Computer graphics processing and selective visual display system – Plural physical display element control system – Segmented display elements
Reexamination Certificate
1998-08-07
2001-03-20
Hjerpe, Richard A. (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Segmented display elements
C345S033000, C345S034000, C345S048000, C345S050000, C345S051000, C345S053000
Reexamination Certificate
active
06204831
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a driving system of a relatively small simple matrix liquid crystal display (hereinafter abbreviated LCD) for remote control devices, electronic calculators, etc.
BACKGROUND OF THE INVENTION
Recently a simple matrix LCD has been widely used for electronic calculators, electric home appliances such as radios and measuring equipment, etc. For application to these devices, it is desirable for the LCD to have less power consumption, less driving voltage, good contrast, less crosstalk, viz., less phenomenon of half-selected segments appearing like those selected because of potentials applied there, and yet less expensive.
A conventional simple matrix driving system for displaying a liquid crystal panel has been a multiplex system, viz., a system of a line sequential AC drive. The system has common electrodes and segment electrodes. The common voltage waveforms are applied to the common electrodes each in a manner of time division line sequences. The signal voltages are each applied to the segment electrodes. Then the selected points are displayed by the combination of these two types of the voltages. The system is widely adopted because less signal lines are needed for driving.
As is well known, electrolysis occurs when a direct current is continuously applied to the liquid crystal. Therefore, the mean value of the electric field applied to the liquid crystal during a certain period needs to be zero in order to prevent the electrolysis.
For obtaining a good display quality, the system described above adopts the system of applying a bias voltage for the proper setting of the effective values “Von” and “Voff”, which are applied to the selected points (an active portion of the liquid crystal) and to the half-selected points (an inactive portion of the liquid crystal) respectively. The system needs three or more values of voltages, viz., voltages of a power source voltage level, zero potential and one or more values of an intermediate level. A popular example is ½ duty-½ bias or ⅓ duty-⅓ bias driving system.
On one hand it is necessary to apply the “Voff” voltages to the half-selected segments for the speed up of the response of the liquid crystal; on the other hand, the larger “Von/Voff” ratio is, the better the contrast is.
The following is an explanation of the example of ½ duty-½ bias or ⅓ duty-⅓ bias driving;
FIG. 4
shows the structural diagram of the liquid crystal display portion of ½ duty-½ bias with seven segments forming a numeric “
8
”. The two common electrodes C
1
and C
2
are commonly coupled with each of the segments, and the four segment electrodes S
1
through S
4
are commonly coupled with each of the segments. The shaded segments in
FIG. 4
are under driving.
FIG. 6
shows the common voltage waveforms of C
1
and C
2
of the conventional liquid crystal driving circuit
1
of FIG.
8
(
a
).
FIG. 6
also shows the segment voltage waveforms of S
1
and S
2
of the same circuit, and the voltage waveforms of the potential differences between the common electrode C
1
and the segment electrodes S
1
and S
2
. This ½ duty-½ bias common voltage waveforms have the three voltage levels of VDD, V
1
and V
2
, and then, the segment voltage waveforms have the two voltage levels of VDD and V
2
. The liquid crystal driving circuit
1
gets these voltages from the voltage dividing circuit
2
. The voltage dividing circuit
2
, having voltage-dividing resistors shown in FIG.
9
(
a
), generates the voltage levels of V
1
and V
2
by dividing the power source voltage VDD which comes from the power source
3
. With a variable resistor Rv in FIG.
9
(
a
), the potential levels between VDD and V
2
are adjusted for the control of the display intensity.
From the voltage waveforms of
FIG. 6
, it is understood that, for example, the voltage of effective value V
1
((1
2
+2
2
)/
½
is applied between the common electrode C
1
and the segment electrode S
1
. Then, the segment
11
between the common electrode C
1
and the segment electrode S
1
is driven because the voltage is higher than the threshold voltage for ON of the liquid crystal. Between the common electrode C
1
and the segment electrode S
2
, the voltage of the effective value V
1
(1
2
/2)
½
is applied. However, the segment
12
between the common electrode C
1
and the segment electrode S
2
is not driven because the voltage is lower than the threshold voltage for ON of the liquid crystal.
FIG. 5
shows structural diagrams of the liquid crystal display portion of ⅓ duty-⅓ bias with seven segments forming a numeric “8”. The system has the common electrodes C
1
through C
3
which are commonly coupled with each of the segments, and the segment electrodes S
1
through S
3
which are commonly coupled with each of the segments. The shaded segments are under driving.
FIG. 7
shows the common voltage waveforms of the common electrodes C
1
through C
3
of the conventional liquid crystal driving circuit
4
of FIG.
8
(
b
).
FIG. 7
also shows the segment voltage waveforms of the segment electrodes S
1
through S
3
of the same circuit, and the voltage waveforms of the potential differences between the common electrodes C
1
, C
2
and the segment electrodes S
1
, S
3
. These ⅓ duty-⅓ bias common voltage waveforms and the segment voltage waveforms have four voltage levels of VDD, V
1
, V
2
and V
3
. The liquid crystal driving circuit
4
of FIG.
8
(
b
)gets these voltages from a voltage dividing circuit
5
. The voltage dividing circuit
5
, having voltage-dividing resistors of FIG.
9
(
b
), generates the voltage V
1
, V
2
and V
3
by dividing the power source voltage VDD from a power source
9
. With a variable resistor Rv in FIG.
9
(
b
), the potential levels between VDD and V
3
are adjusted for control of a display intensity.
From the voltage waveforms of
FIG. 7
, for example, the voltage of the effective value V
1
((1
2
+1
2
+1
2
)/3)
½
is applied between the common electrode C
1
and the segment electrode S
1
. However, the segment
21
of
FIG. 5
between the common electrode C
1
and the segment electrode S
1
is not driven because the effective value is lower than the threshold voltage for ON of the liquid crystal. Between the common electrode C
2
and the segment electrode S
3
, the voltage of the effective value V
1
((1
2
+3
2
+1
2
)/3)
½
is applied. Then, the segment
22
between the common electrode C
2
and the segment electrode S
3
is driven because the effective value is higher than the threshold voltage for ON of the liquid crystal.
As described above, the conventional driving system needs the control of three or more voltages. However, the digital circuits of microcomputer, gate array, etc. are operated on the binary basis of on-off. Therefore, it is practically difficult to adopt the direct control system for the digital circuits like microcomputer, gate array, etc., because a complicated structure is needed for the direct control of three or more voltages on the circuits.
The driving system described above receives a plurality of voltages, in some cases, from the divided voltages which are generated by dividing the power source voltage with the voltage dividing resistors. In these cases, the output impedance of the power source to the LCD depends on the voltage dividing resistors. Then, if the resistance values of the dividing resistors are increased for a purpose of low power consumption, the driving voltage waveforms are distorted by the resistance load and the capacitance of the liquid crystals. Since the capacitance is different by each segment, the display intensity of the selected segments differs by each segment. Then, the uniform contrast is not obtainable and also an uneven crosstalk occurs on the half-selected segments.
The digital circuits have come to be driven with lower and lower voltages and the microcomputers driven with less than two volts are now in use. However, the d
Nishioka Nobuyuki
Yamamoto Osamu
Bowers Benjamin D.
Hjerpe Richard A.
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel L.L.P.
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