Horology: time measuring systems or devices – Chronological – With electro-optical display
Reexamination Certificate
2001-05-01
2003-09-09
Martin, David (Department: 2841)
Horology: time measuring systems or devices
Chronological
With electro-optical display
C368S242000, C345S095000
Reexamination Certificate
active
06618327
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Invention
The present invention relates generally to LCD displays, and more particularly, but not by way of limitation, to a method and system for driving LCD displays.
2. Description of the Related Art
Liquid crystal displays (LCD) are used extensively in electronic devices and displays. The LCD has become part of every day life, being included in devices have become digital in nature, such as automobile dashboards, computer monitors, radios, and watches.
Traditionally, LCDs have been used to display basic information, such as text, numbers, and symbols, mainly due to the limited capability of the LCD (i.e., on/off; black and white). However, more recently LCDs capable of displaying gray scale and color have become available. Further, technical advances in LCDs have provided the ability to use reflective polarizers within the LCDs to allow for screen printed images and colors to be selectively displayed. One such reflective polarizer is described in Ouderkirk et al., U.S. Pat. No. 5,828,488, and issued Oct. 27, 1998. An application of an LCD utilizing reflective polarizers is described in European Patent EP 0 825 477 A3, published Jun. 23, 1999, and issued to applicant Seiko.
An LCD is a passive device that does not generate light, but rather manipulates the ambient light that passes through it. There are many variations of LCD technology, but the most common of these is the field effect twisted-nematic LCD. To provide the reader with a basic understanding of LCDs and their operation,
FIGS. 1
to
3
B are provided and discussed hereinafter.
FIG. 1
is a layered representation of an exemplary LCD
100
. The LCD
100
includes an upper polarizer
105
coupled to an upper glass layer
110
. Beneath the upper glass layer
110
and coupled thereto is an (upper) electrode
115
that is generally transparent. A liquid crystal layer
120
is sandwiched between the upper electrode
115
and a lower electrode
125
, which is coupled to a lower glass layer
130
. A lower polarizer
135
, which may be a reflective polarizer as described in EP 0 825 477 A3, as suggested above, is below the lower glass layer
130
. A reflector
140
may also be located below the lower polarizer
135
. Although not shown, a screen print may be located between the lower reflective polarizer
135
and the reflector
140
. The screen print may show an image or simply reveal a uniform color when a segment is activated or non-activated depending on orientation of polarizers.
FIG. 2
shows selected aspects
200
of the LCD
100
that describe operability of the LCD. A light-molecule or source
205
that is oscillating (i.e., non-polarized) enters the upper polarizer
105
. Because the upper polarizer
105
is polarized in a single plane, only light
210
having its direction vector in the same plane as the upper polarizer
105
passes through the upper polarizer
105
, which generally results in a 50% decrease in light intensity.
Two states of the LCD are shown, (i) voltage applied and (ii) voltage not applied. In the first case, (i.e., voltage not applied), the light
215
a
is rotated in polarity by 90 degrees after passing through the liquid crystal
120
. By not applying a voltage, or applying a voltage below a “turn-on” threshold, to the electrodes
115
and
125
, the crystalline structure
120
a
of the liquid crystal
120
is twisted or rotated by 90 degrees. This 90 degree rotation causes the polarization of the light to be aligned with the lower polarizer
135
such that the light
215
a
passes through the lower polarizer
135
. This light
220
is reflected off of the reflector
140
and a gray-on-gray image is displayed on the LCD as viewed through the upper polarizer
105
. LCDs having a 90 degree twist of the liquid crystal, which are organic molecules, are, generally, twisted nematic (TN) liquid crystals. More recently, super twisted nematic (STN) liquid crystals provide for as much as 360 degrees of twist. The STN liquid crystals provide a much higher response to an applied voltage, thereby allowing for many more segments to be integrated in a display while still producing a high contrast display.
In the second case (i.e., voltage applied), the crystalline structure
120
b
of the liquid crystal
120
becomes aligned in the same direction (i.e., perpendicular to the electrodes
115
and
125
) such that the light
215
b
is not twisted upon exiting the liquid crystal
120
. Because the lower polarizer
135
is oriented perpendicular to the polarization of the incoming light
215
b
, the incoming light is blocked or absorbed by the lower polarizer
135
and is not reflected by the reflector
140
. The image is seen on the LCD as being a “positive” image (i.e., black on gray) as viewed through the upper polarizer
105
.
FIGS. 3A and 3B
are exemplary LCDs
300
a
and
300
b
having seven segments for displaying a digit. In
FIG. 3A
, upper electrodes
115
a
-
115
g
are each applied a voltage so that the digit “
8
” is displayed. In
FIG. 3B
, upper electrodes
115
d
-
115
f
are each applied a voltage so that the digit “7” is displayed. The lower electrode
125
is considered to be a “common” so that a voltage differential is created between the segments connected to the upper electrodes
115
a
-
115
g
having voltage applied thereto. It should be understood that the liquid crystal substantially sandwiched (i.e., within a segment, which is defined by common borders of the upper and lower electrodes
115
and
125
) are affected by the root-mean-square (RMS) voltage applied to the electrodes
115
and
125
.
Driving systems for LCDs generally include specialized circuitry that have standardized functionality. Two conventional approaches using digital circuitry have been taken by designers of driving systems for LCDs; a first approach is a fixed multiplexing approach, and a second approach is a pulse width modulation (PWM) multiplexing approach.
The fixed multiplexing approach operates on the basis of having a fixed number of lower electrodes or backplanes
125
connected to a driving system, where the driving system is configured to drive the upper and lower electrodes with predetermined voltages based on the number of backplanes to turn on and off the segments of the LCDs. A duty cycle is generated by the driving system to create an RMS voltage based on the fixed number of backplanes of the LCD. A limitation of the fixed multiplexing approach is that only two levels can be created on the LCD because the RMS voltage levels produced by the LCD driving system are fixed (i.e., on or off). Once a particular driving system (e.g., driver chip) and the number of backplanes of the LCD are selected or specified, a manufacturer of LCDs selects a liquid crystal fluid that operates within the range of the driving system. Those skilled in the art appreciate that a non-direct current (non-DC) voltage is generated by the driving system and applied to the LCD to avoid damaging the LCD.
Designers who desire gray-scale or color blends (i.e., voltage level changes) displayed on the LCD use pulse width modulation multiplexing. The pulse width modulation multiplexing approach operates on the basis of being able to drive an upper and lower electrode pair using pulse width modulation. One commercially available LCD driving system, SED1767, using conventional PWM is provided by S-MOS Systems, a Seiko Epson affiliate. This LCD driving system provides up to 16 gray-scale levels. However, this driving system requires many inputs, including gray-scale data bits to set gray-scale levels or duty cycles by the LCD driving system.
In general, the LCD driving systems used to generate various gray-scale voltage levels using conventional PWM to produce multi-level displays on LCDs are rather complex and expensive due to their unique functionality. Essentially, these specialty LCD driving systems have been developed for high-end commercial systems. Thus, consumer goods, such as watches, that are sufficiently driven by market considerations, such as price, are cost-prohibited from
Brewer Donald R.
Chun Lee Tak
Carstens David W.
Carstens Yee & Cahoon LLP
Fossil, Inc.
Lindinger Michael L.
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