of a parallel rubbed cell having a layer thickness of 1.8 μm at a temperature of 25 ° C.
The axes of the graph are log ts and log pulse amplitude voltage.

Simulating the addressing waveform shown in FIG. 4, a switching curve in the calibration cell is obtained. 2
Two different addressing waveforms are used. The first curve, Waveform I, is a small (-1) negative pulse applied over time ts, followed by a larger (5) positive pulse applied over time 2ts; That is,
The Lp / Tp ratio is -0.166. In addition, a period of zero volts is followed by a converse, a small (1) positive pulse and a larger (-5) negative pulse. In addition, a 50 KHz square wave signal is superimposed on the addressing waveform to provide an AC bias and to simulate the data waveform. This small pulse is 0.166 times the value of the large pulse at all voltage levels used to provide the curve. This first addressing waveform gives the upper curve. A "time / voltage" value above this curve will cause the cell to switch, whereas a "time / voltage" value below the curve will not.

The second addressing waveform II is a small (1) positive pulse applied initially over time ts, followed immediately by a larger (4) positive pulse applied over time 2ts. After a period of zero volts, the waveform is inverted. The small pulse is 0.25 times the value of the large pulse, ie, Lp / Tp = 0.25. Again, a 50 KHz square wave signal is superimposed on the addressing waveform to provide an AC bias. This second addressing waveform gives the lower curve. The "time / voltage" value above this curve gives the cell switching, while
A "time / voltage" value below the curve does not provide for cell switching. Strobe voltage Vs = 50 volts and data voltage
Operating range for Vd = 10 volts: lower curve, Vs−Vd =
40 gives switching at 52 microseconds, and the upper curve, Vs + Vd = 60, gives switching at about 480 microseconds.

FIG. 17 illustrates the same addressing scheme as used in FIG. 16, ie, the addressing scheme of FIG. 4 modified by using small pre-pulses in the strobe waveform as in FIG. The time-voltage characteristic in the case is shown. FIG. 17 shows that the effect of this pre-pulse is to shift the vertical position of the curve. This is beneficial with respect to temperature compensation, where the movement of the curve due to temperature changes is offset by changing the value of the prepare.

For the upper curve, the simulated addressing waveform is initially time ts
Zero voltage, followed immediately by a larger (6) positive pulse applied for 2 ts of time, ie,
Lp / Tp = 0. After some time interval ts of zero volts, an inverse waveform is applied to maintain a net zero DC voltage. A 50 KHz square wave is superimposed to provide an AC bias.

In the case of the lower curve, the addressing waveform is initially a small (1) positive pulse applied over time ts, followed immediately by a larger (2) applied over 2ts. It is a positive pulse, that is, Lp / Tp = 0.5. This waveform is then inverted. A 50 KHz square wave is superimposed to provide an AC bias.

Operating range when Vs = 50, Vd = 10: lower curve,
Vs-Vd = 40 gives switching at 42 microseconds, and the curve above, Vs + Vd = 60 gives switching at about 500 microseconds.

FIG. 18 is a graph similar to FIG. 16 using the address waveform simulation of FIG. 5 for the same cell. Therefore, the addressing waveform is -1, 6, 4, 6 (Lp / Tp = -0.166) for the upper curve,
In the case of the lower curve, 1, 4, 6, 4 (Lp / Tp = 0.2
5). For Vs = 50, Vd = 10, the lower curve gives switching at 38 microseconds and the upper curve gives switching at about 210 microseconds.

FIG. 19 is a graph similar to FIG. 16 using the address waveform simulation of FIG. 7 for the same cell. Thus, the addressing waveform is -1, 6,..., For a curve having a "+" point, as shown.
6, -6 (Lp / Tp = -0.166), and 1, 4, 4, and -4 in the case of a curve having points marked with "O". The switching is complicated because the upper curve has a concave area where the liquid crystal material switches based on a subsequent pulse rather than a main pulse. When Vs = 50, Vd = 10, the lower curve, Vs−Vd = 40, gives a switch at 58-240 microseconds, and when switching to a subsequent pulse,
Give switching again above 300 microseconds. The upper curve, Vs + Vd = 60, gives no switching at 60 volts. Therefore, the multiplexing operation based on the main pulse occurs in 58 to 240 microseconds,
Multiplexing based on subsequent pulses occurs above 300 microseconds.

For comparison, a conventional logarithmic time / log scheme for a conventional single pulse addressing scheme using the simulation of the strobe and data waveforms of FIG.
The “logarithmic voltage” characteristic is shown in FIG. With respect to the upper curve, the simulated addressing waveform has a negative pulse applied over time ts with an amplitude of 1 "unit" followed by a negative pulse applied over 2 ts with an amplitude of 6 "units". Is a positive pulse. For the lower curve, the addressing waveform has an amplitude applied over time ts of 1 "unit".
Followed by a positive pulse of 4 "units" applied over time ts. The pulse amplitude is shown in “units” to represent a relative value. These curves are obtained at the voltages shown. When Vs = 50, Vd = 10, the lower curve, Vs−Vd = 40, gives switching at 80 microseconds, and the upper curve, Vs + Vd
= 60 gives a switch at about 950 microseconds.

In the following, device characteristics for different liquid crystal materials and different addressing waveforms will be described in detail. A single pixel test cell was assembled, and the test cell was addressed by simulating a 50 column display. To provide an addressing voltage value such that the switching voltage is above the lower curve in FIG. 3 and the non-switching voltage is below the upper curve in FIG. 3, the strobe voltage amplitude (Vs) and the data voltage amplitude ( Vd) to select various values,
In addition, the value of ts (in microseconds) was adjusted to give a clear switching of the display. This ensured that the test cell operated in the area indicated by "hatching" in FIG. The value of the contrast ratio (CR) is the ratio of "light transmitted in one switching state" to "light transmitted in the other switching state", ie a measure of the transparency of the display. . CR is at both ends of the pulse width ts or ts
It is measured at the specified value. The CR is optimized by adjusting one of the switched positions of the long axis of the liquid crystal molecule to correspond to a minimum transmission.

In the table below, the operating range of time ts does not match very well with the information given by the “voltage / time” curves in FIGS. There are three reasons for this. First, FIGS.
The simulation used in 20 is not completely accurate for every situation of the display pattern. Second, at longer pulse widths and correspondingly longer frame times, the operator can identify flicker due to transient switching. Third, at longer pulse widths, the contrast ratio is lower (see FIGS. 14 and 15). For example, 200
In microseconds, the CR is 2, so it is difficult to determine if the material has switched.

Thus, for practical displays, an upper time limit must be taken when the display is no longer effectively switched. This will be significantly smaller than the actual switching time.

Material: SCE8 (1.8 μm thick layer at 25 ° C) Addressing method in Table 1, Figure 4 Vs Vd ts CR 50 5 36−53 8−7 50 7.5 46−115 45−15 40 10 46−88 77 −21.5 50 10 57−140 71− 9.5 Addressing method in Table 2 and Figure 5 50 7.5 40− 73 26−11 40 10 34− 57 64−23 50 10 47−100 67−17 Address in Table 3 and Figure 7 Specification method 50 5 44−280 17.5−5.4 50 7.5 62−225 62− 5 40 10 56−186 87− 5.8 50 10 69−213 79− 4.8 Address specification method in Table 4 and Fig. 11 (single pulse) 50 5 65 −450 23− 3 50 7.5 75−480 65− 2.2 40 10 95−345 49− 2.7 50 10 83−370 63− 2.3 Material: B (1.7 μm thick layer at 30 ° C) Table 5 and Fig. 4 Addressing method Vs Vd ts CR (at minimum ts) 50 10 22− 78 51 50 7.5 17− 82 33 40 10 16− 47 56 Addressing method in Table 6 and FIG. 5 50 10 20− 68 51 50 7.5 14− 62 24 40 10 13− 36 53 40 7.5 10− 37 7.2 45 7.5 10− 42 10 Addressing method in Table 7, Fig. 7 50 10 24− 80 52 50 7.5 19− 98 35 40 10 18−66 68 Table 8, Addressing Scheme of FIG. 4 at Various Temperatures 50 10 39−123 48 25 ° C 50 10 21− 73 59 30 ° C 50 10 12−43 58 35 ° C 50 10 7−25 26 40 ° C 50 10 5−10 5 45 ° C Table 9, Addressing method of FIG. 5 at various temperatures 50 10 18− 64 52 30 ° C 50 10 8−20 13 40 ° C 50 10 8− 37 44 35 ° C 50 10 35− 120 48 25 ° C Table 10 and 30 ° C addressing method of Fig. 11 (single pulse) 50 10 28−93 47 50 7.5 24−148 33 40 10 32−120 44 Material: A (1.7 μm thick layer, 30 4) 10 11 39−100 46 50 10 59−120 26 Table 12, Addressing method of FIG. 5 40 10 33−85 48 50 10 52−110 30 Table 13, FIG. Addressing method 40 10 40−150 46 50 10 64−220 23 Addressing method in Table 14, Fig. 11 (single pulse) 40 10 56−150 32 50 10 66−300 22 Material: Merck Catalog No. 917 Temperature 30 ° C ; Vs = 60V; Vd = 15V Table 15 Addressing method Fig. 11 Fig. 4 Fig. 5 Fig. 7 Shortest slot Time (μs) 27 15 12 17 Longest slot time (μs) 116 37 28 70 Operating range (hours) 4.3X 2.5X 2.3X 4.1X Contrast ratio (CR) 41 84 80 76 Whiteness (%) 64 63 60 63 Operation Range: longest slot time / shortest slot time Whiteness (%) is a comparison with no cell between parallel polarizers.

Material: RSRE A206 (Temperature 30 ° C, Vs = 30V, Vd = 10V) Table 16 Address Figure 11 Figure 4 Figure 5 Minimum slot time (μs) 60 27 20 Operating range (time)> 2X 2.6X 2.2X 1X Contrast ratio ( CR) 14 48 55 Whiteness (%) 77 67 60 Material RSRE A206 = AS500: A151 1: 1 + 5% Dope AS500 contains the following components A151 contains the following components Doping agent 2% chiral 3% racemic "*" represents chirality, and without this "*" the material is racemic.

In nematic liquid crystal devices, it is known to reduce peak row and column voltages by applying additional waveforms to both row and column electrodes.

For example, FIGS. 23 and 24 illustrate two different schemes for reducing the peak voltage of the prior art single pulse drive system of FIG.

In FIG. 23, the strobe (row) waveform is "zero over 1.ts" and "+ Vs positive pulse over 1.ts" in its first field, followed by In the second field, "zero over 1.ts"
And "-Vs negative pulse over 1.ts". Additional waveforms include a positive pulse of Vs / 2 over the first field,
Subsequently, -Vs / 2 over the second field. The resulting strobe waveform varies between Vs / 2 and -Vs / 2 as shown. Data (column)
The waveform is alternating Vd and Vd, each lasting for 1.ts. The additional waveform added to each column is Vs / 2 over the first field, followed by -Vs / 2 over the second field time. The resulting data waveforms are Vd + Vs / 2 and-(Vs / 2 + Vd), as shown.
Vary between. The effect of this additional waveform is to reduce, for example, a peak voltage of 50 volts to 35 volts.

An alternative to FIG. 23 is shown in FIG. As mentioned above, a normal strobe pulse is "zero over 1.ts" and "+ Vs over 1.ts" during its first field time, followed by: Within that second field time, "zero over 1.ts" and "1.ts
-Vs over the range. Additional waveforms are a 2.ts period rectangular waveform applied over the first field time,
Subsequent inverse rectangular waveforms over a second field time, each of which varies between Vs / 2 volts and -Vs / 2 volts. The resulting strobe (row) waveform is as shown. Similarly, the data (column) waveform is also a rectangular waveform that changes between + Vd and -Vd. The additional waveform is identical to the additional waveform applied to the row electrode. The resulting data (column) waveform is as shown, Vd + Vs / 2 and-(Vs
/ 2 + Vd). Again, this additional waveform may be required by the display drive, for example 50
Reduce the peak voltage of volts to 35 volts.

23 and 24 are the same as those in FIGS.
Can be applied to the addressing method. This is shown in FIG.
FIG. 25 shows a modification of the above method. Within the first field time, a 9 zero strobe pulse over 1.ts is followed by a Vs pulse over 3.ts. Within the second field time, a zero strobe pulse for 1.ts followed by a strobe pulse of -Vs for 3.ts. This strobe waveform is shown for row 1, row 2, row 3, row 4 of a four row display. Two different strobe waveforms are shown for row 4 for reasons described below. The additional waveform applied to the row electrodes (and column electrodes) is shown as Vs / 2 over the first field time, followed by -Vs / 2 over the second field time. The resulting row waveforms for row 1 are -Vs / 2 over 1.ts, in the first and second field times, and 3.
Vs / 2 over ts, -Vs / 2 over 4.ts, Vs / 2 over 1.ts, and -Vs / 2 over 3.ts. And Vs / 2 over 4.ts. The combined result of the strobe waveform and the additional waveform for the row represented as row 4a is shown as a waveform with large peak values of + 3Vs / 2 and -3Vs / 2. The reason for this is the extended strobe pulse length that overlaps into adjacent fields. To overcome this, row 4 is left hidden from view or
Or, as shown in row 4b, it is addressed by a strobe voltage of zero.
In a more practical example of a 128 row display, for example, the generated waveform would be programmed for a 128 row display, but only 127 rows are used in the scheme of FIG. For example, as shown in FIG.
If longer strobe pulses are used, one line, or even more, will be left unused. The waveform applied to the column electrodes is shown in FIG. Data 1 and its inverted data 2 are the same as in FIG. The additional waveform is Vs / 2 over the first field time, followed by -Vs / 2 over the second field time. The resulting column waveform is shown as varying between Vd + Vs / 2 and-(Vs / 2 + Vd). Thus, with respect to the scheme of FIG. 5, when Vs = 50 volts and Vd = 10 volts, the schemes of FIGS.
Reduce peak voltage to 35 volts.

────────────────────────────────────────────────── ─── Continuing the front page (72) Inventors Rains, Edward Peter UK, Dubriel R. 14.2. References JP-A-61-286819 (JP, A) JP-A-62-255919 (JP, A) JP-A-64-31129 (JP, A) UK Patent Application Publication 197742 (GB, A) (58) Field (Int.Cl. ⁷ , DB name) G02F 1/133 560

Claims (11)

(57) [Claims] 1. A ferroelectric liquid crystal display formed by a layer of tilted chiral smectic material (7) contained between cell walls (2, 3), wherein the smectic material comprises a voltage-time product and And the cell wall (2, 3) has a matrix of addressable elements (x,
carrying a first set of electrodes (5) on one wall (20) and a second set of electrodes (6) on the other wall (3) arranged to provide y). A method of multiple addressing a ferroelectric liquid crystal display, said method comprising adding two strobe waveforms of opposite polarity,
Or, by adding a blanking waveform followed by a strobe waveform with periodic polarity reversal to maintain a net zero DC value, each electrode in the first set of electrodes (5) is individually And one data waveform is the inverse of the other data waveform, and the period of those data waveforms, 2ts, is twice the period of a single strobe pulse, ts, in the strobe waveform. Applying one of two alternately positive and negative data waveforms (D1, D2) to each electrode in the second set of electrodes (6) in synchronization with the strobe waveform. The combination of one strobe waveform and one data waveform causes switching of the liquid crystal material, thereby addressing the intersection of selected electrodes during a period equal to the data waveform period, but the intersection of unselected electrodes. To The combination of said one strobe waveform and the other data waveform does not cause switching of the liquid crystal material during a period equal to the data waveform period, and the strobe waveform ends at the next electrode (5) in the first set of electrodes. ), While maintaining the same period, 2ts, between the application of the strobe waveform to successive electrodes (5), so that the intersection of each electrode is greater than 2ts The address of each electrode is selectively addressed during time and the intersection of each electrode is addressed by a composite waveform of the appropriate sign, magnitude and time so that that intersection (x, y) requires an overall net zero DC value. The method further comprising the step of: 2. The method of claim 1, wherein the strobe waveform has a zero voltage over an initial ts time period, a non-zero voltage over a period greater than ts, and a subsequent zero voltage representing one frame period. 2. The method according to claim 1, wherein there are several periods ts of, followed by a similar waveform of inverted polarity. 3. The method according to claim 1, wherein the strobe waveform has a first time period ts.
Wherein the non-zero voltage is lower than a subsequent voltage and the amplitude of the non-zero voltage is variable to compensate for the switching characteristics of the material with temperature changes. Item 1. The method according to Item 1. 4. A method according to claim 1, wherein said first set of electrodes and said second set of electrodes both have a reduced peak voltage applied to said electrodes. The method of claim 1, wherein additional waveforms are added. 5. The blanking pulse has two parts of opposite polarities, and the voltage-time product (Vt) of the blanking pulse is equal to the strobe pulse to produce a net zero DC value. 2. The voltage-time product of claim 1
The method described in. 6. A multi-addressed liquid crystal display, comprising: a liquid crystal cell (1) formed by a layer (7) of liquid crystal material contained between two cell walls (2, 3); The liquid crystal material (7) is a graded chiral smectic material having a negative dielectric anisotropy and having switching characteristics determined by both the voltage-time product and the shape of such a product, the cell wall ( 2, 3) carry a first set of electrodes (5) on one wall (2) and a second set of electrodes (6) on the other wall (3), said electrodes ( 5, 6) are arranged so as to collectively form a matrix (x, y) of addressable intersections, wherein at least one of said cell walls (2, 3) is for liquid crystal molecules along a single direction. Said liquid crystal cell (1) being surface treated to provide a surface alignment; A first set of electrodes (5) driver for applying a strobe waveform in sequence to each electrode of the (10), said second set of data waveform to the electrode (6) (D1, D
A driver (11) for adding 2), a waveform generator (12, 13) for generating a strobe waveform and two data waveforms (D1, D2) for adding to the drivers (10, 11). Controlling the order of the data waveforms applied to said second set of electrodes (6) when the strobe waveform is applied sequentially to each electrode (5) in said first set of electrodes (5). (1
4) means for obtaining a required display pattern, and a means for making the time interval at which the strobe waveform is sequentially applied to each electrode (5) equal to the data waveform period; 2 which is the opposite sign
A data waveform generator (13) for generating three sets of data waveforms, each of said data waveforms comprising alternating-sign DC pulses having a period of 2 ts; a first pair of pulses of different amplitudes and different amplitudes; A second pair of pulses of opposite polarity to that of the first pair of pulses, or a blank followed by a pair of pulses of different amplitudes, with periodic polarity reversals to maintain a net zero DC value. And the end of the last pulse of the strobe waveform pulse pair has a duration> ts greater than half the data waveform period 2ts, and the address period of the next electrode (5) And a strobe waveform generator (12) extending up to 2 ts, with a sign and size appropriate to bring the intersection to the required display state once per full display addressing period, and It said display each of the intersection has a structure as addressed by the pulse is a net zero dc value as the body. 7. The strobe waveform generator (12) generates a zero voltage for an initial time period ts, followed by a non-zero voltage for a time period longer than ts, 7. The display of claim 6, further comprising generating several periods of zero voltage representing one frame period, and subsequently generating a similar waveform of inverted polarity. And means for generating an additional waveform and applying said additional waveform to said first set of electrodes and said second set of electrodes. The display according to claim 6, comprising: 9. The display according to claim 6, further comprising a temperature sensor for detecting the display temperature, and means for varying the addressing voltage. 10. The strobe waveform generator (12) is adapted to generate a non-zero voltage over an initial time period ts, wherein the non-zero voltage is lower than a voltage immediately following it, and 10. The display of claim 9, wherein the amplitude and sign of the non-zero voltage are variable to provide compensation for material switching characteristics with temperature changes. 11. The strobe waveform generator (12) is adapted to generate a blanking pulse having two portions having opposite polarities, and a voltage-time product (Vt) of the blanking pulse. 7. The display of claim 6, wherein is combined with a voltage-time product of the strobe pulse to produce a net zero DC value.