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 .mu.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 .mu.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 T1, .+-.Vs is applied in selection period T3, which comes after delay period T2, and the remaining non-selection period T4 is zero potential. The data signal, however, is in phase with the selection pulse of amplitude .+-.Vd shown in 23B 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.