The monitor is an important interface for communication between humans and machines. In the early days, CRT/Cathode Ray Tube displays were mainly used. However, with the continuous progress of technology, various display technologies have sprung up like mushrooms after rain. Recently, due to the advantages of light, thin, short, low power consumption, no radiation risk, flat angle display, and stable and non flickering images, liquid crystal (LCD) displays have been attracted by the continuous decline in prices in recent years, Gradually replacing the mainstream position of CRT, monitors are full of tomorrow's stars. So what are the new features of LCD displays compared to traditional displays?
1、 High display quality
Due to the fact that every point on a LCD display maintains the same color and brightness, emitting constant light after receiving a signal, unlike cathode ray tube displays (CRTs) that require constantly refreshing bright spots. Therefore, LCD displays have high image quality and will never flicker, minimizing eye fatigue.
2、 No electromagnetic radiation
The display material of traditional displays is fluorescent powder, which is displayed by the impact of an electron beam on the fluorescent powder. The moment the electron beam hits the fluorescent powder, it will generate strong electromagnetic radiation. Although many display products have effectively dealt with radiation issues to minimize radiation, it is difficult to completely eliminate it. Relatively speaking, LCD displays have inherent advantages in preventing radiation, as they do not have radiation at all. In terms of electromagnetic wave prevention, LCD displays also have their own unique advantages. They use strict sealing technology to seal a small amount of electromagnetic waves from the driving circuit in the display. In order to dissipate heat, ordinary displays must make the internal circuit in contact with air as much as possible, so that the electromagnetic waves generated by the internal circuit will leak out in large quantities.
3、 Large visible area
For displays of the same size, LCD displays have a larger visible area. The visible area of a LCD display is the same as its diagonal size. The cathode ray tube display has a border of about one inch around the front panel of the picture tube, which cannot be used for display.

4、 Wide application range
The initial LCD displays were usually used in electronic watches and calculators due to their inability to display delicate characters. With the continuous development and progress of LCD display technology, character display has become more delicate, while also supporting basic color display, and gradually used in LCD TVs, camera LCD displays, and handheld game consoles. Subsequently, DSTN and TFT were widely used as LCD display devices in computers, and DSTN LCD screens were used in early laptops; TFT is used both on laptops (most laptops now use TFT displays) and on mainstream desktop displays.
5、 Good visual effect
Compared with traditional displays, LCD displays initially use pure flat glass panels, which have a flat right angle display effect, giving people a refreshing feeling. Moreover, LCD displays are easier to achieve high resolution on small screens, for example, a 17 inch LCD display can achieve 1280 resolution very well × 1024 resolution, while typically 1280 is used on 18 inch CRT color displays × A picture effect with a resolution of 1024 or higher is not entirely satisfactory.
6、 Digital interface
LCD displays are all digital, unlike cathode ray tube color displays that use analog interfaces. That is to say, with LCD displays, graphics cards no longer need to convert digital signals into analog signals and output them as usual. In theory, this will make color and positioning more accurate and perfect.
7、 Well proportioned and small in stature
Traditional cathode ray tube displays always have a bulky tube trailing behind them. LCD displays break through this limitation and give people a brand new feeling. Traditional displays emit electron beams onto the screen through an electron gun, so the neck of the picture tube cannot be made very short. As the screen increases, it will inevitably increase the volume of the entire display. LCD displays control the state of liquid crystal molecules through electrodes on the display screen to achieve display purposes. Even if the screen is enlarged, its volume will not increase proportionally, and it is much lighter in weight than traditional displays with the same display area.
8、 Low power consumption
Traditional displays are composed of many circuits inside, which drive the operation of cathode ray tubes and require a large amount of power consumption. Moreover, as the volume continues to increase, the power consumption of the internal circuits will definitely also increase. Compared to traditional displays, the power consumption of LCD displays is mainly consumed by their internal electrodes and driver ICs, resulting in much lower power consumption.

　　Selection of LCD displays
In the field of flat panel display devices, the currently widely used ones include liquid crystal (LCD), electroluminescent display (EL), plasma (PDP), light-emitting diode (LED), low-voltage fluorescent display devices (VFD), etc.
LCD display devices have the following characteristics
Low voltage micro power consumption; Flat structure; Passive display type (no glare, not irritating to the eyes, not causing eye fatigue); Display a large amount of information (because pixels can be made very small); Easy to color (can be accurately reproduced on chromatography); No electromagnetic radiation (for human safety and conducive to information confidentiality); Long lifespan (This type of device has almost no degradation issues, so its lifespan is extremely long, but the lifespan of the LCD backlight is limited, but the backlight part can be replaced).
8 Elements of LCD Selection
◆ LCD type ◆ Quality assurance ◆ Technical support ◆ Brand and price
◆ Supply chain assurance ◆ Resolution and size ◆ Temperature and brightness ◆ Interface methods
Selection of LCD Display Screen Types
▲ Characters → Determine the number of displayed rows and columns → TN, STN categories → Whether with backlight → Determine size → Determine working and storage temperature range
▲ Graphics → Single color or color (TFT true color or STN false color<usually below 256 colors) → Determine resolution → Determine external dimensions → Backlight type (LED, EL, CCFL) → Determine working and storage temperature range
▲ Customization → Requirements for non-standard modules → Fill out customization form → Sign contract

　　LCD type
In terms of liquid crystal (LCD), from the perspective of selection, we will divide common LCD into the following categories: segment type, character type,
The common segmented LCD consists of 8 segments per word, which are 8 characters and a dot. It can only display numbers and some letters. If it is necessary to display a small number of other characters, Chinese characters, and other symbols, it is generally customized from the manufacturer. The characters, Chinese characters, and other symbols to be displayed can be fixed in designated positions, such as a calculator. For segmented LCD, we offer customized services.
Character LCD, as the name suggests, is used to display characters and numbers, and its display method for graphics and Chinese characters is no different from segment LCD. Character LCD generally has the following resolutions, 8 × 1, 16 × 1. 16 × 2. 16 × 4. 20 × 2. 20 × 4. 40 × 2. 40 × Class 4, where 8 (16, 20, 40) refers to the number of characters (numbers) that can be displayed on a line, and 1 (2, 4) refers to the number of displayed lines.
Graphical dot matrix liquid crystals are further divided into several categories, including TN, STN (DSTN), TFT, etc. This classification needs to start with liquid crystal materials and liquid crystal effects, please refer to the principles of liquid crystal displays.
Due to its limitations, TN type liquid crystals are only used for producing character type liquid crystal modules; STN (DSTN) type liquid crystal modules are generally small to medium-sized, ranging from monochromatic to pseudocolored; TFT type liquid crystals are available from small to large, and almost all colors are true color display modules. Except for TFT type LCD screens, generally small LCD screens have built-in controllers (the concept of controllers is equivalent to the main control chip on the display card), which directly provide MPU interfaces; To control the display of large and medium-sized LCD screens, an additional controller is required.
Therefore, when choosing the LCD screen you need, the following aspects need to be considered in detail:
1、 If only characters and numbers need to be displayed, and the content displayed on one screen does not exceed the maximum limit of a character LCD (such as 40) × 4) You can choose a character LCD and directly connect it to the MPU.
2、 If you need to dynamically display Chinese characters and graphics, you can only choose graphic dot matrix LCD. The next issue to consider is whether to choose STN (DSTN) monochrome, pseudocolor, or TFT true color. In general, if a microcontroller is used for control, due to its control capability limitations, only 640 × Single color below 480, 320 × Select within the range of pseudo colors below 240; If a PC, IPC, or other main control module with strong control capabilities (such as a video input control module) is used, as long as it has a LCD display part or external display control, there can be a large choice. Monochrome, pseudocolor, and true color LCDs without built-in controllers are available. At the same time, consideration should be given to the requirements of external dimensions. Also, please note that the resolution of LCD is physically fixed, and full screen display can generally only be displayed at its inherent resolution, which is different from CRT.

　　3、 When it comes to backlight selection, when it comes to backlight issues, liquid crystals need to be classified from another perspective, namely transparent, reflective, and semi reflective semi transparent liquid crystals. As liquid crystals are passive light-emitting displays, external light sources are necessary for them to display. Transparent liquid crystals must add background light, reflective liquid crystals require strong ambient light, and semi reflective and semi transparent liquid crystals require strong ambient light or backlighting.
Character LCD with backlight is generally LED backlight, with yellow color (red, green tone) as the main color. Generally driven by+5V.
Monochrome STN small dot matrix LCD often uses LED or EL backlight, and EL backlight is common in yellow green (red, green, white tones). Generally, 400-800Hz and 70-100V AC drives are used, and commonly used drives require approximately 1W of power.
Medium to large dot matrix STN liquid crystals and TFT type liquid crystals are mostly cold cathode backlight tubes (CCFL/CCFT), with a white backlight color (red, green, blue tones). Generally, AC drives of 25k-100kHz and above 300V are used.
4、 Temperature range: Many character type liquid crystals and small pattern lattice liquid crystals have room temperature and wide temperature types, while large pattern lattice liquid crystals with wide temperature types are relatively rare in the mainland market. Room temperature generally refers to the working temperature of 0-50 ℃, and wide temperature can reach -20-70 ℃ (some can reach -30 ℃ below zero, such as LQ5AW136 TFT video interface); In addition, there are certain requirements in terms of humidity.
5、 Brightness issue, the brightness unit is cd/m2 or Nit (nit). Most TN and STN (DSTN) liquid crystals have a brightness of no more than 100cd/m2, but the commonly used 5-6 "pseudo color STN screens have a brightness of around 130cd/m2. Kyocera has a 5.7" LCD with a brightness of 200cd/m2, while TFT type liquid crystals have a brightness of over 150cd/m2.
6、 In terms of accessories, due to the lack of international standards for the specifications and interfaces of LCD, the signal interfaces of different manufacturers and types of LCD are often inconsistent. Therefore, when choosing LCD, pay attention to purchasing relevant accessories (including signal connecting devices, inverters, etc.).

　　The driving method of LCD screen
The simple matrix driving method is composed of electrodes in the vertical and horizontal directions. The selection of the part to be driven is controlled by the horizontal voltage, and the vertical electrodes are responsible for driving the liquid crystal molecules.
In TN and STN type liquid crystal displays, the method of simply driving the electrodes is to use the cross mode of the X and Y axes, as shown in the following figure. Therefore, if the display part becomes larger, the reaction time of the electrode in the center part may be longer. In order to ensure consistent screen display, the overall speed will decrease. To put it simply, it's like the screen update frequency of a CRT monitor is not fast enough, which can cause users to feel the screen flicker and jump; Or when fast 3D animation display is needed, but the display speed cannot keep up, there may be a delay in the displayed results. So, early LCD displays had certain limitations in size and were not suitable for watching movies or playing 3D games.
The driving method of the active matrix is to have each pixel correspond to a group electrode, which is constructed in a loop manner similar to DRAM. The voltage is scanned (or charged for a certain time) to represent the state of each pixel. In order to improve this situation, LCD display technology later adopted an active matrix addressing approach to drive, which is currently an ideal device for achieving high data density LCD display effects with extremely high resolution. The method is to use thin film technology to make silicon transistor electrodes, and use scanning method to select the on and off of any display point (pixel). This is actually using the nonlinear function of thin-film transistors to replace the difficult to control nonlinear function of liquid crystals.
In TFT type liquid crystal displays, small grid like lines are drawn on conductive glass, and electrodes are arranged as matrix switches made of thin film transistors. At the intersection of each line, there is a control box. Although the driving signal quickly scans through various display points, only the selected display points in the transistor matrix on the electrode receive enough voltage to drive the liquid crystal molecules, causing the liquid crystal molecule axis to turn and form a "bright" contrast, The unselected display points are naturally a contrast of darkness, thus avoiding the dependence of display functions on the ability of liquid crystal electric field effects.
TFT liquid crystal display principle
TFT type liquid crystal displays are relatively complex, mainly composed of fluorescent tubes, light guide plates, polarizers, filters, glass substrates, directional films, liquid crystal materials, thin mode transistors, and so on. Firstly, the LCD display must first use a backlight, which is a fluorescent tube to project a light source. These light sources will first pass through a polarizing plate and then through the LCD. At this time, the arrangement of the LCD molecules will change the angle of light passing through the LCD. Then these light rays must pass through the colored filter film and another polarizing plate in front. Therefore, by changing the voltage value that stimulates the LCD, we can control the intensity and color of the light that ultimately appears, and thus create color combinations with different shades on the LCD panel.

　　Principle of STN LCD Display
The display principle of STN type is similar to TN, except that the liquid crystal molecules in TN twisted nematic field effect rotate the incident light by 90 degrees, while the STN super twisted nematic field effect rotates the incident light by 180-270 degrees.
It should be noted here that a simple TN LCD display only has two scenarios (black and white), and cannot achieve color changes. STN liquid crystal displays involve the relationship between liquid crystal materials and the interference phenomenon of light, so the displayed color tones are mainly light green and orange. But if a color filter is added to a traditional monochrome STN LCD display, and any pixel in the monochrome display matrix is divided into three sub pixels, the red, green, and blue primary colors are displayed through the color filter, and then the color in full color mode can be displayed by adjusting the proportion of the three primary colors. In addition, the larger the display screen of TN type LCD displays, the poorer the screen contrast will appear. However, with the improvement technology of STN, the insufficient contrast can be compensated for.
TN type LCD display principle
The TN type LCD display technology can be said to be the most basic among LCD displays, and other types of LCD displays can also be said to be improved based on the TN type. Similarly, its operating principle is simpler than other technologies, please refer to the image below for readers. The figure represents a simple construction diagram of TN type liquid crystal display, including vertical and horizontal polarizing plates, directional films with fine grooves, liquid crystal materials, and conductive glass substrates. In the absence of an electric field, the incident light passes through the polarizing plate and passes through the liquid crystal layer. The polarizing light is rotated 90 degrees by the molecular twisted arrangement of the liquid crystal layer. When it leaves the liquid crystal layer, its polarization direction is exactly the same as the direction of the other polarizing plate, so the light can pass smoothly and the entire electrode surface is bright. When an electric field is applied, the optical axis of each liquid crystal molecule turns in the same direction as the electric field, causing the liquid crystal layer to lose its ability to rotate light. As a result, the polarization direction from the incident polarizer is perpendicular to the polarization direction of the other polarizer and cannot pass through, resulting in a dark state on the electrode surface. The imaging principle is to place the liquid crystal material between two transparent conductive glasses attached to a polarizing plate perpendicular to the optical axis. The liquid crystal molecules will rotate and arrange in order according to the direction of the grooves in the film. If the electric field is not formed, the light will smoothly enter from the polarizing plate, rotate in the direction of the liquid crystal molecules, and then exit from the other side. If two conductive glasses are electrified, an electric field will be generated between them, which will affect the arrangement of liquid crystal molecules between them, causing their molecular rods to twist, making it impossible for light to penetrate and thus blocking the light source. The phenomenon of light dark contrast obtained in this way is called the twisted nematic field effect, abbreviated as TNFE (twisted nematic field effect). Almost all liquid crystal displays used in electronic products are made using the principle of twisted nematic field effect

　　Design and Development of LCD Control Driver
For liquid crystal displays, it usually includes a sandwich panel made of glass substrate, ITO (Indium Tin Oxide) film, alignment film, polarizing plate, etc., with a total of two layers on top and bottom. Each interlayer contains grooves formed on electrodes and alignment films, with a 90 degree alignment between the upper and lower glass substrates. Place liquid crystals in the upper and lower layers, and the liquid crystals will align in the direction of the grooves. Overall, the arrangement of liquid crystal molecules looks like a twisted spiral arrangement. When an electric field is applied to the glass substrate, the alignment of liquid crystal molecules changes and becomes an upright state. When liquid crystal molecules stand upright, the light cannot pass through, resulting in black appearing on the display screen. The liquid crystal display (LCD) will control the alignment direction of liquid crystal molecules based on the presence or absence of voltage, so that the panel can achieve the display effect.
There are various classification methods for LCD. Usually, it can be divided into segment, dot character, dot matrix, etc. according to its display method. In addition to black and white display, there are also multi grayscale and color displays.
When driving an LCD, an AC voltage needs to be applied to both the segment electrode and the common electrode. If only DC voltage is applied to the electrode, the liquid crystal itself deteriorates. LCD driving methods include static driving, dynamic driving, and other driving methods.
1) Static drive
All segments have independent driving circuits, indicating the continuous application of voltage between the segment electrode and the common electrode. It is suitable for LCD with simple control.
2) Multi way drive mode
Construct a matrix electrode with n common terminals, and drive the common terminals sequentially in a 1/n sequence. Corresponding to this driving sequence, select and drive all segment signal electrodes. This method is suitable for LCD with more complex control.
In the multi-channel driving mode, pixels can be divided into selection points, semi selection points, and non selection points. In order to improve the contrast of the display and reduce crosstalk, the duty cycle and bias should be reasonably selected.
The relationship between the effective voltage values, duty cycle, and bias voltage when applied to the LCD to indicate ON and OFF is as follows:
Vo: LCD driving voltage
N: Duty cycle (1/N)
a: Bias voltage (1/a)
Multi way driving methods can be divided into point reversal driving and frame reversal driving. Point reversal drive is suitable for low duty cycle applications, as it reverses data during output of each segment. Frame reversal drive is suitable for high duty cycle applications, as it inverts data at each frame output.

　　For the control methods of multi grayscale and color displays, frame rate control (FRC) and pulse width modulation (PWM) methods are usually used. Frame rate control is achieved by reducing the number of frame outputs and controlling the effective value of the output signal to achieve multi grayscale and color control. Pulse width modulation is achieved by changing the pulse width of the segment output signal and controlling the effective value of the output signal to achieve multi grayscale and color control.
The display mode changes from simple segment and dot character to complex dot matrix and step mode. The display color gradually changes from black and white to color. The display screen has gradually shortened response time from small to large, and currently STN displays have advantages in cost and current consumption. TFT displays have advantages in contrast and animation speed.
As a standard circuit manufacturer for LCD drives, there are mainly companies such as NEC, EPSON, and Samsung. At present, the most commonly used driver circuits in the mobile phone market are still black and white circuits. However, four grayscale LCD driver circuits and color LCD driver circuits are gradually being put into the market. In the future, displays with color, large screens, internet access, and fast response will become a popular trend in the development of mobile phones.
The following will take NEC's mPD16682A product as an example to illustrate the main characteristics and design process of LCD control drivers. This chip is suitable for mobile phones, Chinese or Japanese pagers, and other devices that display Chinese or Japanese characters, using 16 x 16 or 12 x 12 dots per character.
*LCD display control/driver with 1/65 minute RAM display
*Using a+3 volt single power supply
*Equipped with a boost circuit (3x and 4x convertible)
*132 x 65 bit RAM for point display
*Output: 132 segments, 65 common terminals
*Used for COG (Chip on Glass)
The basic composition of an LCD driver consists of the following parts:
Control section:
TopDown
logic circuit
RAM section:
Handmade design
Asynchronous 2 PortRAM
I/O port
Output dedicated port
Simulation section:

　　Handmade design
DC/DC converter
da converter
Boost amplifier
Voltage follower
Voltage stabilizing circuit
Temperature compensation circuit
Oscillating circuit
I/O section: handmade design
Taking mobile phones as an example, design and development enterprises should collaborate with domestic chip manufacturing enterprises to design and develop the following series of LCD control drivers for mobile phones that are currently or will soon be needed:
Black and white LCD control driver
Multi grayscale LCD control driver
Color STN-LCD control driver
Color TFT-LCD control driver
1) Determine LCD driver circuit specifications
Determine the specifications for LCD driver circuits based on market demand and development trends.
2) Establish a complete design environment
Due to the involvement of digital, analog, and high-voltage circuits in the LCD control drive circuit. The extraction and validation of SPICE parameters is an important task among them. Therefore, design and process personnel should produce TEG sheets for testing, test them, extract and verify SPICE parameters, and establish a complete design environment.
3) LCD Control Driver Circuit Design
Circuit design includes determining the circuit design scheme, logic synthesis, circuit simulation, and physical implementation.
·Adopting low-power technology requires selecting a low-power power supply; Built in memory and reduced oscillation frequency; Adopting OSO (One Shot Operation) circuit technology; Using MLS (Multi Line Selection) driving method.
·Circuit description and simulation.

　　Digital circuits can be described using HDL language and simulated using HDL. Analog circuits can use schematic input and SPICE simulation.
For the overall circuit simulation, mixed digital and analog simulation technology needs to be adopted, and the verification technology of display images needs to be solved.
·Physical implementation of layout
To ensure design efficiency, the layout of the digital circuit section can be automatically laid out and routed using SE. To achieve high performance, manual layout should be used for analog circuit layout and I/O section layout. Due to the use of different methods for block production throughout the entire chip, it is necessary to utilize full chip synthesis, layout and wiring techniques, as well as DRC technology for partial circuit layouts and full chip layouts.
4) LCD control/driver circuit testing technology. For example, the ability to correspond to multiple pins; High speed data transmission; High precision testing; Corresponding to high voltage.
Explanation of Professional Terminology for LCD
LCD Liquid Crystal Display
LCM Liquid Crystal Module
TN Twisted Neural twisted nematic. Twisted orientation deviation of liquid crystal molecules by 90 °
STN Super Twisted Neural. About 180~270 ° twisted nematic
FSTN Formulated Super Twisted Neural. One layer of optical path compensation film added to STN for monochromatic display
TFT Thin Film Transistor
Backlight - Backlight
Inverter

OSD On Screen Display
DVI Digital Visual Interface (VGA) digital interface
TMDS Transition Minimized Differential Signaling
LVDS Low Voltage Differential Signaling
Panelink -
IC Integrated Circuit
TCP Tape Carrier Package Flexible Circuit Board
COB Chip On Board fixes IC bare chips onto printed circuit boards through bonding
COF Chip On FPC fixes the IC onto the flexible circuit board
COG Chip On Glass fixes the chip onto the glass
Duty - Duty cycle, the ratio of the portion of the voltage that exceeds the threshold for illumination in one cycle
LED Light Emitting Diode
EL Electroluminescence. The EL layer is composed of high molecular weight flakes
CCFL (CCFT) Cold Cathode Fluorescent Light/Tube
PDP Plasma Display Panel
CRT Cathode Radial Tube
VGA Video Graphic Array
PCB Printed Circuit Board
Composite video - Composite video
Component video -
S-video - S terminal, compared to composite video signals, separates contrast and color for transmission
NTSC National Television Systems Committee
PAL Phase Alternating Line
SECAM Sequential Curriculum Avec Memory
VOD Video On Demand
DPI Dot Per Inch points per inch
Analog/Digital Interface for LCD Display
liquid crystal display