Switched mode digital logic method, system and apparatus for directly driving LCD glass

[0002] The present invention relates generally to liquid crystal displays (LCDs), and more particularly to a system, apparatus and method for directly driving LCD glass with a switched mode digital logic circuit.

[0003] Liquid crystal displays (LCDs) are commonly used in consumer electronics such as digital thermostats, alarm control panels, sprinkler system control panels, alarm clock radios, kitchen and laundry appliances, etc. LCDs act in effect as light valves, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission of light. The LCD comprises a thin layer of “liquid crystal material” deposited between two plates of glass, and is often referred to as “glass.” Electrodes are attached to the inside (facing) sides of the plates of the glass. One electrode is referred to as common or backplane, and the other electrodes making up alpha-numeric and/or graphical images on the LCD are referred to as segments or pixels. Segments and pixels will be used herein interchangeably and will designate the LCD electrodes closest to the viewing surface of the LCD, e.g., between the backplane electrode(s) and the front of the LCD.

[0004] The LCD operates by applying a root mean square voltage (V_RMS) between the backplane electrode and the pixel electrode. When a V_RMSlevel of zero volts is applied to the LCD, the LCD is substantially transparent. To turn a LCD pixel “on,” which makes the pixel turn dark or opaque, a V_RMSlevel greater than the LCD threshold voltage is applied to the LCD. Different LCD material have different characteristics but all have in common a minimum RMS voltage that produces 90% contrast, Von, and a maximum RMS voltage that produces 10% contrast, Voff. Contrast is maximized when the LCD pixel is at its darkest or most opaque.

[0005] Many LCDs are multiplexed, that is, they have multiple common lines (also called backplanes) for a given set of segment connections. The timing pattern of sequentially selecting all of the backplanes is called a multiplex frame. Since the commons must multiplex or time-share their LCD segment data on the segment lines, the instantaneous voltage across these segments must be increased. Most LCD driver applications use charge-pump circuits to boost the voltage across the LCD pixels; this technology along with resistor ladders, allow LCD glass to be driven by multiple voltage sources.

[0006] LCD drivers have a high voltage level, Voh, and a low voltage level, Vol. When an LCD has just one backplane, the RMS voltage between the backplane and the segment(s) would be equal to Voh−Vol of the drivers. This is true because all segment(s) of the LCD glass would be constantly driven all the time. But if there are multiple backplanes, all segments cannot be driven concurrently. In order to adhere to the RMS Von spec of the LCD, when a given backplane of a segment(s) must be driven, then a greater voltage must be applied thereto. This is the reason why charge-pumps are traditionally used when driving LCD glass.

[0007] Notice that RMS voltages are specified on LCDs. This is an important requirement; LCD's require zero DC offset. Even a small DC offset (usually greater than 50 mV) across any LCD material can damage that material. In order to keep the LCD glass ‘unpolarized’, all of the asserted signals applied to an LCD must be reversed continually. The polarities between the backplane(s) and segments are typically changed after every multiplex frame. So, each positive frame is followed by a negative one, etc.

[0008] Another technique used by LCD designers is multiple voltage levels, also known as bias. These bias voltages allow the asserted segments in a multiplexed LCD to be driven while the deasserted ones remain at a voltage too low to affect them. A ½ bias drive would consist of two voltage levels above ground; or, Voh, and a mid-level voltage. A ⅓ bias drive would have a fourth voltage level (e.g., two voltages between Voh and Vol). And, a ¼ bias drive would have three mid-voltage levels equally spaced between Voh and Vol. And so on for other bias ratios. Charge pumps are often also used to generate a greater supply voltage which, through the use of resistor ladder networks, create the desired middle voltages.

[0009] The asserted common line is at either Voh or Vol (depending if this is a positive or negative multiplex frame). The segment lines are brought to either Voh or Vol so as to produce the segment pattern desired. The non-asserted common lines must also have a certain voltage value for proper operation of the LCD. If the voltage driven on them was Voh or Vol, some other (e.g., non-selected) pixels would be affected since the segment lines are being driven. The unused commons cannot be left floating because a DC bias can result on the deasserted ones (e.g., one leg of these capacitors are tied to the pixel lines being driven and the other legs are tied to a floating common). The solution is to bring the deasserted commons to a mid-voltage. This voltage must be such that the voltage across a deasserted segment is LOWER than Voff and the voltage across an asserted segment is HIGHER than Von. When there are multiple backplanes present, sometimes it is easier to implement this with higher order bias ratios (⅓, ¼, ⅕, ⅙ or more).

[0010] More detailed descriptions of LCD operation and technologies are disclosed in Application notes AN563 and AN658 by Microchip Technologies Inc., 2355 West Chandler Blvd., Chandler, Ariz. 85224-6199. These application notes are incorporated herein by reference for all purposes.

[0011] Because of consumer product cost constraints, ease of manufacture, miniaturization, improved reliability, etc., it is desirable for a digital circuit (e.g., microcontroller, microprocessor, programmable logic array (PLA), application specific integrated circuit (ASIC) and the like) to directly drive LCD glass. An added benefit would be the ability to directly drive LCD glass having a plurality of backplanes and be able to also control contrast of the LCD without additional hardware components or manual adjustments.

[0012] What is needed is a system, method and apparatus for directly driving LCD glass with digital logic while retaining the capabilities of using multiplexed multiple backplanes with associated pixels and, in addition, being able to control LCD segment or pixel contrast.

[0013] The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing hardware and software methods, and an apparatus for directly driving liquid crystal display (LCD) glass with a digital logic circuit (e.g., microcontroller, microprocessor, programmable logic array (PLA), application specific integrated circuit (ASIC) and the like). Software programs, firmware in EEPROM, mask ROM or a hardwired state machine, etc., may be used for control of the digital logic circuit. An exemplary software program for a microcontroller is attached hereto as “Appendix A” and is incorporated herein by reference for all purposes.

[0014] Advances in LCD technology allow the design engineer to specify lower voltage chemistries in their LCD displays and thus avoid using costly charge-pump circuits and power consumptive resistor ladders in their designs. This can be done via switch-mode techniques that need only a single supply voltage. The digital logic circuit has a plurality of digital outputs coupled to the backplane(s) and pixels of the LCD glass and functions as a “switched mode” LCD driver having the following features: 1) Substantially no DC bias of the LCD glass by continually reversing voltage polarity across the LCD material, 2) maintaining minimum refresh rate so as to avoid flicker, 3) the resultant RMS voltage across a deasserted pixel is less than Voff, and 4) the resultant RMS voltage across an asserted pixel is greater than Von.

[0015] Low power and voltage, e.g., 3.3 volts or lower, product applications using a microcontroller and an LCD will especially benefit from the present invention. For example, battery operated devices do not require power consuming charge pumps or resistor networks when using the embodiments of the present invention.

[0016] LCD contrast may be controlled by adjusting the time interval of the phases wherein all segments have no RMS voltage potential The longer the LCD segments remain at a zero potential, the lower the bias on all of the segments, hence contrast is lowered.

[0017] According to the present invention, different segments may have different contrast or shading. This is accomplished as follows, for a high contrast segment more phases are at Von for that segment. For a lower contrast segment, less phases are at Von.

[0018] The present invention is directed to an apparatus for driving a liquid crystal display (LCD), said apparatus comprises a liquid crystal display (LCD) having N backplanes and a plurality of segments; and a microcontroller having a plurality of digital outputs, wherein the N backplanes and the plurality of segments of the LCD are coupled directly to the plurality of digital outputs of the microcontroller. Wherein N is a positive integer number. The microcontroller and LCD may be adapted to be powered from a battery power supply. The LCD driving voltages are pulses from the microcontroller having voltage amplitudes substantially the same as a supply voltage of the microcontroller. The microcontroller drives the LCD according to an LCD driver program. The LCD driver program controls the microcontroller to produce a series of pulses from the plurality digital outputs coupled to the N backplanes and the plurality of segments of the LCD so as to control the LCD. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that there is a continually reversing voltage polarity across the LCD material. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that there is substantially no noticeable flicker of the LCD. The LCD driver program controls amplitudes and duration of the series of pulses from the microcontroller so that a resultant RMS voltage across an asserted one of the plurality of segments is greater than Von. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that a resultant RMS voltage across a deasserted one of the plurality of segments is less than Voff. The LCD driver program varies some of the duration's of the series of pulses from the microcontroller so as to control the segment contrast of the LCD. The LCD driver program varies some of the amplitudes of the series of pulses from the microcontroller so as to control contrast between the plurality of segments of the LCD. A temperature sensor may be coupled to the microcontroller, wherein the temperature sensor supplies ambient temperature information to the microcontroller so that the LCD operating parameters may be adjusted for changes in the ambient temperature.

[0019] The present invention is also directed to a system using a microcontroller and a liquid crystal display (LCD), said system comprising: a liquid crystal display (LCD) having N backplanes and a plurality of segments; a microcontroller having a plurality of digital outputs, wherein the N backplanes and the plurality of segments of said LCD are coupled directly to some of the plurality of digital outputs of said microcontroller; and a control program for controlling said microcontroller, wherein said microcontroller performs a function and controls said LCD. The LCD may display parameters and information relating to the function.

[0020] The function performed by the microcontroller may include, but is not limited to, control of temperature (thermostat), humidity, sprinkler, alarm and security system, alarm clock, timer, clothes dryer, washing machine, toaster, microwave, oven, cooktop, clothes iron, water heater, tankless water heater, solar heating, swimming pool, jacuzzi, answering machine, pager, telephone, intercom, caller identification, electronic address book, treadmill, stationary bicycle, exercise machine, torque wrench, depth gauge, scale, speedometer, automobile tire condition status, anti-skid and anti-lock brakes, fuel gauge, engine monitoring, operation of luminaries (lights) in a building, power load management, video cassette player, DVD player, uninterruptable power supply (UPS), dictaphone, tape recorder, MP3 music player, video game toy, calculator, personal digital organizer, etc.

[0021] The present invention is further directed to a method of operation for driving a liquid crystal display (LCD) having a backplane and a plurality of segments, said method comprising the steps of:

[0022] applying a high level to a backplane, a low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;

[0023] applying the high level to the backplane, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0024] applying the high level to the backplane, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0025] applying the low level to the backplane, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;

[0026] applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0027] applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0028] applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period; and

[0029] applying the high level to the backplane, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period.

[0030] The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:

[0031] a) applying a high level to an asserted backplane i of N backplanes, the high level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;

[0032] b) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0033] c) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0034] d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;

[0035] e) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0036] f) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0037] g) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0038] h) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of LCD segments for the second time period; and

[0039] i) incrementing i by 1 then repeating steps a) through h) until i=N.

[0040] The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:

[0041] a) applying a low level to an asserted backplane i of N backplanes, the low level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and a high level to deasserted ones of the plurality of segments for a first time period;

[0042] b) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0043] c) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0044] d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;

[0045] e) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0046] f) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0047] g) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0048] h) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of LCD segments for the second time period; and

[0049] i) incrementing i by 1 then repeating steps a) through h) until i=N.

[0050] The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:

[0051] a) applying a high level to an asserted backplane i of N backplanes, the high level to deasserted backplanes of the N backplanes, a low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;

[0052] b) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0053] c) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;

[0054] d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0055] e) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0056] f) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period; and

[0057] g) incrementing i by 1 then repeating steps a) through f) until i=N.

[0058] The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having a backplane and a plurality of segments, said method comprising the steps of:

[0059] a) applying a low level to an asserted backplane i of N backplanes, a high level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;

[0060] b) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;

[0061] c) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;

[0062] d) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0063] e) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;

[0064] f) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period; and

[0065] g) incrementing i by 1 then repeating steps a) through f) until i=N.

[0066] A technical advantage of the present invention is low cost and reduced number of parts.

[0067] Another technical advantage is operation at low operating voltage levels.

[0068] Another technical advantage is no external or additional parts are required to directly drive the LCD.

[0069] Still another technical advantage is correction of the LCD bias voltages based on temperature.

[0070] A feature of the present invention is directly driving LCD glass without resistor networks or charge pumps.

[0071] Another feature is software control of contrast and brightness of LCD segments.

[0072] Another feature is operation at low voltage and/or system voltage.

[0073] Another feature is adjustment of LCD bias voltages based on temperature.

[0074] Features and advantages of the invention will be apparent from the following description of the embodiments, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.

[0075] A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawing, wherein:

[0076] FIG. 1 illustrates a schematic block diagram of an exemplary embodiment of a directly driven LCD;

[0077] FIG. 2 illustrates an exemplary schematic timing diagram of an embodiment of the invention;

[0078] FIG. 3 illustrates an exemplary schematic timing diagram of another embodiment of the invention;

[0079] FIG. 4 illustrates an exemplary schematic timing diagram of still another embodiment of the invention;

[0080] FIG. 5 illustrates an exemplary schematic timing diagram of yet another embodiment of the invention;

[0081] FIG. 6 illustrates a schematic block diagram of an exemplary embodiment of a temperature compensated directly driven LCD; and

[0082] FIG. 7 illustrates a schematic block diagram of an exemplary embodiment of a system application using the microcontroller and having a directly driven LCD.

[0083] While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawing and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0084] The present invention is directed to a method, system and apparatus for directly driving liquid crystal display (LCD) glass with a digital logic circuit (e.g., microcontroller, microprocessor, programmable logic array (PLA), application specific integrated circuit (ASIC) and the like) running in a switched mode of operation. The invention comprises a digital circuit having a plurality of digital outputs coupled to backplane(s) and pixels of a LCD glass in combination with a software program so as to function as a “switched mode” LCD driver. Battery powered devices comprising a microcontroller directly coupled to an LCD have improved battery life and reduced costs because of simplification of the LCD driver circuits, according to the present invention.

[0085] Referring now to the drawing, the details of exemplary embodiments of the present invention are schematically illustrated. Like elements in the drawing will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.

[0086] Referring to FIG. 1, depicted is a schematic block diagram of an LCD system represented by the numeral 100. The LCD system 100 comprises an integrated circuit microcontroller 104 adapted for driving an LCD glass 102 without requiring intermediate circuitry between the microcontroller 104 and the LCD glass 102 (represented herein by backplane(s) 102aand segments 102b). The microcontroller 104 in combination with a software program is configured to function as a switched mode—adjustable duty cycle driver. The microcontroller 104 is coupled to backplanes 102aand segments 102bof the LCD glass 102. Digital outputs RA0 and RA1 of the microcontroller 104 are coupled to COM 0 and COM 1, respectively, of the backplanes 102a. Digital outputs RB0, RB1, RB2 and RB3 of the microcontroller 104 are coupled to SEG 0, SEG 1, SEG 2 and SEG 3, respectively, of the segments 102b. The microcontroller 104 is programmed to drive the LCD glass so that there is substantially no DC bias because the voltage polarities between electrodes (backplane(s) 102aand segments 102b) are being continually reversed but remain equal in magnitude, a minimum refresh rate is maintained to avoid flicker of the LCD, the resultant RMS voltage across a deasserted segment is less than Voff, and the resultant RMS voltage across an asserted segment 102ais greater than Von.

[0087] The discrete equation for RMS voltage is: $\begin{array}{cc}{V}_{\mathrm{RMS}}=\sqrt{\frac{1}{N}\sum _{n=1}^N{v}_n^2}& \mathrm{Equation}1\end{array}$

[0088] Referring now to FIG. 2, depicted is an exemplary schematic timing diagram of a 1:3 Voff biased switched mode embodiment of the present invention. Eight phases from the microcontroller 104, represented by Phase A₁through Phase H₁, sequentially drive the LCD glass 102 for each multiplex frame. The first four phases are the “Positive Multiplex Subframe” and the last four phases are the “Negative Multiplex Subframe.”

[0089] These sequence of eight phases are repeated for each backplane 102aon the LCD glass 102. The following steps are performed (and depicted in FIG. 2) in eight phases (A₁-H₁) for Backplane N and then repeated for each subsequently asserted backplane. Note that phase D₁and phase H₁have all segments 102bdriven with zero potential and that the time duration of these two phases is different than all the others. These may be seen in the following phase state descriptions:

[0090] Backplane N is ASSERTED & High

[0091] All other backplanes are DEASSERTED & High

[0092] Asserted segments are Low

[0093] Deasserted segments are High

[0094] Timeout period=t₁

[0095] Backplane N is ASSERTED & High

[0096] All other backplanes are DEASSERTED & Low

[0097] Asserted segments are Low

[0098] Deasserted segments are High

[0099] Timeout period=t₁

[0100] Backplane N is ASSERTED & High

[0101] All other backplanes are DEASSERTED & Low

[0102] Asserted segments are Low

[0103] Deasserted segments are Low

[0104] Timeout period=t₁

[0105] Backplane N is ASSERTED & Low

[0106] All other backplanes are DEASSERTED & Low

[0107] Asserted segments are Low

[0108] Deasserted segments are Low

[0109] Timeout period=t₂

[0110] Backplane N is ASSERTED & Low

[0111] All other backplanes are DEASSERTED & Low

[0112] Asserted segments are High

[0113] Deasserted segments are Low

[0114] Timeout period=t₁

[0115] Backplane N is ASSERTED & Low

[0116] All other backplanes are DEASSERTED & High

[0117] Asserted segments are High

[0118] Deasserted segments are Low

[0119] Timeout period=t₁

[0120] Backplane N is ASSERTED & Low

[0121] All other backplanes are DEASSERTED & High

[0122] Asserted segments are High

[0123] Deasserted segments are High

[0124] Timeout period=t₁

[0125] Backplane N is ASSERTED & High

[0126] All other backplanes are DEASSERTED & High

[0127] Asserted segments are High

[0128] Deasserted segments are High

[0129] Timeout period=t₂

[0130] These eight phases translate to the states of a ‘state machine’ programmed on the microcontroller. That is, there are eight states per backplane or 8N total states in the machine). The order of the aforementioned phase states is exemplary and it is contemplated and within the scope of the present invention that these states may be executed in any order so long as the resulting RMS voltages are the same, any phase state sequence is appropriate.

[0131] The waveforms depicted in FIG. 2 show these multiplex frame phases graphically along with the resulting voltage waveforms on four classes of pixels. These four classes of pixels are herein defined:

[0132] 1. Asserted pixels wired to the current asserted backplane signal

[0133] 2. Asserted pixels wired to a deasserted backplane signal

[0134] 3. Deasserted pixels wired to the current asserted backplane signal

[0135] 4. Deasserted pixels wired to a deasserted backplane signal

[0136] These four classes of pixels may be further defined as subsets of the set of all pixels on the LCD. Furthermore, the union of these four subsets equals the set of all LCD pixels. So, at each point in time in the waveforms of FIG. 2, the instantaneous voltage on every pixel in the LCD can be determined. Furthermore, all of these instantaneous voltages on a given pixel can be used to calculate the resulting RMS voltage of that pixel.

[0137] LCD contrast may be controlled by adjusting the time interval of the phases wherein all segments have no RMS voltage potential The longer the LCD segments remain at a zero potential, the lower the bias on all of the segments, hence contrast is lowered.

[0138] The voltage potential and polarity are dependent upon the relationship between the voltage levels applied to the backplanes and segments and the time durations thereof. As depicted in FIG. 2, the asserted segment(s) on Backplane N (I.-III.) is at a positive logic high for three phase intervals (Phases A₁, B₁and C₁) and is at a negative logic high for three phase intervals (Phases E₁, F₁and G₁). During Phases D and H the voltage potential is at a logic low (approximately zero volts). Therefore, the RMS waveform across the asserted segments and the Backplane N is RMS symmetrical and therefore leaves no DC component on the segments. Having a logic high on the asserted segments for three time periods, t₁, positive and three time periods, t₁, negative, assures that the active segment is at or above Von.

[0139] The deasserted segment(s) on the N Backplane (I.-IV.) is at a positive logic high for only one phase interval (Phase C₁) and is at a negative logic high for one phase interval (Phase G₁). During Phases A₁, B₁, D₁, E₁, F₁and H₁the voltage potential is at a logic low (approximately zero volts). Therefore, the RMS waveform across the deasserted segment(s) and the N Backplane is RMS symmetrical and therefore leaves no DC component on these segments. Since the logic high is only for one time period, t₁, positive and one time period, t₁, negative, the deasserted segment(s) of the N Backplane never go above Voff.

[0140] In a similar fashion to the deasserted segment(s) described above, the asserted segment(s) on the other backplane(s) (II.-III.) are at a logic high for only one time interval at a positive polarity (Phase A₁) and one time interval at a negative polarity (Phase E₁). Likewise, the deasserted segment(s) on the other backplane(s) (II.-IV.) are at a logic high for only one time interval at a positive polarity (Phase E₁) and one time interval at a negative polarity (Phase B₁). During the remaining time intervals, the voltage is at a logic zero. All RMS waveforms are symmetrical and thus there is substantially no DC component buildup on the segments.

[0141] As depicted in the waveforms of FIG. 2, the voltages on the backplane and segment lines are either Vol or Voh. Furthermore, since the LCD pixels are typically modeled as capacitors and so, require almost no current drive once charged, a CMOS device driving a pixel effectively generates 0 volts for Vol and V_DDvolts for Voh. This range of voltage, Voh−Vol is defined herein as Vr.

[0142] The potentials across the pixels can be Vr, 0, or −Vr. The first requirement for directly driving LCD's, that is, no DC bias on the pixels is met and can be verified visually by the waveforms of FIG. 2. For every positive pulse on a pixel there is a corresponding negative one. Therefore, the voltage across all pixel classes averages to zero. Note that the asserted pixels connected to the currently asserted backplane drives three times as long as the other pixel classes. This added amount of time is what makes the visual difference between an opaque pixel and a transparent one.

[0143] Where, the voltage v is measured over N intervals. Defining V_Loas the RMS voltage of a deasserted pixel, the following Equation 2 may be derived: $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\left(\frac{1}{T}\right)·\left[2{\mathrm{Nt}}_1\right]·V_r^2}& \mathrm{Equation}2\end{array}$

[0144] Where T is the total time interval of the multiplex frame, N is the number of backplanes, and Vr is +/− (Voh−Vol). Time interval t₂can be represented as a function of ti, via the use of a constant, C, so that:

[0145] t₂=C·t₁

[0146] From this T can be found as a function of t₁:

[0147] T=(6t₁+2t₂)·N=2N(3t₁+Ct₁)=2Nt₁(3+C)  Equation 3

[0148] Substituting this into Equation 2 yields: $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\frac{2{\mathrm{Nt}}_1{V}_r^2}{2{\mathrm{Nt}}_1\left(3+C\right)}}=\sqrt{\frac{1}{3+C}}·V_r& \mathrm{Equation}4\end{array}$

[0149] Similarly, V_Hican be calculated: $V_{\mathrm{Hi}}=\sqrt{\frac{1}{T}\left[6t_1{V}_r^2+2\left(N-1\right)t_1{V}_r^2\right]}$

[0150] Substituting the relation for T from Equation 3 and reducing yields: $V_{\mathrm{Hi}}=\sqrt{\frac{6t_1+2\left(N-1\right)t_1}{2{\mathrm{Nt}}_1\left(3+C\right)}}·V_r=\sqrt{\frac{6+2N-2}{2N\left(3+C\right)}}·V_r$

[0151] Simplifying: $\begin{array}{cc}{V}_{\mathrm{Hi}}=\sqrt{\frac{2+N}{N\left(3+C\right)}}·V_r& \mathrm{Equation}5\end{array}$

[0152] Equations 4 and 5 for V_Loand V_Hi, respectively, may be used to calculate the lower and upper possible voltages of any segment on the LCD glass according to the present invention.

[0153] Note that all the phases have the same time interval, t₁, except for phases D₁and H₁; the time interval for these phases is t₂. Also, note that during phases D₁and H₁, all of the backplane and segment lines are at the same voltage. This means that the potential across all segments is zero volts during these time intervals. Therefore, V_Loand V_Hican both be adjusted by varying the duration of time t₂. Using Equations 4 and 5, ‘operating points’ may be determine for the LCD drive by choosing an effective value for the constant C. Also the value of C determines the voltage setpoint, or bias, of Voff.

[0154] Setting V_Loequal to the LCD specification, Voff (the 10% operating point), in Equation 4, dividing both sides by V_r, and squaring both sides: $\frac{V_{\mathrm{off}}^2}{V_r^2}=\frac{1}{3+C}$

[0155] Rearranging, $\begin{array}{cc}3+C=\frac{V_r^2}{V_{\mathrm{off}}^2}& \mathrm{Equation}6\end{array}$

[0156] Solving for C, yields: $\begin{array}{cc}C=\frac{V_r^2}{V_{\mathrm{off}}^2}-3=\frac{V_r^2-3V_{\mathrm{off}}^2}{V_{\mathrm{off}}^2}& \mathrm{Equation}7\end{array}$

[0157] One of the requirements stated herein for driving an LCD is that a minimum refresh rate be maintained on the LCD to avoid flicker. Since C in Equation 7 determines the relationship between t₁and t₂, and LCD manufacturers specify an operating frequency for their LCD glass, t₁and t₂may be calculated: $\begin{array}{cc}{t}_1=\frac{1}{\left[2N\left(\mathrm{freq}\right)\left(3+C\right)\right]}& \text{Equation 8}\end{array}$

[0158] And as defined before, t₂is:

[0159] t₂=Ct₁  Equation 9

[0160] Therefore, t₁and t₂may be chosen so that the resulting V_LoRMS voltage across any pixel is being biased to the specification, Voff. Preferably all of the pixels on the LCD are being biased to a voltage that just barely has them turned on (remember, Voff is the 10% contrast point). Any added bias to any pixel will make it darker and given enough, (V_Hi) make it visible (opaque). Thus, the reference of this exemplary embodiment to 1:3 Voff Biased.

[0161] However, there is a limit since an endless number of backplanes cannot be multiplex at a given V_DD, or for that matter, support V_DDlevels below a certain point. These limitations may be calculated. From Equation 6, the minimum V_DDlevel possible can be determined. The time interval t₂will be shorter and shorter for lower values of V_r(which is effectively equal to V_DDsince there is virtually no current flowing into the LCD). At some value of V_DD, the time interval t₂may be substantially zero, so setting C equal to zero and defining V_requal to V_{DD min}yields:

[0162] V_{DD min}={square root over (3)}·V_off  Equation 10

[0163] Equation 10 indicates that the present invention can generate bias voltages high enough to be able to turn on a segment as long as the supply voltage is above V_{DD min}. Calculation of how high a bias may be given to an asserted pixel is a function of the number of backplanes that need to be supported. This can be determined by using Equation 5 for calculating V_Hi. By setting V_Hiequal to Von, dividing both sides by V_r, squaring both sides and substituting (3+C) from Equation 6, yields: $\frac{V_{\mathrm{on}}^2}{V_r^2}=\frac{2+N}{N\left(3+C\right)}=\frac{\left(2+N\right)V_{\mathrm{off}}^2}{{\mathrm{NV}}_r^2}$

[0164] Multiplying both sides by (V_r)²and dividing both sides by (V_off)²gives: $\frac{V_{\mathrm{on}}^2}{V_{\mathrm{off}}^2}=\frac{2+N}{N}=\frac{2}{N}+1$

[0165] Or, after solving for N and setting N equal to Nmax: $\begin{array}{cc}{N}_{\max }=\frac{2V_{\mathrm{off}}^2}{V_{\mathrm{on}}^2-V_{\mathrm{off}}^2}& \mathrm{Equation}11\end{array}$

[0166] Note that Equation 11 is only a function of the characteristics of the LCD, there is no V_ror V_DDterm in it. Furthermore, because of this restriction, not all LCDs may support a high number of backplanes. However, there are LCD manufacturers whose fluid chemistries can support a large number of backplanes according to the present invention. For example, referring to Table 1:

[0167] The two fluids listed in Table 1 by LXD Inc., may support up to ten backplanes at 4.76 volts. Since the backplane and segment signals are generated by general purpose I/O pins, any digital logic, e.g., microcontroller or programmable logic device may be used to drive up to 10 backplanes of LCD glass at V_DDof 5 volts.

[0168] This embodiment of the invention is biased to the parameter Voff. What is meant by the term ‘Voff Biased’ is this: Regardless of the supply voltage or the number of backplanes to be multiplexed, all pixels will be able to meet the Voff specification. The Von specification, however, will be affected by the number of backplanes to be supported and by the supply voltage.

[0169] Referring now to FIG. 3, depicted is an exemplary schematic timing diagram of a 1:3 Von biased switched mode embodiment of the present invention. If the polarities of the backplane signals were inverted from those illustrated in FIG. 2 (1:3 Voff Biased) it is possible to drive the LCD glass biased to the parameter Von. In the ‘Von Biased’ embodiment regardless of the supply voltage or the number of backplanes to be multiplexed, all pixels will be able to meet the Von specification. The Voff specification, however, may be affected by the number of backplanes to be supported and by the supply voltage.

[0170] Eight phases from the microcontroller 104, represented by Phase A₂through Phase H₂, sequentially drive the LCD glass 102 for each multiplex frame. These sequence of eight phases are repeated for each backplane 102aon the LCD glass 102. The following steps are performed (and depicted in FIG. 3) in eight phases (A₂-H₂) for Backplane N and then repeated for each subsequently asserted backplane. Note that phase D₂and phase H₂have all segments 102bdriven with zero potential and that the time duration of these two phases is different than all the others.

[0171] Backplane N is ASSERTED & Low

[0172] All other backplanes are DEASSERTED & Low

[0173] Asserted segments are Low

[0174] Deasserted segments are High

[0175] Timeout period=t₁

[0176] Backplane N is ASSERTED & Low

[0177] All other backplanes are DEASSERTED & High

[0178] Asserted segments are Low

[0179] Deasserted segments are High

[0180] Timeout period=t₁

[0181] Backplane N is ASSERTED & Low

[0182] All other backplanes are DEASSERTED & High

[0183] Asserted segments are Low

[0184] Deasserted segments are Low

[0185] Timeout period=t₁

[0186] Backplane N is ASSERTED & Low

[0187] All other backplanes are DEASSERTED & Low

[0188] Asserted segments are Low

[0189] Deasserted segments are Low

[0190] Timeout period=t₂

[0191] Backplane N is ASSERTED & High

[0192] All other backplanes are DEASSERTED & High

[0193] Asserted segments are High

[0194] Deasserted segments are Low

[0195] Timeout period=t₁

[0196] Backplane N is ASSERTED & High

[0197] All other backplanes are DEASSERTED & Low

[0198] Asserted segments are High

[0199] Deasserted segments are Low

[0200] Timeout period=t₁

[0201] Backplane N is ASSERTED & High

[0202] All other backplanes are DEASSERTED & Low

[0203] Asserted segments are High

[0204] Deasserted segments are High

[0205] Timeout period=t₁

[0206] Backplane N is ASSERTED & High

[0207] All other backplanes are DEASSERTED & High

[0208] Asserted segments are High

[0209] Deasserted segments are High

[0210] Timeout period=t₂

[0211] These eight phases translate to the states of a ‘state machine’ programmed on the microcontroller. That is, there are eight states per backplane or 8N total states in the machine). The order of the aforementioned phase states is exemplary and it is contemplated and within the scope of the present invention that these states may be executed in any order so long as the resulting RMS voltages are the same.

[0212] The voltage potential and polarity are dependent upon the relationship between the voltage levels applied to the backplanes and segments and the time durations thereof. As depicted in FIG. 3, the asserted segment(s) on Backplane N (I.-III.) is at a logic low for all phase intervals (Phases A₂through H₂). Therefore, the RMS waveform across the asserted segments and the Backplane N is RMS symmetrical and therefore leaves no DC component on the segments.

[0213] The deasserted segments on the N Backplane (I.-IV.) are at a negative logic high for Phases A₂and B₂, at a logic low for Phases C₂, D₂, G₂and H₂and at a positive logic high for Phases E₂and F₂. Therefore, the RMS waveform across the deasserted segment(s) and the N Backplane is RMS symmetrical and therefore leaves no DC component on these segments. Since the logic high is only for two time intervals, t₁, positive and two time intervals, t₁, negative, the deasserted segment(s) of the N Backplane never go above Voff.

[0214] In a similar fashion to the deasserted segment(s) described above, the asserted segment(s) on the other backplane(s) (II.-III.) are at a logic high for only two time intervals at a positive polarity (Phases B₂and C₂) and two time intervals at a negative polarity (Phases F₂and G₂). Likewise, the deasserted segment(s) on the other backplane(s) (II.-IV.) are at a logic high for only two time intervals at a positive polarity (Phases C₂and E₂) and two time intervals at a negative polarity (Phases A₂and G₂). During the remaining time intervals, the voltage is at a logic zero. All RMS waveforms are symmetrical and thus there is substantially no DC component buildup on the segments. $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\left(\frac{1}{T}\right)·\left[2·2\left(N-1\right)t_1\right]·V_r^2}& \text{Equation 12}\end{array}$

[0215] Where T is determined to be the same as in Equation 3, and t₂is defined in the same manner as Equation 10; this yields: $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\frac{4\left(N-1\right)t_1{V}_r^2}{2{\mathrm{Nt}}_1\left(3+C\right)}}=\sqrt{\frac{N-1}{N}·\frac{2}{3+C}}·V_r& \text{Equation 13}\end{array}$

[0216] And, for V_Hi, after using Equation 3 and 9 again yields: $\begin{array}{cc}{V}_{\mathrm{Hi}}=\sqrt{\left(\frac{1}{T}\right)·\left[2·2{\mathrm{Nt}}_1\right]·V_r^2}=\sqrt{\frac{2}{3+C}}·V_r& \text{Equation 14}\end{array}$

[0217] Biasing for Von requires setting V_Hi=Von in Equation 14 and solving for 3+C and C: $\begin{array}{cc}3+C=\frac{2V_r^2}{V_{\mathrm{on}}^2}& \text{Equation 15}\\ C=\frac{2V_r^2}{V_{\mathrm{on}}^2}-3=\frac{2V_r^2-3V_{\mathrm{on}}^2}{V_{\mathrm{on}}^2}& \text{Equation 16}\end{array}$

[0218] As disclosed hereinabove, the values for t₁and t₂can be calculated as a function of the LCD manufacturer's ‘Refresh Rate’ specification. Equation 8 and Equation 9, apply to this embodiment as well.

[0219] V_{DD min}for this embodiment may be calculated. As the supply voltage is reduced, the constant C must be reduced to allow the LCD's maximum and minimum contrast to be maintained. At some point, C will equal zero, and the supply voltage will not be allowed to be reduced any further. So, setting C=0, in Equation 15, solving for Vr and then setting Vr equal to V_{DD min}yields: $\begin{array}{cc}{V}_{\mathrm{DD}\min }=\sqrt{\frac{3}{2}}·V_{\mathrm{on}}& \text{Equation 17}\end{array}$

[0220] The number of backplanes may be derived from Equation 13. Substituting Equation 15 into Equation 13, eliminating Vr, solving for N and setting N=Nmax, yields: $\begin{array}{cc}{N}_{\max }=\frac{V_{\mathrm{off}}^2}{V_{\mathrm{on}}^2-V_{\mathrm{off}}^2}& \text{Equation 18}\end{array}$

[0221] The ratio 1:3 described in the previous two embodiments, relate to the number of states that the asserted backplane line is driven to the number that the deasserted backplane lines are driven in any multiplex frame.

[0222] The following exemplary embodiments will disclose a six phase state switched-mode technique for driving LCD glass. Again, the term ‘Voff Biased’ is defined as: Regardless of the supply voltage or the number of backplanes to be multiplexed, all pixels will be able to meet the Voff specification. The Von specification, however, will be affected by the number of backplanes to be supported and by the supply voltage.

[0223] In the following exemplary embodiments, the first three phase states comprise the positive multiplex Subframe and the last three, the negative multiplex Subframe. These can be seen more readily in FIGS. 4 and 5 and the detailed descriptions thereof in the state descriptions that herein follow:

[0224] Referring now to FIG. 4, depicted is an exemplary schematic timing diagram of a 1:2 Voff biased switched mode embodiment of the present invention. Six phases from the microcontroller 104, represented by Phase A₃through Phase F₃, sequentially drive the LCD glass 102 for each multiplex frame. These sequence of six phases are repeated for each backplane 102aon the LCD glass 102. The following steps are performed (and depicted in FIG. 4) in six phases (A₃-F₃) for Backplane N and then repeated for each subsequently asserted backplane.

[0225] The currently asserted backplane is designated Backplane N. 1:2 indicates that the asserted backplane is driven in 2 states while the deasserted backplane is only driven in one. These may be seen in the following phase state descriptions:

[0226] Backplane N is ASSERTED & High

[0227] All other backplanes are DEASSERTED & High

[0228] Asserted segments are Low

[0229] Deasserted segments are High

[0230] Timeout period=t₁

[0231] Backplane N is ASSERTED & High

[0232] All other backplanes are DEASSERTED & Low

[0233] Asserted segments are Low

[0234] Deasserted segments are High

[0235] Timeout period=t₁

[0236] Backplane N is ASSERTED & Low

[0237] All other backplanes are DEASSERTED & Low

[0238] Asserted segments are Low

[0239] Deasserted segments are Low

[0240] Timeout period=t₂

[0241] Backplane N is ASSERTED & Low

[0242] All other backplanes are DEASSERTED & Low

[0243] Asserted segments are High

[0244] Deasserted segments are Low

[0245] Timeout period=t₁

[0246] Backplane N is ASSERTED & Low

[0247] All other backplanes are DEASSERTED & High

[0248] Asserted segments are High

[0249] Deasserted segments are Low

[0250] Timeout period=t₁

[0251] Backplane N is ASSERTED & High

[0252] All other backplanes are DEASSERTED & High

[0253] Asserted segments are High

[0254] Deasserted segments are High

[0255] Timeout period=t₂

[0256] These six phases translate to the states of a ‘state machine’ programmed on the microcontroller. That is, there are six states per backplane or 6N total states in the machine). The order of the aforementioned phase states is exemplary and it is contemplated and within the scope of the present invention that these states may be executed in any order so long as the resulting RMS voltages are the same, any phase state sequence is appropriate.

[0257] These states, as stated hereinbefore, do not need to be executed in any particular order as long as they are executed once per multiplex frame. The RMS calculations will work out the same. In this exemplary embodiment there are six phase states, and each backplane line needs its own set of six phase states (per activation); therefore, a system with N number of backplanes requires 6N states running on a digital logic circuit, e.g., microcontroller or programmable logic device.

[0258] The total time for a multiplex frame is different for a six phase embodiment than for the eight phase embodiments described herein. Using the definition for the constant C, from Equation 9:

[0259] T=(4t_i+2t₂)·N=2N(2t₁+Ct₁)=2Nt₁(2+C)  Equation 19

[0260] Similarly, as before, V_Loand V_Himay be calculated by substituting in Equation 19 as needed: $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\left(\frac{1}{T}\right)·\left[2\left(N-1\right)t_1\right]·V_r^2}=\sqrt{\frac{N-1}{N·\left(2+C\right)}}·V_r& \text{Equation 20}\\ \begin{array}{c}{V}_{\mathrm{Hi}}=\sqrt{\left(\frac{1}{T}\right)·\left[4t_1+2\left(N-1\right)t_1\right]·V_r^2}\\ =\sqrt{\frac{N+1}{N·\left(2+C\right)}}·V_r\end{array}& \text{Equation 21}\end{array}$

[0261] Since this is a Voff biasing embodiment, from Equation 20, set V_Lo=Voff and solve for C and 2+C: $\begin{array}{cc}2+C=\frac{V_r^2}{V_{\mathrm{off}}^2}·\frac{N-1}{N}& \text{Equation 22}\\ C=\frac{V_r^2}{V_{\mathrm{off}}^2}·\frac{N-1}{N}-2& \text{Equation 23}\end{array}$

[0262] From Equation 22, the minimum V_DDthat can be supported may be calculated. Setting C equal to zero, and setting Vr equal to V_{DD min}gives: $\begin{array}{cc}{V}_{\mathrm{DD}\min }=\sqrt{\frac{2N}{N-1}}·V_{\mathrm{off}}& \text{Equation 24}\end{array}$

[0263] The maximum number of backplanes supportable may be calculated from Equation 21 by setting V_Hiequal to Von, substituting 2+C from Equation 22 and solving for N, then setting N equal to Nmax yields: $\begin{array}{cc}{N}_{\max }=\frac{V_{\mathrm{off}}^2+V_{\mathrm{on}}^2}{V_{\mathrm{on}}^2-V_{\mathrm{off}}^2}& \text{Equation 25}\end{array}$

[0264] In this embodiment, V_{DD min}is a function of N, the number of backplanes. Furthermore, the value of V_{DD min}will go down with increasing N; that is, this embodiment becomes more robust with a larger number of backplanes. Nmax has Von squared in its numerator, so the number of backplanes supportable should be greater here than the eight phase embodiments described herein.

[0265] In order to avoid flicker, the minimum time period of t₁must be determined. As before, t₁needs to be a function of the refresh frequency, the number of backplanes in the LCD and the relation of the t₁phase states to t₂phase states. To calculate t₂, use Equation 9: $\begin{array}{cc}{t}_1\le \frac{1}{\left[2N\left(\mathrm{freq}\right)\left(2+C\right)\right]}& \text{Equation 26}\end{array}$

[0266] Referring now to FIG. 5, depicted is an exemplary schematic timing diagram of a 1:2 Von biased switched mode embodiment of the present invention. Six phases from the microcontroller 104, represented by Phase A4 through Phase F₄, sequentially drive the LCD glass 102 for each multiplex frame. These sequence of six phases are repeated for each backplane 102aon the LCD glass 102. The following steps are performed (and depicted in FIG. 5) in six phases (A4-F4) for Backplane N and then repeated for each subsequently asserted backplane.

[0267] This exemplary embodiment is the second of the 1:2 Switched-Mode digital logic circuit drivers for LCD glass. There are only six phase states required per backplane N. The term ‘Von Biased’ is defined herein as: Regardless of the supply voltage or the number of backplanes to be multiplexed, all pixels will be able to meet the Von specification. The Voff specification, however, will be affected by the number of backplanes to be supported and by the supply voltage.

[0268] The currently asserted backplane is designated Backplane N. 1:2 indicates that the asserted backplane is driven in 2 states while the deasserted backplane is only driven in one. These may be seen in the following phase state descriptions:

[0269] Backplane N is ASSERTED & Low

[0270] All other backplanes are DEASSERTED & High

[0271] Asserted segments are Low

[0272] Deasserted segments are High

[0273] Timeout period=t₁

[0274] Backplane N is ASSERTED & Low

[0275] All other backplanes are DEASSERTED & Low

[0276] Asserted segments are Low

[0277] Deasserted segments are High

[0278] Timeout period=t₁

[0279] Backplane N is ASSERTED & Low

[0280] All other backplanes are DEASSERTED & Low

[0281] Asserted segments are Low

[0282] Deasserted segments are Low

[0283] Timeout period=t₂

[0284] Backplane N is ASSERTED & High

[0285] All other backplanes are DEASSERTED & Low

[0286] Asserted segments are High

[0287] Deasserted segments are Low

[0288] Timeout period=t₁

[0289] Backplane N is ASSERTED & High

[0290] All other backplanes are DEASSERTED & High

[0291] Asserted segments are High

[0292] Deasserted segments are Low

[0293] Timeout period=t₁

[0294] Backplane N is ASSERTED & High

[0295] All other backplanes are DEASSERTED & High

[0296] Asserted segments are High

[0297] Deasserted segments are High

[0298] Timeout period=t₂

[0299] The aforementioned phase states, do not need to be executed in any particular order as long as they are executed once per multiplex frame. The RMS calculations will work out the same. In this embodiment, there are six phase states per backplane N, therefore, a system with N number of backplanes requires 6N phase states running on the digital logic circuit, e.g., microcontroller or programmable logic device.

[0300] The total time for a multiplex frame is the same as before and Equation 19 may also be used for this embodiment. V_Loand V_Hi, by substituting into Equation 19 as needed: $\begin{array}{cc}{V}_{\mathrm{Lo}}=\sqrt{\left(\frac{1}{T}\right)·\left[2\left(N-1\right)t_1\right]·V_r^2}=\sqrt{\frac{N-1}{N·\left(2+C\right)}}·V_r& \text{Equation 27}\\ \begin{array}{c}{V}_{\mathrm{Hi}}=\sqrt{\left(\frac{1}{T}\right)·\left[4t_1+2\left(N-1\right)t_1\right]·V_r^2}\\ =\sqrt{\frac{N+1}{N·\left(2+C\right)}}·V_r\end{array}& \text{Equation 28}\end{array}$

[0301] Note that Equations 27 and 28 are the same as Equations 20 and 21. This is true because the inversion of the backplane waveforms in phases A, B, D and E made no change to the resultant RMS voltage of the bottom four timing diagrams shown FIG. 4. According to the Von biasing embodiment herein, from Equation 28, set V_Hi=Von and solve for C and 2+C: $\begin{array}{cc}2+C=\frac{V_r^2}{V_{\mathrm{on}}^2}·\frac{N+1}{N}& \text{Equation 29}\\ C=\frac{V_r^2}{V_{\mathrm{on}}^2}·\frac{N+1}{N}-2& \text{Equation 30}\end{array}$

[0302] From Equation 29, the minimum V_DDthat can be supported may be calculated. Setting C equal to zero, and setting Vr equal to V_{DD min}yields: $\begin{array}{cc}{V}_{\mathrm{DD}\min }=\sqrt{\frac{2N}{N+1}}·V_{\mathrm{on}}& \text{Equation 31}\end{array}$

[0303] The maximum number of backplanes supportable, may be calculated using Equation 26 by setting V_Loequal to Voff, substituting 2+C from Equation 29, solving for N and setting N equal to Nmax yields: $\begin{array}{cc}{N}_{\max }=\frac{V_{\mathrm{off}}^2+V_{\mathrm{on}}^2}{V_{\mathrm{on}}^2-V_{\mathrm{off}}^2}& \text{Equation 32}\end{array}$

[0304] V_{DD min}is a function of N, the number of backplanes. Furthermore, the value of V_{DD min}will go down with decreasing N; that is, this embodiment will becomes more robust with a smaller number of backplanes. Nmax has Von squared in its numerator, so the number of backplanes supportable may be greater here than the first two embodiments disclosed herein. Use Equation 25 for calculating t₁and Equation 9 for t₂.

[0305] The 1:2 embodiments may be easier to implement than the 1:3 embodiments since there are two less phase states per multiplex frame. Requiring less phase states implies less coding in a microcontroller or programmable logic device. Also, having less phase states means a pixel that needs to be driven high, does not have to wait as long to be recharged; thus the 1:2 embodiments may be able to support a greater number of backplanes.

[0306] Table 2 represents a summary of the equations needed to implement the exemplary embodiments described herein. Table 3 represents a list of liquid crystal material manufacturers, the Von and Voff characteristics, and Nmax and V_{DD min}calculations. Note that V_{DD min}is shown with the maximum Nmax backplanes possible and with a nominal of 4 backplanes.

[0307] Table 3 herein shows the performance comparisons of the four exemplary embodiments described herein. Note the backplane maximizing superiority of LCD fluids with small differences in their Von vs. Voff RMS voltages and the support of low V_{DD min}, voltages when a fluid with a low Voff specification is used.

[0308] Global Pixel Digital Contrast Control is a feature of the present invention. This feature comprises digital control of the LCD contrast. Traditional LCD circuits used potentiometers on the resistor ladder chain to adjust the overall contrast of the LCD device. Contrast control is now possible by merely adjusting the time interval, t₂. The mathematical effect of doing this alters the value of the constant, C. The higher the value of C, the longer the state machine stays in the t₂interval; and so, as explained previously herein, the longer all the LCD pixels remain at a zero potential. This drops the bias on all the pixels, hence lowering the contrast of all of them.

[0309] Referring to FIG. 6, depicted is a schematic block diagram of an exemplary embodiment of a temperature compensated directly driven LCD system. The LCD 102 is driven by a microprocessor 104. The microprocessor 104 comprises a central processing unit (CPU) 608, a random access memory (RAM) 612, and a read only memory (ROM) 610. The ROM 610 may be for example, but is not limited to, an electrically erasable and programmable ROM (EEPROM). A control program for the microcontroller 104 may be stored in the ROM 610, or may be stored in as firmware in a mask programmable ROM. A temperature sensor 606 may be used for measuring the environmental temperature of the LCD 102.

[0310] Global control of the LCD's contrast also allows for easy temperature compensation of the LCD glass. A restriction often encountered by LCD designers is that the values of Voff and Von drift with the ambient temperature. A microcontroller or programmable logic system equipped with a temperature sensor can compensate for variations in temperature by re-biasing the operating points of the LCD to the optimum for the current ambient temperature. Temperature compensation of the LCD glass in this manner is contemplated in the present invention and incorporated by reference herein.

[0311] Referring to FIG. 7, depicted is a schematic block diagram of an exemplary embodiment of a system application using the microcontroller and having a directly driven LCD. The LCD 102 is driven by a microprocessor 104. The microprocessor 104 comprises a central processing unit (CPU) 608, a random access memory (RAM) 612, and a read only memory (ROM) 610. The ROM 610 may be for example but not limited to an electrically erasable and programmable ROM (EEPROM). A control program for the microcontroller 104 may be stored in the ROM 610, or may be stored in as firmware in a mask programmable ROM. The control program may be adapted for controlling the LCD and a system application (function) 720.

[0312] The system application 720 may be controlled by the microcontroller which may include, but is not limited to, control of temperature (thermostat), humidity, sprinkler, alarm and security system, alarm clock, timer, clothes dryer, washing machine, toaster, microwave, oven, cooktop, clothes iron, water heater, tankless water heater, solar heating, swimming pool, Jacuzzi, answering machine, pager, telephone, intercom, caller identification, electronic address book, treadmill, stationary bicycle, exercise machine, torque wrench, depth gauge, scale, speedometer, automobile tire condition status, anti-skid and anti-lock brakes, fuel gauge, engine monitoring, operation of luminaries (lights) in a building, power load management, video cassette player, DVD player, uninterruptable power supply (UPS), Dictaphone, tape recorder, MP3 music player, video game toy, calculator, personal digital organizer, etc.

[0313] Individual Pixel Digital Contrast Control is another feature of the present invention. It is possible for a particular pixel to have a mid-contrast gray level while others are at other Grey levels. It is quite easy to do this by dynamically modifying which segment lines are to be driven during the currently executing multiplex frame. That is, before each new multiplex frame is to begin, the microcontroller or programmable logic system determines if any given pixel that is to be asserted, should or should not actually be driven during this frame. This has the effect of time-domain ‘dithering’ individual pixels.

[0314] If an OFF pixel can be defined as having a contrast ratio of 0 and an ON pixel having a contrast ratio of 1, then this time domain dithering technique can produce pixel Grey levels with fractional contrast ratios between 0 and 1. For example, if a given pixel is to have a contrast ratio of ½ then it will be driven during every other multiplex frame; a pixel with a contrast ratio of ⅔, would be driven for 2 frames every 3. Two registers per pixel can be maintained to produce unique contrast levels for each pixel. The first register would hold the number of ON multiplex frames while the other held either the number of OFF frames or the total number of ON+OFF frames.

[0315] Referring now to Table 4, examples of current production microcontrollers are illustrated. Assume in Table 4 that ten I/O's are used for driving the backplane lines of an LCD. All of the other I/O's are assumed to drive segment signals. From this an exemplary number of supportable segments may be calculated. Note that in real applications not all pins of the microcontroller would be dedicated for LCD support; so, these are maximum numbers:

[0316] As Table 4 illustrates, the present invention can support an LCD with over 500 segments. An advantage of the present invention is that no charge-pumps or resistor ladder network are required to drive the LCD glass.

[0317] The invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

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