Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-01-07
2001-06-26
Shankar, Vijay (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S208000, C345S210000
Reexamination Certificate
active
06252571
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a bistable liquid crystal display device that uses a chiral nematic liquid crystal and has a memory effect and its drive method and the drive circuit used therein. This invention also relates to a liquid crystal display device that sets a total of eight or more voltage levels suited to driving chiral nematic liquid crystal and the power supply circuit device used therein.
BACKGROUND OF THE INVENTION
A bistable liquid crystal display that uses chiral nematic liquid crystal is disclosed in Japanese Laid-Open Patent Application 1-51818, which describes the initial orientation condition, the two stable states, the method whereby the stable states are achieved, etc.
However, the description in the aforementioned Japanese Laid-Open Patent Application 1-51818 notes only the action or phenomenon of two stable states and does not disclose any means offered for the practical application as a display member. Further, the above publication contains no description of a matrix display, which currently has the greatest potential for application as a display member, or any disclosure of a drive method.
In Japanese Laid-Open Patent Application 6-230751, which we have submitted, we proposed a method that controls back-flow which occurs in a liquid crystal cell for improved practical application. This method first provides a period wherein a Frederick's transition is generated by applying a high voltage for about 1 ms, and then it either creates a 0-degree uniform state by immediately applying a constant voltage pulse following the Frederick's transition voltage whose polarity is reversed from or the same as the previous pulse and whose threshold value is greater, or it realizes a 360-degree-twisted state by similarly providing a pulse period with a lower threshold value immediately following the Frederick's transition voltage. In this method, the writing time per line of the matrix display is 400 &mgr;s, thus requiring a total of more than 160 ms (6.25 Hz) to write more than 400 lines, which results in flickering of the display and therefore presents a problem in practical application.
The inventors submitted in Japanese Laid-Open Patent Application 5-37057 a means of further improving the writing time. This, as shown in FIG. 2 and FIG. 4 of such application, provides a delay time after the reset pulse that generates the Frederick's transition and then applies an ON or OFF selection signal. By doing this, a writing time several times faster, e.g., 50 &mgr;s, than previously can be realized.
However, these drive methods require that a large reset voltage exceeding 20 V, an OFF voltage of 1 to 3 V for achieving two stable states and a selection voltage ranging from an ON voltage of several volts to 6 or 7 volts all efficiently coexist on the circuitry and that alternating current be used to achieve a longer liquid crystal life.
FIG. 23
of this application shows a 7-level drive method that creates a drive waveform for bistable display in accordance with the voltage averaging method.
FIG. 23A
is the waveform of the scanning signal, wherein Vr, which exceeds 20 V, is applied in reset period T
1
, ±Vs is applied in selection period T
3
, which comes after delay period T
2
, and the remaining non-selection period T
4
is zero potential. The data signal, however, is in phase with the selection pulse of amplitude ±Vd shown in
23
B of the same figure and switches display ON and OFF by applying a negative-phase AC pulse. Also, the voltage of the difference signal, like that shown in
FIG. 23C
, of the scan signal and the data signal is applied to the liquid crystal.
Here, since the bias voltage Vd need only be about 1 V, a large voltage difference occurs between the scanning signal waveform and the data signal waveform. Particularly since a voltage difference close to 20 V occurs between Vr and Vs in the scanning signal waveform, this is not desirable in a circuit configuration.
Since the ratio between the scanning voltage and the ON/OFF signal voltage during matrix drive in a bistable liquid crystal display is greatly unbalanced, this unbalance may become a major obstacle to configuring a specific drive circuit or configuring the circuit as an IC.
This is not that extreme even in the voltage-averaging drive method of prior art matrix type liquid crystal display elements, but based on similar conditions a six-level method was proposed (
Liquid Crystal Device Handbook,
Nikkan Kogyo, p. 401). However, though this is effective in balancing the drive voltages of the scanning waveform and the signal waveform and increasing the ratio of the ON voltage and the bias voltage, when a reset voltage with a large voltage difference like that in the present invention is also applied, it is impossible to apply it as is to drive a chiral nematic liquid crystal, which is an object of the present invention.
Since the number of levels of the drive voltage is large in the above method, adjustment of the optimum drive voltage becomes extremely difficult and presents a problem in practical application.
Further, since the threshold voltage and saturation voltage of the bistable liquid crystal are temperature dependent and fluctuate inside the liquid crystal panel, it was shown that it would be difficult to achieve a stable display characteristic.
A purpose of this invention is to offer a liquid crystal display device and its drive method and the drive circuit used therein which are capable of not generating a large voltage difference in the scanning signal waveform and the data signal waveform while still improving the display characteristic.
Another purpose of the invention is to offer a liquid crystal display device and its power supply circuit device capable of accurately generating a plurality of voltage levels greater than eight levels and also easily adjusting a plurality of levels by an easy operation.
SUMMARY OF THE INVENTION
This invention is a drive method for a liquid crystal display device that applies the voltage of the difference of a data signal and a scanning signal having at least a reset period, a selection period and a non-selection period in one frame on a chiral nematic liquid crystal having at least two stable states, wherein
a total of eight or more voltage levels made up of a plurality of levels of a first group on the low voltage side and a plurality of levels of a second group on the high voltage side are provided,
the voltage levels of the scanning signal and the data signal are alternated between the first group and second group every mH (where, m is an integer that is 2 or greater and mH≠1 frame period), which is an integral multiple of the unit time (1H) equivalent to the selection period of the scanning signal,
when the data signal is a voltage level of the first group, the voltage level of the reset period in the scanning signal is selected from the second group, and when the data signal is a voltage level of the second group, the voltage level of the reset period in the scanning signal is selected from the first group,
when the data signal is a voltage level of the first group, the voltage levels of the selection period and non-selection period in the scanning signal are each selected from the same first group, and when the data signal is a voltage level of the second group, the voltage levels of the selection period and non-selection period in the scanning signal are each selected from the same second group, and
the polarity of the voltage applied to the liquid crystal is reversed every mH.
The liquid crystal display device related to the method of this invention comprises,
a liquid crystal panel wherein a chiral nematic liquid crystal having at least two stable states is infused between a first substrate whereon a plurality of scanning electrodes are formed and a second substrate whereon a plurality of data electrodes are formed,
a scanning electrode drive circuit that outputs a scanning signal having at least a reset period, a selection period and a non-selection period in one frame to each of the scan
Inoue Akira
Nomura Hiroaki
Frenel Vanel
Seiko Epson Corporation
Shankar Vijay
Watson Mark P.
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