SE515073C2 - Method for driving liquid crystals - Google Patents

Abstract

A method for changing a reflectance state of a row and column array of bistable liquid crystals making up picture elements of a liquid crystal display is provided. The method is based on the insight, that less power is consumed when driving bistable cholesteric liquid crystals if the picture of the display is analysed and the driving adapted to the nature of the picture. Thus, the number of desired state changes between picture elements of adjacent rows and in the same column is counted and the polarity of the control voltage is adapted thereto.

Description

70 75 20 25 30 35 515 D75 f _ 2 _ Fig. 1 shows the reflectance as a function of applied voltage.

When applying voltage pulses with a square shape, there are four important voltage thresholds: V1, V2, V3 and V4. The figure shows two initial states, focalconic state and planar state. Voltages below V1 do not affect the crystal.

When the voltage rises to V2, the crystal approaches the focal conical state, ie with low reflectance (black), regardless of the initial state. focal calcium state after the pulse. Between V3 and V4 increases Between V2 and V3 the crystal enters the reflectance to the planar state. Typical values for the voltages are: V1 = 6 volts, V2 = 15 volts, V3 = 30 volts and V4 = 40 volts. To put the crystal in the reflective (green) state, a voltage pulse of about 40 volts is thus required. To put the crystal in the non-reflective (black) state, a voltage pulse of about 15-30 volts is required.

The basic difference between driving crystals off (TN) compared to BCLCs is that these BCLCs need to be refreshed type twisted nematic and super twisted nematic (STN) or updated only when the screen image changes. Between changes, BCLC monitors do not consume any energy at all.

(LCD module), which is operated as a passive matrix consisting of m rows and n pcs Fig. 2 shows a liquid crystal display module columns. Thus, the module m * n has pixels (pixels), where m and n are integers greater than zero. Each pixel is in contact with a row and a column driver, which generate the relatively high voltages required to change the polarity of BCLCs.

PCT Publication No. WO 98/55987 discloses a method and circuit for operating a monitor consisting of BCLCs. However, one purpose of the described method and circuit is to provide a video speed update of moving images, slow, ie energy consumption is not taken into account. OBJECT OF THE INVENTION An object of the present invention is to provide a method for driving car-stable cholesteric liquid crystals which is less energy-intensive than known methods.

Another purpose is to provide an apparatus which realizes the energy-saving method.

The invention in summary The invention is based on the insight that a less energy-intensive method can be achieved for driving bistable cholesterol liquid crystals if the screen image is analyzed and the drive is adapted to the nature of the image.

According to the invention there is provided a method of changing the reflectance state of a row and column arrangement of bistable liquid crystals constituting pixels in a liquid crystal display, the pixels having a reflective and a transparent state, the method comprising the following steps: arranging voltage control address relation to the pixels for applying a control voltage across controlled pixels during control periods, the control voltage being the difference voltage between a row and column voltage, and the invention being characterized by the following steps: determining the next desired state of the pixels; count the number of changes of the desired state between the pixels in adjacent rows and in the same column; determine whether there are more state changes than there are identical states; in case there are more state changes than there are identical states, the control voltage applies a polarity change at the beginning and substantially in the middle of each_rad line control period, and in case there are more identical states than there are state changes, the control voltage applies a polarity change only substantially in the middle of the control period. An apparatus for carrying out the method according to the invention is also provided together with a display, which consists of such an apparatus.

Preferred embodiments are set out in the dependent claims.

Brief Description of the Drawings The invention is described below by way of example with reference to the drawings, Fig. 1 Fig. 2 Fig. 3 Fig. Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. Fig. In which: is a diagram showing the reflectance of BCLCs as a function of the applied voltage ; is an overview view of an LCD module including drive stages; is an equivalent circuit to a liquid crystal display; 4a and 4b are diagrams showing an RC circuit used as a model for a pixel of a liquid crystal display before and after polarity change, respectively; is a diagram of the different voltage levels used to drive a liquid crystal display; is a diagram showing an example of driving a 4x4 matrix of liquid crystal pixels; is a curve diagram showing the driving curve, which is used to drive a contentless image according to model A and a checkered image according to model B; is a curve diagram showing the driving curve, which is used to drive a checkered image according to model A and a meaningless image according to model B; and 10 is a graph showing the driving curve used to drive a contentless image or a checkered image according to model C; ll is a graph showing the drive current, which is used to drive a contentless image according to model D and a checkered image according to model E; and 12 is a graph showing the drive curve used to drive a checkered image of model D and a blank image of model E. Detailed Description of the Invention The following is a detailed description of a preferred embodiment. of the invention. A model of a liquid crystal is shown in Fig. 3. In simplified form, the crystal can be considered as a capacitance in series with a resistance. Typical values for a pixel in a monitor are then R = 10 kohm and C = 1 nF.

Since the energy consumption of an individual pixel is of crucial interest in determining the energy consumption of a liquid crystal display, this pixel consumption will be described below. The energy for charging a capacitor in an RC circuit is calculated as follows (see Fig. 4a); uc = E (1__ ef / Rc) ut = E_ e-uxc E. -I / RC Losses in R are calculated as follows: Ere-zx / Rc Pr = '2 t ~ R =? - R () 1 ( ) Rz <==> w 2_ -zunc 2 °° 2 2 W = pmmt:;, _ E ß dt = -ä.i 0 o RR 2 Û 2 2 Energin i C är dà: R °° ww _ -a / Rc 2 °° w = jpmdt = Jin) - imdt = jan _ e - = 'R °> E_e-dt = [í (-11c.e ~ = f * = ° ifåí. E- * ffwn 0 0 0 2 2 R 2 2 The total energy consumed W is EZC.

The consumed energy for polarity change of a capacitor in an RC circuit, see Fig. 4b, is calculated in the following way with the initial value uc = -E at t = 0. 70 15 20 25 30 515 073 '"_6_ Il u E 1- ze- ”RÜ C u = zE-eI / RC l '_ -z / RC icozå: 2E I: Losses in R: W e ïPmcu alu -i (r) <1t = cja fl zwfzmwdf = _ 452 .Bg e-” RC w = 2E2c 0 0 r 0 RR 2 Total energy consumed: === ww 2_ -z / Rc 2 2 “° W = [P (:) ar = jE-1 (:) dt = jE --_-- 2E ° d : = - E (Rc-e- "RÜ = 2E2c 0 0 ORR 0 Charging a capacitor requires energy W = Q * U. Half of this energy is stored in the capacitor. The losses are always 50% regardless of the value of R. When polarity changes, the energy is consumed W = 2 * Q * U. All this energy disappears as losses in R.

The term EP for "energy package" is used when comparing consumption for different methods of operating a liquid crystal display. An EP is equal to the product QU = CU2. In the examples described below, C is the sum of the capacitances constituted by the crystals in a shield row.

All liquid crystals must be operated with alternating current to avoid ionic drift in the crystal, which eventually destroys the IAPT drive (IATP = Pleshko technique) utilizes 5 voltage levels and the ground crystal. The s.k. Improved Alt- friend and described in a publication by Scheffer, T (1997): "Addressing of Passive Matrix, RMS Responding Liquid Crystal Displays", Handbook of Liquid Crystal Research, Oxford pp. 445-470, cleans. The relationship between the levels and the input signals are shown University Press, which is hereby introduced as a graphical figure in Fig. 5. 70 75 20 25 515-073 get -7- Row drive stages output one of the voltages VDDH, V1, V4 or earth, and column drive stages output one of the voltages VDDH, V2, V3 and earth The voltages used here are: VDDH = 4O V V1 = 35 V V2 = 3O V V3 = 10 V V4 = 5 V The row and column drive stages are controlled by signals DATA row, DATA column and an image field signal (FR) This makes it possible to generate eight different voltage differences (black arrows) which normally have three different amplitudes.The voltage differences are divided into two groups of four voltage differences, the groups having different polarity.Fr ~ signal performs polarity change by changing group.Table I shows the resulting voltages as a result of the above control signals.

Tabe fl l FR HHHHLL DATAkmumn HHLLHH DATA line HLHLHL Kdumnmwm fi nmng VDDH VDDH V2 V2 EARTH EARTH V3 V3 Frame fl wq fi nmng EARTH V1 EARTH V1 VDDH V4 VDDH V4 No green 5-result 30-voltage 30 green none black none Here, H and L denote high and low signal, respectively.

Table I shows that changing the FR signal only changes the polarity of the resulting voltage, not its amplitude. This is fundamental to understanding the present invention.

Fig. 6 shows an example of drive voltages for monitor drive with the FR signal low. The number of pixels is 4 * 4 = 16.

In the example of Fig. 6, all pixels are initially in the planar state. All possible voltage transitions are described in Table 515 075 '_8_ II below, where the levels are written in the form of row level: column level and EA means "not addressed" and A means "addressed": Table II FR transition state before state after polarity change? voltage change diff 10 75 20 25 30 35 EA green EA green LH V4, earth V1, VDDH yes (5 to -5) -10 HL V1, VDDH V4, earth yes (-5 to 5) 10 LL V4, earth V4, earth HH V1, VDDH V1, VDDH EA black EA black LH V4, V3 V1, V2 yes (-5 to 5) 10 HL V1, V2 V4, V3 yes (5 to -5) -10 LL V4, V3 V4, V3 HH V1, V2 V1, V2 EA green EA black LH V4, ground V1, V2 HL V1 VDDH V4, V3 LL V4, ground V4, V3 yes (5 to -5) -10 HH V1, VDDH V1, V2 yes (-5 to 5) 10 EA black EA green LH V4, V3 V1, VDDH HL V1, V2 V4, ground LL v4, vs v4, ground yes (-5 rm s) 10 HH V1, V2 V1, VDDH yes (5 to -5 ) -10 EA green A green LH V4, soil soil, VDDH yes (5 to -40) -45 HL V1, VDDH VDDH, soil yes (-5 to 40) 45 LL V4, soil VDDH, soil (5 to 40) 35 HH V1, VDDH earth, VDDH (-5 to -40) -35 EA black A black LH V4, V3 earth, V2 (-5 to -30) -25 HL V1, V2 VDDH, V3 (5 to 30) 25 LL V4, V3 VDDH, V3 yes (-5 to 30) 35 HH V1, V2 earth, V2 yes (5 to -30) -35 EA green A black LH V4, earth earth, V2 yes (5 to -30) - 35 HL V1, VDDH VDDH, V3 yes (-5 to 30) 35 10 15 20 25 30 35 LL HH LH HL LL HH LH HL LL HH LH HL LL H ~ H HL LL HH LH HL LL LH HL LL HH LH LL V4, earth V1, VDDH EA black V4, V3 V1, V2 V4, V3 V1, V2 A green VDDH, soil soil, VDDH VDDH, soil soil, VDDH A black VDDH, V3 soil, V2 VDDH, V3 soil, V2 A green VDDH, soil soil, VDDH VDDH, soil soil, VDDH A black VDDH, V3 soil, V2 VDDH, V3 soil, V2 A green VDDH, soil soil, VDDH VDDH, soil soil, VDDH A black VDDH, V3 soil, V2 VDDH, V3 515 073 VDDH, V3 soil, V2 A green soil, VDDH VDDH, soil VDDH, soil soil, VDDH EA green V1, VDDH V4, soil V4, soil V1, VDDH EA black V1, V2 V4, V3 V4. V3 V1, V2 EA black V1, V2 V4, V3 V4, V3 V1, V2 EA green V1, VDDH V4, soil V4, soil V1, VDDH A green soil VDDH VDDH, soil VDDH, soil soil VDDH A black soil V2 VDDH, V3 VDDH, V3 9 ._ (s nu 30) (-s nu -30) (-s nu -40) (s nu 40) (-s nu 40) (s nu _40) (40 nu -s) (- 40 nu s) (40 nu s) (40 nu -s) (30 nu s) (-30 nu ss) (30 nu -s) (-30 nu s) (40 nu ss) (-40 nu -s) (40 nu -s) (-40 nu ss) (30 nu -s) (-30 nu ss) (30 nu ss) (-30 nu -s) (40 nu _40) (40 nu 40) (30 more 30) (-30 to 30) 80 60 10 15 20 25 30 35 515 073 _ l 0 _ jH-H soil, V2 soil, V2 A green A black LH VDDH, soil soil, V2 yes (40 to -30) - 70 HL soil, VDDH VDDH, V3 yes (-40 to 30) 70 LL VDDH, soil VDDH, V3 (40 to 30) -10 HH soil, VDDH soil, V2 (-40 to -30) 10 A black A green LH VDDH, V3 soil VDDH yes (30 to -40) -70 Hl. soil, V2 VDDH, soil yes (-so: in 40) 10 LL VDDH, V3 VDDH, soil (30 to 40) 10 HH soil, V2 soil, VDDH (-30 to -40) -10 Depending on the image written will the voltage across each pixel to be unique to each image. This is very important when carrying out the method according to the invention. The voltage sequence of each pixel is made up of the above-mentioned eight differential voltages.

To prevent ion drift, the drive curve must change polarity at least once every row. If this requirement is not met, no other level changes are preferably performed. Very large power is consumed due to unnecessary level changes and the capacitor discharge. Short-circuiting the capacitor before the polarity change saves 50%, since a polarity change requires 2 EPs while charging requires only one EP.

When operating a monitor, the line that is written does not require the most energy, but the others, because there are so many more.

Assuming that a row of 200 consumes ten times as much as the others, the energy consumption of the fear in question becomes only 5% of the total energy consumption. You thus gain the most from optimizing the consumption of the lines that are not being written.

When studying the voltage transitions for pixels that are not addressed, it can be seen that a polarity change can be obtained in two different ways, either by changing the FR signal or by changing the pixel content, ie from green to 10 75 20 25 30 35 515 073 * _11- black or vice versa. This means that a change of the pixel content at the same time as a change of the FR signal does not result in a polarity change. Patterns with many changes of pixel content, such as grid patterns, can thus be "neutralized" by the FR signal. Again with reference to the example in Fig. 6, the energy consumption will be further explained. When writing in line 1, the 2-4 pixel lines of the lines have the voltage plus or minus five volts, ie their current state does not change. They are thus disabled for writing. However, the pixels of the first and fourth columns have negative voltage, while the pixels of the second and third rows have positive voltage. This depends on the voltage of the column drivers, ie which colors are currently written to the first line.

When writing in the second line, the lines disabled for writing have changed for columns 1 and 3. This is because the color of pixels in these columns changes from line 1 to line 2. When writing on the third line, three columns change polarity, that is, three pixels in row 3 have changed color compared to the color of pixels immediately above row 2. When finally writing takes place in the fourth row, only one column changes polarity because only the pixel contained in column 1 of row 4 has a color that deviates from the color in the same column in row 3.

Each time a column changes polarity, the voltage of the respective pixel deactivated changes by ten volts corresponding to 2 EP. It can be mentioned that all pixel voltages are zero between writing one image field and the next. This means that when writing in the first line, all columns consume one EP.

Below is a description of different types of drive curves and their energy consumption. The number of energy packets (EPs) required to drive an image depends on the nature of the image, as indicated above. The energy consumption is calculated for 70 75 20 25 30 35 515 073 in «_12- two extreme cases: when the image is completely empty, ie when all pixels are either black or green, and when these pixels change content from each line to the next, as in a checkered image or an image with horizontal lines one pixel wide. All other cases lie somewhere between these two extreme cases.

In the description below, "n" denotes the number of lines written, and "EP" means energy packets.

Model A: Model A is a conventional drive method that changes the polarity twice per row. Initially, FR is always high and becomes low at "half time", ie after half the line update period. The FR signal thus has the same position for each line.

Fig. 7 shows the voltage over for writing deactivated pixels for model A, ie a square voltage with an amplitude of five volts. Initially, ie before the first line is written, the voltage is always zero volts, as mentioned above.

The EP consumption is shown in the diagram. The first EP is consumed when the pixel in question is charged to 5 volts, positive or negative. Then the polarity of all pixels changes, consuming 2 EP. The writing of the first line thus consumes 3 EP. When the second line is written, the polarity changes twice, at the beginning and at half time, consuming four more EPs. Each line after a first consumes four EPs.

The energy consumption of a contentless image then becomes W = [3 + 4 (n-1)] * EP. For 200 lines, this amounts to 799 EP. For the checkered pattern, this energy consumption is W = [3 + 2 (n-l)] * EP. For 200 lines this amounts to 401 EP, see fig 8. 70 15 20 25 30 35 515 0.73 - ~ _13 ..

Model B: In Model B, the polarity changes once per line. This is achieved by changing the FR signal only at "half time" and not at the beginning of each line. The exception is the first line, where the pixel in question is charged from a grounded output state. The number of energy packets consumed to [3 + 2 (n-l)] * EP. 200 lines consume 401 EP. This curve shape is identical to writing the contentless image is then W = the curve shape with checkered pattern and model A (see Fig. 8).

For the checkered pattern, the energy consumption is W = [3 +4 (n-l)] * EP. 200 lines consume 799 EP. This curve shape is identical to the curve shape with a contentless image and model A (see Fig. 7).

In the three models C, D and E below, the capacitor is discharged between each polarity change, ie another energy-saving measure is used.

Model C: The C model uses return to earth between changes.

The sausage shapes are changed from models A and B to curve shows 9 driving for a merna in Figs. 9 and 10, of which Fig. Is empty image and Fig. 10 shows driving for a checkered picture. The number of EPs required for the non-stop image and the checkered one is W = requires 400 EPs. writing of the inne- 2n * EP. 200 rows In Fig. 10, however, there are unnecessary discharges on two occasions, namely at the beginning of the second and subsequent rows. If these are removed, model D is obtained.

Model D: Model D has only one polarity change per row and return to ground at "half time". The number of energy packets required to write a meaningless image is then (W = [2 + (n-1)] * EP. 200 lines require 201 EP, see fig 11. 70 75 20 25 30 515 073 -i _14- For the grid pattern is the energy consumption W = [2 + 3 (nl)] * EP 200 lines require 599 EP, see fig 12.

Model E: Model E has two polarity changes per row and return to ground at "half time". writing a blank image is W = The number of EPs required to [2 + 3 (n-1)] * EP. 200 lines require 599 EP. This curve shape is identical to the curve shape with a checkered image and model D, see Fig. 12. [2 + (n-1) 1 * EP. 200 lines require 201 EP. This curve shape is identical to For the grid pattern, the energy consumption is W = the curve shape with a contentless image and model D, see fig ll.

The energy consumption of the various models A-E is summarized below in Table III.

TabeHlH Image type Model A Model B Model C Model D Model E hmehäßßs 799EP 401EP 400EP 201EP 599EP Checkered 401 EP 799 EP 400 EP 599 EP 201 EP Table III shows that the energy consumption of model C is independent of image type. To minimize energy consumption, one must also know the nature of the image, ie the type of test images, meaningless and checkered, which the image in question is most similar to. However, models D and E are always preferable to models A and B, since another power consumption measure has been introduced there, ie the discharge.

In the method according to the invention, in a first step the image to be written is analyzed and accordingly the drive method A-E is selected.

Alternatively, each row is analyzed, and the optimal drive method for this particular row is used only for this row.

With line-by-line optimization, the maximum energy consumption for models A and B is 600 EP and for models D and E 400 EP.

Tests have shown that almost all images benefit from being operated with methods B or D. The savings are about 45% and 72% for typical screen images. In a few cases, however, line-by-line optimization provides a significant advantage over the more uncomplicated optimization of the entire image.

By using the methods of the invention, a current saving of about 200 uA at 40 volts, corresponding to 1.6 mA at 5 volts, can be significant. In today's applications, where every energy saving that is dragged on for a few seconds does not seem to count, however, this reduction in energy consumption is not negligible.

An apparatus for realizing the method according to the invention comprises a conventional LCD drive module with electronic circuits, which implements the method according to the invention.

No special arrangements need be made other than an image analysis body.

The present invention can be varied within the scope defined by the claims below. Thus, reference has been made here to rows and columns. One skilled in the art will recognize that these expressions may be interchangeable.

Cholesterol crystals have been described in connection with the method of the invention. Any crystal exhibiting similar properties can be used in connection with the present invention.

Claims (8)

10 15 20 25 30 35 5145 075 _ 15 _ Patent claim A method for changing the reflectance state of a row and column arrangement of bistable liquid crystals constituting the pixels of a liquid crystal display, the pixels having a reflective and a transparent state, the method comprising the steps of: arranging voltage control addressing electrodes the pixels for applying a control voltage across controlled pixels during control periods, the control voltage being the difference voltage between a row and column voltage, characterized by the following steps: - determining the next desired state of the pixels; count the number of changes of the desired state between the pixels in adjacent rows and in the same column; - determine whether there are more state changes than there are identical states; - in the event that there are more state changes than there are identical states, the control voltage causes a polarity change at the beginning and substantially in the middle of each row control period, - and in the case there are more identical states than there are state changes, the control voltage has one pole change in size only substantially at the middle of the control period. Method according to claim 1, characterized by the further step of discharging the picture elements at the beginning and substantially at the middle of each control period. Method according to claim 1, characterized by the further step of discharging the image before each polarity change of the control voltages. Method according to one of Claims 1 to 3, characterized in that the crystals consist of bistable, cholesteric liquid crystals. 10 75 20 515 0.73 _17- Method according to one of Claims 1 to 4, characterized in that the method is applied line by line. Method according to one of Claims 1 to 4, characterized in that the method is applied once for the entire image. Apparatus for changing the reflectance state of a row and column arrangement of bistable liquid crystals constituting the pixels of a liquid crystal display, said pixels having a reflective and a transparent state, comprising row and column drive stages connected to voltage control addressing electrodes in relation to the picture elements for applying a control voltage over controlled picture elements during control periods, characterized in that the row and column drive stages are arranged to perform a method according to one of the preceding claims. Display comprising a row and column arrangement of bistable liquid crystals, which constitute picture elements having a reflective and a transparent state, characterized in that the display comprises an apparatus according to claim 7.