Computer graphics processing and selective visual display system – Plural physical display element control system – Segmented display elements
Reexamination Certificate
2001-07-31
2004-01-27
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Segmented display elements
C345S034000, C345S053000
Reexamination Certificate
active
06683587
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION TECHNOLOGY
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.
The LCD operates by applying a root mean square voltage (V
RMS
) between the backplane electrode and the pixel electrode. When a V
RMS
level 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
RMS
level 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.
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.
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.
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.
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.
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).
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.
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.
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.
SUMMARY OF THE INVENTION
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.
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.
Low power and voltage, e.g., 3.3 volts or lower, product applications using a micro
Baker & Botts L.L.P.
Dinh Duc Q
Hjerpe Richard
Microchip Technology Incorporated
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