Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-09-11
2001-08-21
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S208000, C345S210000, C349S096000, C349S098000
Reexamination Certificate
active
06278429
ABSTRACT:
TECHNICAL FIELD
This invention relates to the operation of liquid crystal displays. In particular, the present invention relates to a driving circuit for operating a cholesteric liquid crystal display. Specifically, the present invention relates to a driving circuit employing specially modified drivers that are normally used in super twisted nematic liquid crystal displays.
BACKGROUND ART
Cholesteric liquid crystal materials are known and disclosed in U.S. Pat. Nos. 5,437,811; 5,695,682; 5,453,863; and 5,691,795, all of which are assigned to the assignee of the present invention and which are incorporated herein by reference. The primary advantage of the bistable cholesteric liquid crystal materials disclosed in these patents is that they can be driven to a desired texture with application of a voltage and remain in that texture after removal of the applied voltage. As seen in
FIG. 1
, bistable cholesteric liquid crystal materials are known to exhibit at least four states or textures: homeotropic, focal conic, transient planar, and planar. Both the homeotropic and transient planar textures are considered transitory and do not remain after removal of an electric field. These transitory textures are employed to facilitate the transformation of the cholesteric liquid crystal material into either a weakly light scattering, transmissive focal conic texture or a reflective planar texture.
The next step in the development of bistable cholesteric liquid crystal devices was focused on how to drive the cholesteric liquid crystal material quickly between the focal conic and planar textures. This development is necessitated by the desire to provide efficient operation of the device, with as fast as possible update rates. Such driving schemes are found in U.S. Pat. No. 5,748,277, and in U.S. patent application Ser. No. 08/852,319, both of which are owned by the assignee of the present invention and which are incorporated herein by reference. Initially, a three phase dynamic drive scheme, as shown in
FIGS. 2A and 2B
, was employed to control the appearance of the cholesteric device. As is discussed in the above patents, the liquid crystal material is disposed between two substrates, one of which has a plurality of row electrodes and the other which has a plurality of column electrodes orthogonal to the row electrodes. Application of voltage waveforms to the electrodes is multiplexed or applied in a predetermined sequence. Hence, these displays are sometimes referred to as multiplexed displays. Those skilled in the art will appreciate that multiplexed displays are not limited to “row and column” electrode patterns. Segmented liquid crystal displays, such as clock faces and calculator displays, may also be multiplexed. In either type of display the term “common electrode” may be used to refer unto a row electrode, and the term “segment electrode” may be used to refer to a column electrode.
The liquid crystal material in between the intersecting electrodes form a pixel. As shown in
FIGS. 2A and 2B
, the appearance of each pixel is controlled by a pixel voltage waveform which comprises a sequence of three RMS voltages: V
preparation
, V
select
on-select
, and V
evolve
. V
preparation
or V
p
drives the cholesteric liquid crystal material into the homeotropic texture regardless of its initial texture. Application of V
select
on-select
or V
s
s
determines if the homeotropic texture relaxes into the planar (V
select
) or the focal conic texture (V
non-select
). The evolution voltage or V
e
serves two functions. First, it permits the focal conic texture to evolve from the transient planar texture that results from applying V
non-select
. The evolution voltage also restores and maintains the homeotropic texture after V
select
is applied allowing relaxation to the planar texture which occurs when V
evolve
is removed. It has been determined that display update speed can be increased by applying the voltages V
preparation
and V
evolve
across many rows simultaneously. Once V
evolve
is removed from the last addressed row, all power is removed from the display and the desired indicia appears on the display.
Implementation of such a drive scheme has proven to be quite costly. In particular, previous displays required 50-60V (RMS) to drive the cholesteric liquid crystal material into the homeotropic texture from which it relaxes into the reflective planar texture. Since the use of cholesteric liquid crystal materials in displays is relatively new, there are no commercially available electronic driving circuits uniquely designed to apply the necessary voltage waveforms to a display.
One option that was initially investigated was to employ a multiplexed super twisted nematic (STN) display driver. STN displays are addressed constantly so that each pixel always has an applied voltage across it that is the combination of waveforms being applied to the appropriate intersecting electrodes. The “state” or texture of a particular pixel (on or off, light or dark) depends on the average voltage across the pixel during a single scan or update of the display. The difference between the average voltages of these two pixels states is small, on the order of about 0.1 volt. This difference is generated entirely by the choice of voltage, either high or low, applied to the pixel while it is selected for update. The number of DC voltage levels required to drive a STN display is relatively small. Four voltage levels are required for each common/row and segment/column waveform and typically, two of these voltage levels are common to both. Accordingly, only six distinct DC voltage levels, which are separate from the logic voltage inputs, are required to address an STN display. STN driver chips also include a data input called the frame line that selects between two fixed pairs of display voltage inputs for all the outputs on a chip. For example, if the display voltage inputs are labeled V
1
, V
2
, V
3
, and V
4
, the frame line can select between either the pair V
1
and V
2
or the pair V
3
and V
4
. No other selections are possible. Of course other label designations could be used for the voltage inputs. Each STN driver chip also includes a shift register containing one data bit per chip output. Each bit selects one of the two display voltage inputs selected by the frame line. Accordingly, each bit can select between V
1
and V
2
or between V
3
and V
4
. Once again, no other selections are possible. The voltages applied to the display voltage inputs must obey strict rules. At a minimum, the rule (V
4
≧V
3
≧V
2
≧V
1
) must be obeyed. Moreover, it is typical to require two of the four display voltage inputs (V
3
and V
4
) to be set very near to the chip's upper supply voltage while the other two display voltage inputs (V
1
and V
2
) are set very near the chip's lower supply voltage. These requirements are intended to ensure proper chip operation and are primarily a function of the chip design.
Although it was desired to employ the STN driver chips to drive the cholesteric liquid crystal display because of their relatively low cost (about 2 cents per output), it was readily apparent that the drive scheme requirements of cholesteric liquid crystal displays were significantly more severe than super twisted nematic displays. The state of a STN pixel depends only on the average voltage across the pixel during a single update of the display and not on the specific sequence of voltages applied to the pixel. While cholesteric liquid crystal displays respond to the average voltages applied to them, the state of a pixel depends on the sequence of RMS/average voltages applied during an update. As noted previously, the dynamic address scheme requires the proper application of RMS voltages V
p
, V
s
s
, and V
e
in order to select between the two stable cholesteric liquid crystal textures. The only known way to address cholesteric liquid crystal displays with the dynamic drive scheme was to employ high voltage analog switches to generate the necessary row waveforms.
A first attempt at employing STN dr
Bos Philip J.
Hewitt Richard
Ruth Jonathan C.
Hjerpe Richard
Kent State University
Nguyen Kimnhung
Renner Kenner Greive Bobak Taylor & Weber
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