Method for driving and liquid crystal display device including dot matrix display part and fixed pattern display port

A method is disclosed for driving a liquid crystal display provided, in the same liquid crystal panel, with a dot matrix display part and a fixed pattern or segment display part. The dot matrix display part is driven in a time-multiplexed fashion, while the fixed pattern or segment display part is driven in a static fashion using driving voltage waveforms in common with the dot matrix display part.

BACKGROUND OF THE INVENTION 
The present invention relates to a driving method for a liquid crystal 
display device, and more particularly to a driving method suitable for 
improving the contrast characteristics when a liquid crystal display which 
is provided with a dot matrix display part and a fixed pattern display 
part, or with a segment display part which permits display of letters, 
numbers and special symbols with stroke or line segments in a single 
liquid crystal panel, is driven in a highly time-multiplexed fashion. This 
invention also relates to a liquid crystal display device employing a 
method according to the present invention. 
FIG. 1 is a perspective view of an example of a conventional liquid crystal 
display device. In FIG. 1, an upper glass substrate 1 is provided with 
electrodes 3 formed of a transparent conductive coating and a lower glass 
substrate 2 is also provided with electrodes 4 formed of a transparent 
conductive coating. A liquid crystal layer 5 is inserted into a space 
formed between the glass substrates 1 and 2 which is several or several 
tens of microns thick. In this structure, the electrodes 3 and 4 form a 
matrix, and the intersections of the electrodes 3 and 4 act as picture 
elements. The transmitted light intensity of transmissive-type liquid 
crystal display devices or the reflected light intensity of 
reflective-type liquid crystal display devices gnerally depends on the 
effective value of the voltage applied to the liquid crystal. A liquid 
crystal display is based on principles such as the dynamic scattering mode 
(DSM) or the field effect mode (FEM). Twisted nematic liquid crystal 
display devices are currently the most generally used devices. In such a 
device, as is disclosed in U.S. Pat. Nos. 3,918,796 and 3,731,986, the 
liquid crystal layer 5 is composed of a nematic liquid crystal material 
which has a positive dielectric anisotropy and which is formed into a 
twisted structure in which the major axes of liquid crystal molecules 
adjacent to the upper glass substrate 1 is perpendicular to the major axes 
of the liquid crystal molecules adjacent to the lower glass substrate 2. 
The electro-optic change in the twisted structure which is produced by the 
application of a voltage across the liquid crystal layer 5 is detected by 
a pair of polarizing plates disposed in front of and behind the twisted 
structure. 
A line sequential scanning method is generally used as a method for driving 
the liquid crystal matrix shown in FIG. 1. FIG. 2 shows the state of the 
display device at a certain time in which scanning electrodes are 
represented as X.sub.1, X.sub.2, and X.sub.3, while signal electrodes are 
represented as Y.sub.1, Y.sub.2 and Y.sub.3, though the number of 
electrodes are six for the sake of simplicity, it can be larger in a 
practical device. The scanning electrodes X.sub.1, X.sub.2, and X.sub.3 
are sequentially selected to provide scanning; in FIG. 2, the electrode 
X.sub.2 is selected, as indicated by the hatching. 
A display signal is applied to the signal electrodes, and in this case, the 
electrode Y.sub.2 is selected, as indicated by the hatching. 
Hereunder, the state of a picture element 11 which is the intersection of 
the selected scanning electrode X.sub.2 and the selected signal electrode 
Y.sub.2 is referred to as the "selected state", the state of a picture 
element 12 which is the intersection of the selected scanning electrode 
X.sub.2 and a non-selected signal electrode Y.sub.1 or Y.sub.3, or the 
intersection of a non-selected scanning electrode X.sub.1 or X.sub.3 and 
the selected signal electrode Y.sub.2 is referred to as the "half-selected 
state", and the state of a picture element 13 which is the intersection of 
a non-selected scanning electrode X.sub.1 or X.sub.3 and a non-selected 
signal electrode Y.sub.1 or Y.sub.3 is referred to as the "non-selected 
state". 
In an amplitude-selective addressing scheme used as a method of driving the 
liquid crystal matrix display device, driving voltage are given by 
EQU V.sub.121 =V.sub.111 +2/a V.sub.0 ( 1) 
EQU V.sub.211 =V.sub.111 +V.sub.0 ( 2) 
EQU V.sub.221 =V.sub.111 +1/a V.sub.0 ( 3) 
EQU V.sub.122 =V.sub.112 -2/a V.sub.0 ( 4) 
EQU V.sub.212 =V.sub.112 -V.sub.0 ( 5) 
EQU V.sub.222 =V.sub.112 -1/a V.sub.0 ( 6) 
Where 
V111 and V112 are voltages alternately applied to a selected signal 
electrode during a predetermined period, 
V121 and V122 are voltages alternately applied to a non-selected signal 
electrode during the predetermined period, 
V.sub.211 and V.sub.212 are voltages alternately applied to a selected 
scanning electrode during the predetermined period, 
V221 and V222 are voltages alternately applied to a non-selected scanning 
electrode during the predetermined period, 
V0 is an amplitude of an alternating voltage applied across liquid crystal 
layer at a selected picture element, 
a is an arbitrary constant larger than 3, 
V111 and V112 are arbitrarily given and the condition that V111.noteq.-1/a 
V0 and V112.noteq.-1/a V0. 
For example, driving voltages applied to the electrodes alternate between 
V111 and V112, between V121 and V122, between V211 and V212, and between 
V221 and V222 in one scanning period for one scanning electrode, or V111, 
V121, V211 and V221 are applied to the electrodes in one frame period 
required for scanning all the scanning electrodes and V112, V122, V212 and 
V222 are applied to the electrodes in the next frame period. Thus the 
polarity of applied voltages across the liquid crystal layer is 
periodically reversed so that the liquid crystal layer has no mean DC 
level applied across it. A reference signal for reversal of polarity of 
voltages applied across the liquid crystal to yield AC operation of the 
cell is hereinafter called a control signal M. 
The operational margin has its maximum value when the constant a is such 
that 
##EQU1## 
Where 
N is the number of scanning lines. This kind of method is disclosed in U.S. 
Pat. No. 3,976,362. This driving method is referred to as an "a:1 
selection scheme" hereinafter. The effective values V.sub.s1, V.sub.s2 of 
voltage applied at the selected point and the half-selected point is 
represented by the following formulae and they are constant irrespective 
of changes in the display pattern. 
##EQU2## 
It is obvious from the formulae (8) and (9) that the contrast between the 
selected point and the non-selected point becomes smaller as the number N 
of time-multiplexing increases. 
There are several methods for displaying letters, numerals and figures by 
means of a liquid crystal display device. For example, in a dot matrix 
display system using the liquid crystal display device shown in FIG. 1, by 
combinations of intersections of the stripe-shaped electrodes 3 and 4, 
letters, numerals and figures are displayed. When the display is limited 
to the display of numerals, the numerals 0 to 9 are displayed in a segment 
display system (U.S. Pat. No. 3,781,863), by selecting appropriate 
electrodes from among seven segment electrodes arranged in the shape of 
the figure 8. When the same letter, numeral or figure is displayed 
constantly, a fixed pattern display system is employed in which electrodes 
are formed into the shape of the relevant letter, numeral or figure, and 
voltage is constantly applied to the electrodes. 
Because of the recent trend toward the diversification of usages of liquid 
crystal display devices and the contents of their displays, a need has 
arisen for displaying on a single liquid crystal display surface, i.e. a 
liquid crystal panel, by a combination of the dot matrix display system 
and the segment display system, the dot matrix display system and the 
fixed pattern display system, or the segment display system and the fixed 
pattern display system. 
For example, as is shown in FIG. 3, a liquid crystal panel 6 is provided 
with a dot matrix display part 7 and a fixed pattern display part 8. In 
this example, a group of numerals or a graph is shown in the dot matrix 
display part 7, and the fixed pattern display part 8 is provided with 
display parts 8a, 8b and 8c which show that the group of numerals or graph 
is, for example, the maximum value (MAX), medium value (MED), and minimum 
value (MIN), respectively. 
In the dot matrix display part 7, n scanning electrodes X.sub.1, X.sub.2, . 
. . X.sub.n are subsequently selected by a scanning electrode driving 
circuit 9, and a selected signal electrode voltage or a non-selected 
signal electrode voltage is applied to the signal electrodes Y.sub.1, 
Y.sub.2 . . . Y.sub.m by a signal electrode driving circuit 10 according 
to information to be displayed. 
In the fixed pattern display part 8, an electrode 8M which is common to the 
display parts 8a, 8b and 8c is selected as the scanning line X.sub.n+1 
which follows the scanning line X.sub.n of the dot matrix display part 7 
by the scanning electrode driving circuit 9 in the order of X.sub.1, 
X.sub.2, . . . X.sub.n, X.sub.n+1. Fixed pattern electrodes 8P, 8Q and 8R 
which are used exclusively for the display parts 8a, 8b and 8c, 
respectively, are connected to one of the signal electrode Y.sub.1, 
Y.sub.2, . . . Y.sub.m of the dot matrix display part 7, and a signal 
electrode voltage is applied by the signal electrode driving circuit 10. 
Consequently, the fixed pattern display parts 8a, 8b and 8c are driven in 
the same way as the picture elements of the dot matrix display part. 
In other words, when a conventional liquid crystal panel is provided with 
both dot matrix display system and fixed pattern display system, the fixed 
pattern display system is also driven in time-multiplexed fashion with the 
same duty ratio 1/n+1 as that of the dot matrix display system. 
This kind of driving method, however, is disadvantageous in that as the 
number of time-multiplexing, namely the number of scanning lines (n +1) 
increases, and the contrast decreases as described before. 
The case will now be considered in which the liquid crystal display device 
shown in FIG. 3 is formed into a reflective type display device by 
disposing a reflector behind the liquid crystal panel 6, and is driven by 
the a:1 selection scheme. In the dot matrix display part 7 each selected 
point is surrounded by a non-selected part, i.e. half-selected elements, 
and the effective value of voltage applied to the half-selected elements 
is constant irrespective of changes in the display pattern. 
Accordingly, the display luminance of the dot matrix display part 7 
consists of only two kinds of luminance, i.e., the luminance of selected 
points and that of half-selected points, so that there is little 
possibility of misreading the display, even if the number (n+1) of 
time-multiplexing increases and the contrast between the selected points 
and the half-selected points thereby drops. Since the fixed pattern 
display part 8 is surrounded by an area 14 where there are no scanning 
electrodes or signal electrodes (hereinafter referred to as a 
"no-electrode area"), there are three kinds of luminance in the fixed 
pattern display part 8 and its vicinity, namely, the luminance of selected 
points; the luminance of non-selected points, i.e., half-selected points, 
and the luminance of the no-electrode area 14, which is not influenced by 
the applied voltage at all. As a result, when the number (n+1) of 
time-multiplexing increases, and there is little difference in luminance 
between the selected points and the half-selected points, the contrast 
between selected points and no-electrode area 14 appears to be 
approximately the same contrast between half-selected points and 
no-electrode area 14, so that there is a strong possibility of misreading 
a half-selected point as a selected point. For example, in the liquid 
crystal panel shown in FIG. 3, even when the "MED" in the display part 8b 
of the fixed display pattern part 8 is selected, if the difference between 
the effective voltage applied across the liquid crystal layers of the 
display parts 8a and 8c and that applied across the liquid crystal layer 
of the display part 8b becomes very small, because of the large number 
(n+1) of time-multiplexing, the display parts 8a, 8b and 8c appear to have 
approximately the same contrast with respect to the no-electrode area 14, 
which acts as a background, so that all of the display parts 8a, 8b and 8c 
appear to be in the selected state. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
for driving a liquid crystal display device which maintains high contrast 
in a fixed pattern display system and a segment display system even when 
the device is driven in highly time-multiplexed fashion and thereby 
preventing misreading, thereby eliminating the above-described problems in 
the prior art. 
It is another object of the present invention to provide a liquid crystal 
display device employing the method according to the present invention. 
To achieve this aim, according to the present invention, a common electrode 
of a fixed pattern display part and a segment display part are driven 
independently of a dot matrix display part.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 4 shows a liquid crystal display device employing an embodiment of a 
driving method according to the present invention. In the dot matrix 
display part 7, n scanning electrodes X.sub.1, X.sub.2, . . . X.sub.n are, 
as is the case with the display device shown in FIG. 3, sequentially 
selected and scanned by a scanning electrode driving circuit 29, and a 
selected signal electrode voltage or a non-selected electrode voltage is 
applied to the signal electrodes Y.sub.1, Y.sub.2, . . . Y.sub.m by a 
signal electrode driving circuit 30 according to information to be 
displayed. The display device shown in FIG. 4 is different from that shown 
in FIG. 3 in that the electrode 8M which is common to the display parts 
8a, 8b and 8c of the fixed pattern display part 8 is connected to a common 
electrode driving circuit 15, in that the fixed pattern electrodes 8P, 8Q 
and 8R which are exclusively used for the display parts 8a, 8b and 8c, 
respectively, are not connected to any of the signal electrodes Y.sub.1, 
Y.sub.2, . . . Y.sub.m of the dot matrix display part 7, and in that a 
selected signal electrode voltage or a non-selected signal electrode 
voltage is applied to the fixed pattern electrodes independently of each 
other by the signal electrode driving circuit 30. 
The dot matrix display part 7 is driven on the basis of the formulae (1) to 
(6) which are related to the a:1 selection scheme, and 
the selected signal electrode voltages V.sub.111, V.sub.112 are such that 
EQU V.sub.111 =0 
EQU V.sub.112 =V.sub.0 
the non-selected signal electrode voltages V.sub.121, V.sub.122 are such 
that 
EQU V.sub.121 =2/a V.sub.0 
EQU V.sub.122 =(1-2/a) V.sub.0 
the selected scanning electrode voltages V.sub.211, V.sub.222 are such that 
EQU V.sub.211 =V.sub.0 
EQU V.sub.212 =0 
and the non-selected scanning electrode voltages V.sub.221, V.sub.222 are 
such that 
EQU V.sub.221 =1/a V.sub.0 
EQU V.sub.222 =(1-1/a)V.sub.0 
The alternation of driving voltages between V.sub.111 and V.sub.112, 
V.sub.121 and V.sub.122, V.sub.211 and V.sub.212, and V.sub.221 and 
V.sub.222 is conducted by the control signal M. 
On the other hand, when the fixed pattern 8 is driven, the non-selected 
signal electrode voltages V.sub.121, V.sub.122, which are the same as the 
one applied to the signal electrodes Y.sub.1, Y.sub.2, . . . Y.sub.m, are 
inputted to the common electrode driving circuit 15. As is shown in FIGS. 
5A and 5B, the common electrode driving circuit 15 alternately applies the 
voltages V.sub.121, V.sub.122 to the common electrode 8M by the 
alternation of driving voltages carried out by the control signal M. The 
selected signal electrode voltage V.sub.111 and V.sub.112 are alternately 
and constantly applied to a selected one of the fixed pattern electrodes 
8P, 8Q and 8R in the display parts 8a, 8b and 8c of the fixed pattern 
display part 8 as shown in FIG. 5C. FIG. 5D shows the voltages applied 
across the liquid crystal layer. 
As is clear from FIG. 5, in the fixed pattern display part 8, the display 
device is driven in a static driving fashion, not in a time-multiplexed 
fashion. Therefore, no voltage is applied across the liquid crystal layer 
at a non-selected electrode part of the fixed pattern display part 8. 
Consequently, the fixed pattern display part 8 is free from the 
half-selected points which are inherent in the dot matrix display part 7, 
and there is no possibility of misreading. 
It is possible to use a pair of transfer gates for the common electrode 
driving circuit 15. 
In this embodiment, a combination of the dot matrix display system and the 
fixed pattern display system is employed, but the present invention is not 
restricted to this combination. A combination of the segment display 
system and the dot matrix display system is adaptable to the present 
invention. For example, it is possible to drive the display device by 
connecting the common electrodes arranged in the shape of the figure 8 as 
described above to the common electrode driving circuit 15 shown in FIG. 
4, and each of the segment electrodes to the signal electrode driving 
circuit 30. 
The present invention is realized simply by attaching a pair of transfer 
gates to a conventional device in place of commonly driving by a scanning 
electrode driving circuit, and thereby the contrast is improved several 
fold. 
As has been described above, it is possible in accordance with the present 
invention to greatly improve the contrast of the fixed pattern display 
part and the segment display part, and a method for driving a liquid 
crystal display device providing a good display quality and a liquid 
crystal display device employing the method is thereby obtained.