Method of driving a liquid crystal display device

• US 6,236,385 B1
• Filed: 10/29/1998
• Issued: 05/22/2001
• Est. Priority Date: 02/25/1993
• Status: Expired due to Fees

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Momose, Kenichi, Inoue, Akira, Tanaka, Takaaki, Sato, Yuzuru, Nomura, Hiroaki
• Primary Examiner(s): Mengistu, Amare
• Assistant Examiner(s): Nguyen, Jimmy H.
• Application Number:
  US09/183,602
• Time in Patent Office: 936 Days
• Field of Search: 345/94, 345/95, 345/97, 345/96, 345/99, 345/101, 345/208, 349/33
• US Class Current: 345/95
• CPC Class Codes: G09G 2300/0486 Cholesteric liquid crystals, including chiral-nematic liquid crystals, with transitions between focal conic, planar, and homeotropic states
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 2310/065 Waveforms comprising zero voltage phase or pause
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/10 Special adaptations of display systems for operation with variable images
• CPC Class Codes: G09G 3/3629 using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
• CPC Class Codes: G09G 3/3681 suitable for passive matrices only
• CPC Class Codes: G09G 3/3692 suitable for passive matrices only

Abstract

A method of driving a liquid crystal that has a memory capability, wherein a reset voltage is initially applied to the liquid crystal during a reset period, to bring about a Freédericksz transition in the liquid crystal. During a subsequent selection period, a selection voltage that is selected on the basis of a critical value that brings about one of two metastable states in the liquid crystal is applied to the liquid crystal, and, during a non-selection period that follows the selection period, a non-selection voltage that is less than or equal to a threshold value that maintains two metastable states is applied to the liquid crystal. A delay period is provided between the reset period and the selection period, in order to gain effective timing for applying the selection voltage to the liquid crystal after the application of the reset voltage has been turned off. This shortens the length of the selection period, and hence the write time.

Claims

1. A method of driving a liquid crystal display device that uses a chiral nematic liquid crystal medium wherein liquid crystal molecules interposed between two substrates have a predetermined angle of twist in an initial state and said chiral nematic liquid crystal medium has two metastable states differing from said initial state as relaxation states achieved after a voltage that brings about a Freé

dericksz transition has been applied in said initial state, said liquid crystal display device comprising a plurality of row electrodes formed on a first one of said two substrates, each of said row electrodes being supplied with a row electrode signal, and a plurality of column electrodes formed on a second one of said two substrates, each of said column electrodes being supplied with a column electrode signal, wherein intersections between said row electrodes and said column electrodes form pixels and the voltage of a difference signal between said row electrode signal and said column electrode signal is applied to said liquid crystal corresponding to each of said pixels, said method comprising the steps of;

a) during a reset period, applying to said liquid crystal a reset voltage greater than or equal to a threshold value that brings about said Freé

dericksz transition;

b) during a first selection period after said reset period, applying to said liquid crystal a selection voltage that is selected on the basis of a critical value that brings about one of said two metastable states;

c) during a non-selection period following said selection period, applying to said liquid crystal a non-selection voltage that is less than or equal to a threshold value that maintains said two metastable states;

d) during a delay period between said reset period and said selection period, applying to said liquid crystal a delay voltage that is less than or equal to said critical value that brings about one of said two metastable states;

e) in said column electrode signal, setting four types of potential for applying positive and negative ON selection voltages and positive and negative OFF selection voltages to said liquid crystal as said data potential;

f) in said row electrode signal, setting two types of potential for applying a positive or negative reset voltage to said liquid crystal during said reset period as said reset potential, two types of potential for applying a positive or negative selection voltage to said liquid crystal during said selection period as said selection potential, and two types of potential for imposing a bias potential on said four data potentials during said non-selection period and said delay period; and

g) setting either two types of said selection potential or two potentials among said four types of said data potential to be the same as said two types of said reset potential;

wherein said difference signal includes within one frame said first selection period that is set to be shifted for each of said row electrodes, said non-selection period following thereafter, said reset period set before said selection period, and said delay period inserted between said reset period and said first selection period;

wherein said row electrode signal is at a reset potential during said reset period, a selection potential during said first selection period, and a non-selection potential during said delay period and said non-selection period;

wherein said column electrode signal is set to a data potential which includes either an ON selection potential or OFF selection potential in synchronism with said first selection period; and

whereby at least eight potential levels are used for driving said liquid, crystal.

2. The method of claim 1, including the steps of:

dividing said eight potential levels into two groups such that four levels are in a first, low-voltage group (V1, V2, V3, and V4, where;

V1<

V2<

V3<

V4) and four levels are in a second, high-voltage group (V5, V6, V7, and V8, where;

V4<

V5<

V6<

V7<

V8);

selecting said reset potential from the group consisting of said second group, when said data potential of said column electrode signal lies within said first group, and selecting said reset potential from the group consisting of said first group when said data potential of said column electrode signal lies within said second group; and

in said periods other than said reset period, selecting one potential each from said first group when said data potential of said column electrode signal is within said first group, and selecting one potential each from said second group when said data potential of said column electrode signal is within said second group.

3. The method of claim 2, including the steps of:

driving said liquid crystal in an alternating manner such that within the period of one frame, an ON selection potential of said column electrode signal is set by alternating pulses of V4 and V5 and an OFF selection potential of said column electrode signal is set by alternating pulses of V2 and V7; and

, in a sequence corresponding thereto, said reset potential of said row electrode signal is set by alternating pulses of V8 and V1, said selection potential is set by alternating pulses of V1 and V8, and said non-selection potential is set by alternating pulses of V3 and V6, whereby the polarity of each voltage applied to said liquid crystal is inverted at each pulse.

4. The method of claim 3, wherein said voltages are set to be in the relationship:

V4−

V3=V3−

V2=V7−

V6=V6−

V5.

5. A method of driving a liquid crystal display device wherein said liquid crystal display device comprises:

a chiral nematic liquid crystal medium wherein liquid crystal molecules interposed between two substrates have a predetermined angle of twist in an initial state and said chiral nematic liquid crystal medium has two metastable states differing from said initial state as relaxation states achieved after a voltage that brings about a Freé

dericksz transition has been applied in said initial state;

a plurality of row electrodes formed on one of said substrates, each being supplied with a row electrode signal; and

a plurality of column electrodes formed on the other of said substrates, each, being supplied with a column electrode signal;

wherein intersections between said row electrodes and said column electrodes form pixels and the voltage of a difference signal between said rout electrode signal and said column electrode signal is applied to said liquid crystal corresponding to each of said pixels to drive said pixels;

said method being characterized in using at least eight potential levels to drive said liquid crystal and comprising the steps of;

a) including within one frame of said difference signal a selection period that is set to be shifted for each of said row electrodes, a non-selection period following thereafter, and a reset period set before said selection period;

b) setting said column electrode signal to data potentials that include a potential that is either an ON potential or an OFF potential for each occurrence of said selection period corresponding to each of said pixels on the same column electrode, and setting four types of potential for applying positive and negative ON selection voltages and positive and negative OFF selection voltages to said liquid crystal, as said data potentials of said column electrode signal;

c) setting said row electrode signal to a reset potential during said reset period, a selection potential during said selection period, and a non-selection potential during a non-selection period, said reset potential being two types of potential for applying positive and negative reset voltages to said liquid crystal during said reset period, said selection potential being two types of potential for applying positive and negative selection voltages to said liquid crystal during said selection period, and said non-selection potential being two types of potential for imposing a bias potential on said four types of data potential during said non-selection period; and

d) setting either two types of said selection potential or two potentials among said four types of said data potential to be the same as said two types of said reset potential.

6. The method of claim 5, including the steps of:

dividing said eight potential levels into two groups such that four levels are in a first, low-voltage group (V1, V2, V3, and V4, where;

V1<

V2<

V3<

V4) and four levels are in a second, high-voltage group (V5, V6, V7, and V8, where;

V4<

V5<

V6<

V7<

V8);

selecting said reset potential from amongst said second group when said data potential of said column electrode signal lies within said first group, or from amongst said first group when said data potential of said column electrode signal lies within said second group; and

in said periods other than said reset period, selecting one potential each from the same first group when said data potential of said column electrode signal is within said first group, or selecting one potential each from the same second group when said data potential of said column electrode signal is within said second group.

7. The method of claim 6, further comprising the steps of:

setting, within the period of one frame, an ON selection potential of said column electrode signal by alternating pulses of V4 and V5;

setting, within the period of one frame, an OFF selection potential of said column electrode signal by alternating pulses of V2 and V7; and

in a sequence corresponding thereto, said reset potential of said row electrode signal is set by alternating pulses of V8 and V1, said selection potential is set by alternating pulses of V1 and V8, and said non-selection potential is set by alternating pulses of V3 and V6, whereby the polarity of each voltage applied to said liquid crystal is inverted at each pulse.

8. The method of claim 7, wherein:

said voltages are set to be in the relationship;

V4−

V3=V3−

V2=V7−

V6=V6−

V5.