In the ventricle of the reptile heart. Who has a three-chambered heart? Amphibians and reptiles
Liquid crystal indicators (LCDs) are based on the use of so-called liquid crystals (LCs), which are some organic liquids with an ordered arrangement of molecules characteristic of crystals. Liquid crystals are transparent to light rays, but under the influence of an electric field of 2 - 5 kV/cm, their structure is disrupted, the molecules are arranged randomly and the liquid becomes opaque.
These indicators can have various designs and work either in transmitted light created by some special source, or in the light of any source (artificial or natural) reflected in the indicator.
In Fig. 40 shows an LCD operating on reflection. Indicators of this type are used in wrist electronic watch, microcalculators and other devices. Between two glass plates 1 and 3, glued together with polymer resin 2, there is a layer of liquid crystal 4 with a thickness of 10 - 20 microns. Plate 3 is covered with a continuous conductive layer (electrode 5) with a mirror surface. Transparent layers are applied to plate 1 - electrodes A, B, C, from which conclusions are drawn that are not shown in the figure. These electrodes are in the form of numbers or letters or segments for synthesizing various characters.
Figure 40 – Liquid crystal display, reflective
If voltage is not applied to the sign electrodes, then the LCD is transparent, the light rays of external natural lighting pass through it, are reflected from electrode 5, come back out and no signs are visible. But if voltage is applied to some electrode, for example A, then the liquid crystal under this electrode becomes opaque, light rays do not pass through this part of the liquid (6), and then a dark sign is visible on a light background.
Liquid crystal indicators are very economical and durable. To control LCDs, rather complex devices are used, usually based on integrated circuits. They are widely used as portable and stationary displays electronic devices– communications equipment, measuring equipment, computer equipment. At the same time, today they are the main type of monitors and television receivers.
The efficient and reliable use of many industrial electronics systems is impossible without the participation of a human operator in control, who must receive necessary information about the operation of the system and controlled parameters. This purpose is served by devices designed to convert various data into a visible image and called information display devices.
Information display devices can solve the simplest, but very important tasks monitoring the system status: “Working”, “Not working”, “On”, “Off”, “Stop”, etc. In more complex cases, they are assigned the function of displaying digital, text and graphic information characterizing the technological process, the operation of the production facility and the entire system.
In Fig. 8.8, a shows the circuit for excitation of segments with an alternating voltage signal. The device consists of two logic circuits AND with two inputs OO2, 003, an inverter 001 and a driver switch on a transistor UT. A voltage equal to double the amplitude of the nominal alternating excitation voltage of a given liquid crystal indicator is applied to the transistor collector.
An excitation voltage with a frequency = (30...50) Hz is applied to input 002, and a damping voltage with a frequency /g = (10...40) kHz is applied to the input £ШЗ. When the logic level of the control signal is low, 002 opens, and the transistor operates in pulse mode with a frequency corresponding to the excitation frequency of the LC segment. A control signal with a high logical level, coming from the decoder to the control input, opens B03. As a result, the device generates a voltage of increased frequency, to which the LCD segment does not respond. Taking into account the fact that the control device must be comparable in power consumption to the LCD indicator, all logic circuits are made on the basis of KMDP structures.
In addition to what is described, another type of device for exciting LCD indicators is also used (Fig. 8.8, b). At the input of logic circuits 002 and 003, pulse voltages with frequency/v = (15...20) Hz, shifted in phase relative to each other by 180°, are supplied from an external generator. Depending on the level of the control signal, voltage is applied to the indicator segment through the driver switch (transistor UT) rectangular shape, direct or phase shifted. A signal of one phase is constantly supplied to the common electrode of the indicator through another switch-former (transistor UT2).
When the phases on the electrodes of a segment coincide, the latter is not excited; when the phases differ, the segment is excited. Note that the phase control method makes it possible to reduce the indicator supply voltage by half.
When using multi-digit indicators, it is required big number external connections required to manage segments. This forces us to resort to creating multiplexer control. In Fig. Figure 8.9 shows the principle of controlling a four-digit indicator with separated common electrodes for each digit, which consists in combining identical segments across all digits and sequentially addressing data to the corresponding digits. The process of displaying a four-digit number is carried out in clock cycles. In each cycle, an alternating control voltage is applied to the control bus of the segments and to the line of the common electrode of the discharge that is excited in this path. Due to the long relaxation time of liquid crystals, the discharge numbers continue to be read in the period between excitation cycles without applying voltage.
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- Study the connection diagram of the liquid crystal indicator (LCD) to the microcontroller.
- Study the features of the symbolic LCD.
- Study the features of parallel synchronous data transmission.
- Learn to display information on the LCD.
2 Preliminary preparation for work
- Using the lecture notes and recommended literature, study the principle of operation of a symbolic liquid crystal indicator.
- Using the lecture notes and recommended literature, study the principle of operation of parallel I/O ports of a microcontroller.
- Create an algorithm for the program according to the task.
- Write a program in the C programming language.
3 Brief theoretical information
3.1 Design and principle of operation of a symbolic liquid crystal indicator
Currently, microprocessor systems widely use liquid crystal indicators (LCD). Conventionally, all LCDs can be divided into two categories: symbolic, or sign-synthesizing, and graphic. Graphic indicators are a matrix of m lines and n columns at the intersection of which there are pixels. A pixel is an indivisible rectangular or round object with a specific color; pixel – smallest unit raster image. If an electrical signal is applied to a certain column and row, the pixel at their intersection will change its color. By applying a group of signals to columns and rows, you can form an arbitrary graphic image point by point. This is how a graphical LCD works. In a character LCD, the pixel matrix is divided into submatrices, each submatrix is designed to form one character: a number, a letter or a punctuation mark. Typically, a matrix of eight rows and five columns is used to form one character. Character indicators come in one-, two- and four-line formats.
To simplify interaction microprocessor system and LCDs use a specialized chip - an LCD controller (driver). It controls the pixels of the liquid crystal display and the interface part of the indicator. Typically, such a controller is included in the indicator. In general, the liquid crystal indicator is a printed circuit board on which the display itself, the controller and the necessary additional electronic components are mounted. Appearance The LCD is shown in the figure below.
Figure 1 – Appearance of the liquid crystal indicator
4 Assignment for work in the laboratory
4.1 Displaying a symbol on the LCD
- Develop a program algorithm that displays your name in a given line on the LCD screen. The LCD operating mode and line number are determined according to the task option (Table 2).
- Using the circuit diagram of the LESO1 training stand, determine which pins of the ADuC842 microcontroller the LCD is connected to. Using the SFR table, determine the addresses of the input/output ports used.
- Develop and enter the program text in accordance with the created algorithm.
- Translate the program and correct syntax errors.
- Make sure that the required symbol appears on the display screen at the specified position.
4.2 LCD control via the serial port of a personal computer (optional)
- Change the program so that the LCD screen displays information transmitted from a personal computer via UART. The command is sent through the nwFlash terminal. The choice of synchronization source and data transfer rate is at the discretion of the student.
- Upload the resulting *.hex file to the LESO1 laboratory bench.
- Pass the symbol codes through the nwFlash terminal, make sure that the corresponding symbols are displayed on the indicator screen.
Table 2 - Task options
option number | line number | cursor mode |
1 | first | switched off |
2 | second | on, flickering |
3 | first | on, no flickering |
4 | second | switched off |
5 | first | on, flickering |
6 | second | on, no flickering |
7 | first | switched off |
8 | second | on, flickering |
9 | first | on, no flickering |
10 | second | switched off |
11 | first | on, flickering |
12 | second | on, no flickering |
13 | first | switched off |
14 | second | on, flickering |
15 | first | on, no flickering |
5 Instructions for preparing a report
The report must contain:
- Goal of the work.
- Schematic diagram connecting the LCD to the control microcontroller.
- Block diagram LCD
- Diagrams of data transfer over a parallel interface.
- Calculation of timer parameters.
- Graphic diagram of the program algorithm.
- Source text of the program.
- Contents of the software project listing file.
- Conclusions from the laboratory work performed.
The schemes, as well as the report as a whole, are carried out in accordance with the standards of the ESKD.
Liquid crystal indicators appeared recently (70s) and began to be widely used as digital indicators. LCD indicators are passive devices. They do not generate light and require additional illumination; they themselves act as a modulator, operating in the mode of transmitting or reflecting light.
Liquid crystals (LCs) are organic liquids having elongated rod-shaped molecules. There are three types of FAs (Fig. 5.2): smectic, nematic and cholesteric.
In smectic liquid crystals, highly elongated molecules are arranged in layers of equal thickness, close to the length of the molecules. The molecules are oriented parallel to each other. Nematic liquid crystals do not have a layered structure, and the molecules are also oriented parallel to each other with their long axes. Cholesteric FAs have a layered structure, but in each layer the molecules are elongated in a certain preferential direction.
Rice. 5.2 - Types of liquid crystal indicators:
A- smectic; b- nematic; V- cholesteric
The orientation of an individual LC molecule is subject to continuous thermal fluctuations, but at any point in the liquid there is an average orientation characterized by a unit vector called director D. When an LC substance occupies a large volume, regions with independent director orientations appear in the molecule. To impart the same orientation throughout the entire working space, the LC is enclosed in a narrow (several tens of micrometers) space between the substrates. As a result, the specific orientation of LC molecules is determined by both neighboring molecules and the boundary surface of the substrate. The orienting effect is achieved by sputtering thin films of SiO 2 onto the substrate.
LC molecules are individual dipoles. The orientation of molecules can change as a result of various electrohydrodynamic effects caused by the flow of even a small current or under the influence of an electric field.
The design of the unit cell of the LCD indicator is simple and contains two glass plates with inside transparent conductive coating. LCD is poured between the plates. The thickness of the LC ranges from 6 to 25 µm. This design is essentially a flat capacitor. In the absence of voltage on the cell, the LC substance is homogeneous and transparent. When a threshold voltage is applied to the cell, a wavy domain structure appears. When the threshold voltage is exceeded, the domain structure turns into a cellular structure, then a vortex movement. LCD loses optical uniformity and scatters light in all directions. This effect is called dynamic scattering. Currently, indicators based on the dynamic dispersion effect are common, as well as indicators using the field twist effect (twisting) and the “guest-host” effect.
Currently, the most common indicators are those that use the field twist effect (from the English twist - twisting). The operation of a cell with crossed polarizer P and analyzer A is shown in Fig. 5.3.
In the absence of supply voltage to the cell, the LC molecules are twisted by approximately 90° due to the orienting effect of the P and A substrates.
A polarizer is an optical element that transmits light polarized in one direction and rejects light polarized in the opposite direction, depending on the orientation of the polarizer. If the axes of the second polarizer, called the analyzer, are parallel to the axes of the first, then the light passes through the second polarizer; if the axes of the analyzer are perpendicular, the radiation is extinguished.
Rice. 5.3 - Operation of an LCD indicator based on the twist effect at voltages:
A- zero; b- exceeding the threshold
Light incident from above is polarized in such a way that its polarization vector coincides with the direction of director D at the upper substrate. When passing through an LC, the plane of polarization of light rotates (like the director of LC molecules) and the light passes through the analyzer. When the cell is powered with a voltage above the threshold, the LC polarization vector acquires a vertical direction and the LCs do not rotate the plane of polarization, and the analyzer does not transmit light.
LCD indicators have advantages over indicators based on the dynamic dissipation effect (lower operating currents 1-3 µA/cm 2 instead of 10 µA/cm 2, and therefore greater durability). The performance of LCDs using the twist effect is much higher than when using dynamic scattering.
The disadvantages of LCD indicators based on the twist effect include a smaller viewing angle than indicators based on the dynamic scattering effect, which is associated with the narrow directional pattern of light during the twist effect and the influence of polarizers. The use of polarizers leads to losses of up to 50% of light and also increases the cost of indicators.
Indicators without polarizers can be created based on the “guest-host” effect. Rod-shaped dye molecules (guest) are introduced into the LC (host). The dye molecules tend to be oriented parallel to the axes of the FA molecules (Fig. 5.4).
Rice. 5.4 - Operation of an LCD cell using the “guest-host” effect at voltages:
A- zero; b- exceeding the threshold; 1 - dye molecules; 2 - liquid crystal molecules
In the initial state, at zero voltage on the LC cell, light with any direction of polarization is absorbed (Fig. 5.4, A). When a sufficiently strong electric field is applied, the LC substance goes into a state in which all the dye molecules are oriented vertically, and the light incident on the cell passes freely through it (Fig. 5.4, b).
The described system is promising, as it allows one to obtain an almost black positive image on a white background at high brightness and a fairly wide viewing angle. The contrast of indicators based on the “guest-host” effect is somewhat worse due to the absorption of light by the dye.
The advantages of LCD indicators are as follows:
Low power consumption (110 μW/cm2);
Work at high level external lighting;
Simplicity of design and manufacturing technology;
Low cost, low operating voltage.
The main disadvantages of LCD indicators include a narrow range of operating temperatures (from -10 to +60 ° C), long transient processes, which also depend on temperature.
In table 5.5 shows the parameters of some LCD indicators produced in our country.
Table 5.5
Currently, work is underway to create matrix LCD indicators. Significant progress has been made in the creation of multicolor LCD indicators using color filters.
LIQUID CRYSTAL INDICATORS
Liquid crystal displays (LCDs) control the reflection and transmission of light to create images of numbers, letters, symbols, etc. Unlike Light-Emitting Diodes (LEDs), LCDs do not emit light.
The basis of LCDs are liquid crystals (LCs), the molecules of which are ordered layer by layer in a certain way between two glass plates. In each layer, the cigar-shaped LC molecules line up in one direction, their axes becoming parallel (Fig. 1).
rice. 1 One layer of LC molecules. All molecules are parallel to each other.
The glass plates have a special coating such that the orientation of the molecules in the two outer layers is perpendicular. The orientation of each LC layer changes smoothly from the top to the bottom layer, forming a spiral (Fig. 2). This spiral "twists" the polarization of light as it passes through the display.
rice. 2 Several layers of LC molecules, ordered in such a way
that polarized light "twists" as it passes through them.
Molecules in different layers line up in a spiral.
Under the influence of an electric field, the LC molecules are reoriented parallel to the field. This process is called twisted nematic field effect (TNFE). With this orientation, the polarization of light does not twist when passing through the LC layer (Figs. 3a and 3b). If the front polarizer is oriented perpendicular to the back polarizer, light will pass through the display when it is turned on, but will be blocked by the rear polarizer. In this case, the LCD acts as a light blocker.
The display of different characters is achieved by selectively etching a conductive surface previously created on the glass. The non-etched areas become the characters, and the etched areas become the background of the display.
rice. 3a "Off" state of the LCD.
LC molecules form a spiral, twisting the polarization of light.
rice. 3b "On" state.
The electric field reorients the LC molecules so
that they do not change the polarization of light.
Symbols are created from one or more segments. Each segment can be addressed (powered) individually to create a separate electrical field. In this way, the passage of light is controlled electrically, turning on and off the necessary segments. In the inactive part of the display, the orientation of the molecules remains spiral, forming the background. The powered segments make up characters that contrast with the background.
Depending on the orientation of the polarizer, the LCD can display a positive or negative image. In a positive image display, the front and rear polarizers are perpendicular to each other, so that the unpowered segments and background allow the polarized light to pass through, while the energized segments block the passage of light. The result is dark characters on a light background.
In a negative image display, the polarizers are parallel, "in phase", blocking the passage of reversed polarization light, so that the unpowered characters and background are dark, and the powered characters are light.
A reflective LCD has a reflector behind the rear polarizer that reflects light passing through the unpowered segments and the background. In negative reflective displays, light is reflected through energized, "on" segments. Transmissive LCDs use the same principles, but the background or segments are made brighter by using backlighting.
rice. 4 Main components and design of reflective LCD.
LCD display modes determine how the indicator controls light to create an image. To choose optimal mode For a specific application, typical indicator lighting conditions must be considered (see Table 1).
Table 1. LCD display modes
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Reflective (working on reflection) indicators
Typically, reflective LCDs use a display mode with dark characters on a light background (the so-called positive image).
In a positive image indicator, the front and rear polarizers are out of phase, or cross-polarized by 90°.
If the segment is "off", the external light follows the following path: passes through the vertical polarizer, through the transparent electrode of the segment, through the LCD molecules that twist it 90 °, through the transparent common electrode, through the horizontal polarizer, and hits the reflector, which sends out the light back along the same path (Fig. 5a).
rice. 5a Reflective indicator is off.
Light passes through a horizontal polarizer and is reflected back.
If the segment is "on", external light does not change its polarization when passing through the liquid crystal layer. Thus the polarization of the light is opposite to the rear polarizer, which prevents light from passing to the reflector. Since light is not reflected, a dark segment is obtained (Fig. 5b).
rice. 5 B
with a horizontal polarizer so that it does not reach the reflector.
The reflective indicators are very bright, have excellent contrast and have a wide viewing angle. They require good ambient lighting and do not use artificial backlighting (although some models use overhead lighting). Due to their low current consumption, reflective indicators are often used in battery-powered devices.
Transmissive (transmitting) indicators
Transmission LCDs do not reflect light. Instead, they create an image by manipulating the light from an artificial light source located behind the indicator.
In transmission indicators, the front and rear polarizers are “in phase” with each other (parallel). In the off segment, the polarized backlight light is twisted by 90° by the LC molecules and is out of phase with the front polarizer. The polarizer blocks light, creating a dark segment.
rice. 6a When turned off, no light passes through
through the transmission display.
When the segment is turned on, the light is not twisted, being in phase with the front polarizer, and passes through it, creating a light pattern. In this way, the transmissive display creates a light image on a dark background (negative image).
rice. 6b When turned on, the light is out of phase
with a horizontal polarizer, the fact that it does not reach the reflector.
Transmission indicators must be backlit to ensure that the segments illuminate evenly. They are good for use in dim or low light environments. Under direct conditions sunlight backlight can't overcome sun rays and the image is not noticeable.
Transreflective (transmission and reflection) indicators
Transreflective indicators use a white or silver translucent material that reflects some ambient light and also transmits light from the backlight. Because these indicators both reflect and transmit light, they can be used in a wide range of lighting brightnesses. An example would be indicators mobile phones- they are readable both in bright light and in complete darkness. Transflective displays have lower contrast than reflective displays because some of the light passes through the reflector.
Backlight options
Below are the LCD backlight options.
rice. 7
Table 2. Comparison of illumination methods
Property | LED | Incandescent lamps | Electroluminescent |
Brightness | Average | High | Small - Medium |
Color | Red - Amber - Green | White | White |
Size | Small | Small - Medium | Thin |
Fastening | SMD - Radial | Radial - Axial | Axial |
Voltage | 5 Volt | 1.5 V - 28 V | 45 V - 100 V |
Current at 5V (per sq.in.) | 10 - 30 mA | 20 mA | 1 - 10 mA |
Temperature | Warm | Hot | Cold |
Cost (per sq. inch) | $0.10 - $1.00 | $0.10 - $0.80 | $0.50 - $2.00 |
Spread of light | Directed | Spherical | Lambertskoye |
Impact resistance | Excellent | Low | Excellent |
Service life (hours) | 100 000 | 150 - 10 000 | 500 -15 000 |
Use and storage temperature
Analysis of the temperature range is very important when describing an LCD.
All LC materials have a strictly defined upper operating temperature limit, or isotropic limit. Above this limit, the LC molecules take on a random orientation. Isotropic conditions make the positive image completely dark and the negative image transparent. The isotropic temperature is called the nematic-isotropic transition temperature, or N-I transition.
LCDs can recover after short exposure to isotropic temperatures, although temperatures above 110°C will destroy the display's internal coating.
The lower limit of the LCD temperature range is not as well defined as the upper limit. At low temperatures the response time of the indicator increases, as the movement of molecules slows down and the viscosity of the liquid crystal substance increases.
At very low temperatures, the liquid crystal substance turns into a solid, or crystalline state. This temperature is called the crystalline-nematic transition temperature, or C-N transition. However, the LCD material is “super-cold”, it perceives temperatures lower C-N limit, actually turning the crystals of the substance. (Typically for exposures down to -60°C). As a result, LCDs are often operational at temperatures below their C-N junction.
The effect of low temperatures is usually reversible. For example, an LCD immersed in liquid nitrogen returns to its normal state after short period heating
In addition, LCD materials have low temperature coefficient. This coefficient is important for multiplex indicators because low value effective control voltage. Outside the temperature range, temperature compensation may be required.
Heaters
Indicators with integral heaters can operate at temperatures down to -55°C. Heaters require a temperature-controlled power supply. When used with heaters, the indicator response time at low temperatures remains the same as at 0°C. Increasing the heater power reduces the heating time. Typically a power requirement of between 2 and 3 watts per square inch of indicator surface is required.
External lighting
As already discussed, the brightness of the external indicator light is very important. The choice of indicator type is made based on the external lighting conditions.
External influences
There are many LCD modifications that are resistant to various kinds external influences, as required by military standards. For example, there is a “highly stable” coating to protect against high temperature and humidity. The “barrier” coating prevents contamination by conductive substances that could cause a short circuit in the indicator. Thin film heaters can be used in low temperature applications. Right choice The connector also helps overcome external influences.
Viewing angle and direction
rice. 8 The cone of vision describes the area
within which the observer can read the information on the display.
When choosing an LCD, you should determine how the observer will look at the indicator: Will he sit or stand? At what angle is the display? What viewing angle width is required? The fact is that the contrast of the image on the indicator depends on the relative position of the display and the observer.
Typically, the direction of vision is described in a similar way to the face of a watch. If the observer is looking from above, it is called 12 o'clock, from below - 6 o'clock, from the right - 3 o'clock, from the left - 9 o'clock. Critical viewing angles (indicator tilts) depend on the viewing direction and can be illustrated by isocontrast curves on a graph in the polar coordinate system (Fig. 9).
The viewing angle also depends on the thickness of the LCD layer. Most LCDs are manufactured according to the second class with a thickness of 6 to 8 microns. The first class has a thickness of 3 to 4 microns. The widest viewing angle (up to 165°) is achieved with 4-micron technology. This also reduces the response time of the LCD.
rice. 9 Isocontrast LCD curve.
Objective measurement of image contrast from different angles.
Image Contrast
Contrast is mainly determined by ambient lighting conditions and whether the image is positive or negative. As the effective rms voltage increases, the contrast increases. The efficiency of the polarizer and LCD liquid also contribute to better contrast.
LCD segments
The parts of the LCD that act as shutters, turning on and off to form images, are called segments.
The segments are created by transparent indium tin oxide electrodes deposited on LCD glass. Numbers from 0 to 9 and some letters can be displayed on a seven-segment display. The sixteen-segment indicator can display numbers, all Latin and almost all Russian letters (except Y, Ts, Shch). To make the symbols less angular and more natural, matrix indicators are used. They can also be used to display small images. The number of indicator segments affects the method of managing it.
rice. 10 Seven segment display,
sixteen segment display
5x7 matrix display
In addition to alphanumeric characters, the LCD can display small pictures, or icons. For example, the display in Fig. 11 shows the functions of the copier. These images do not change - they can only be turned on or off.
rice. eleven Functional display of the copier.
Response time
The LCD typically has a response time of 50 ms at 20°C, and best models- up to 10 ms. Standard LCD can display signal up to 10Hz if required; It is difficult for the naked eye to track data with such frequency.
Color images
There are several methods to create a color image on an LCD (Table 3).
Table 3. Color in LCD
Dual-In-Line (DIL)
The double-row pin arrangement is convenient for use in harsh conditions. DIL ensures quick, smooth installation of the indicator. The pins can be soldered into a printed circuit board or inserted into a connector. These highly conductive, stainless steel leads provide a rigid connection, even when subjected to shock or vibration.
rice. 12 DIL conclusions
Rubber connector (Elastomeric, rubber connector)
The rubber conductor is a flexible rubber block with big amount transverse conductive veins (like a comb) with a very small pitch. It allows for quick installation/disassembly without solder joints or abrasive contacts, self-aligning. This connection is often used in small instruments where size is limited. Although it is resistant to shock and vibration, the rubber compound should not be used in particularly aggressive environments without increased attention to LCD protection.
rice. 13 Rubber connector
Flex, heat seal connector
Both the PCB and LCD are attached to the flex cable by heating under pressure. This connection is used in most moving devices where displacements can cause breakage of rigid leads. A flexible connection is often used in very large LCDs or applications that require a separate controller board. The popularity of this connection method is growing and developers are finding new applications for it.
rice. 14 Flexible connection
General principles
There are two types of LCD controllers: direct and multiplex. Both types have their advantages and disadvantages.
Table 4. Comparison of direct and multiplex controllers
Multiplex control
Multiplex (MUX) control reduces the number of LCD pins required. Multiplex displays have more than one common terminal (COM). Multiplexing means that each segment pin (SEG) addresses a segment on each of the COM pins. Quantity general conclusions is called the LCD multiplex value.
rice. 15 Option for organizing COM and SEG pins
Energy consumption
Typically, LCDs require very little power to operate - 5 to 25 µA at 5 V (per square inch) for a TN indicator. Artificial lighting or heating requires additional energy.
All LCDs require pure AC control voltage. Random DC voltage, such as a DC component in the signal, can significantly reduce the life of the indicator and should be limited to 50 mV.