LCD basics and S3C2410 LCD Controller

Source: Internet
Author: User
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I. The era of ultra-thin flat display is approaching
The CRT (cathode ray tube) used by the TV has the disadvantages of large size, heavy weight, and limited size. With the development of electronic technology, there are more and more requirements for Mobile Display. the inherent limitations of CRT have hindered the miniaturization and mobility of CRT. This makes it more necessary to develop a new generation of display technology! The next generation of display focuses on a few points: the plane is right angle, the screen display is not deformed, light, short, and less energy consuming, convenient to carry and must be compatible with the existing image signal technology. At present, we have talked about the technology of ultra-thin display, which is the most popular application of tft LCD. For example, electronic products that require complex information display, such as digital cameras, notebook computers, and PDAs, are indispensable. Tft LCD technology also includes low-temperature multi-silicon crystal tft LCD, reflective tft LCD, and many other different display technologies. Next we will look at the history and Principles of the LCD.
Ii. invention and discovery of Liquid Crystal
The birth of Liquid Crystal came from the discovery of a very special substance. As early as 1850, Virchow, mettenheim, And Valentin found that the identifier of nerve fiber contained this unusual thing. In 1877, German physicist Otto Lehmann observed Liquid Crystallization for the first time using a polarization microscope, but he did not understand the causes of this phenomenon. Until February 1888 AD, an Austrian plant scientist, Friedrich reinitzer (1857-1927), discovered cholesteryl benzoate, a chemical that had two melting points at different temperatures during heating, in these two different temperature points, the state is between the general liquid and the solid material, similar to the plastic, but within a certain temperature range it has both liquid and crystalline properties, because of its special status. Reinitzer later visited Lehmann to explore the performance of the substance, which was named "Liquid Crystal" by the two later. Reinitzer and Lehmann are known as the father of liquid crystal.
Like the CRT cathode ray tube, LCD was discovered as early as 1888 years ago (in fact, it has been 80 years since it was actually used around the world. As LCD did not help much in military purposes in the two wars, it lags far behind CRT. What is important is oseen and Z? Cher, the two scientists, established the equation of state change for liquid crystal. By 1968, engineers at the American company of RCA used liquid crystal molecules to change the arrangement of their molecules due to voltage influences, and the principle that it can cause deflection of the incident light, made the world's first LCD screen. As a result, the 1970 s of Japanese Sony and sharp companies on the Comprehensive Development and Application of Liquid Crystal Display Technology, LCD display is successfully integrated into modern electronic products.
To describe the physical properties of a liquid crystal, you must first understand the directionality of the general solid crystal. The special material such as a liquid crystal not only has the directionality of the general solid crystal, but also has the fluidity of the liquid. To change the direction of a solid crystal, the entire crystal must be rotated. changing the direction of a liquid crystal does not require any trouble. Its directionality can be controlled by an electric field or a magnetic field.
Changing the direction of the liquid crystal varies depending on the liquid crystal composition. Some liquid crystal and the electric field parallel time can be low, so when the electric field is applied, it will rotate in the direction of the electric field, in addition, the liquid crystal corresponds to a low vertical time field of the electric field. Because the liquid crystal is sensitive to the external power (electric field or magnetic field), it presents a direction effect, and when the light is incident to the liquid crystal, it will inevitably follow the arrangement of liquid crystal molecules, generates a natural deflection image (as shown in Figure 3-1 ).

Figure 3-1

The electronic structure of some liquid crystal molecules has a strong electronic binding ability. Therefore, when the liquid crystal molecules are affected by the external electric field, it is easy to be polarized to generate an inductive dipolar, which is also the source of interaction between liquid crystal molecules. The liquid crystal display used in general electronic products is the use of the photoelectric effect of liquid crystal, through the external voltage control, and then through the refraction characteristics of liquid crystal molecules, and the ability to rotate the light to obtain the bright and dark conditions, and then achieve the purpose of imaging.

When the power supply is off, the liquid crystal has a polarized Effect
Turns the incident light and passes through the gate to show light color.
LCD does not have polarized function when the power supply is turned on
Therefore, the light cannot be rendered dark through a grid.

Iii. Types of liquid crystal displays a liquid crystal display (Liquid Crystal Display) made of liquid crystal ). There are three types of drivers: static, simple matrix, and active matrix. Among them, the pure matrix type is also known as passive (passive), which can be divided into torsion column type (twisted nematic, TN) and super twisted nematic (STN) the active matrix is based on thin film transistor (thin film transistor; TFT. TN type TN Liquid Crystal Display technology can be said to be the most basic liquid crystal display, other types of liquid crystal display can also be said to be based on TN type for improvement. Similarly, its operating principle is simpler than other technologies. The structure of TN includes the vertical direction and the horizontal direction of the light board (polarizer), which has fine groove, Intermediate Mixed Liquid Crystal Material and conductive glass substrate (glass ). The display principle of the STN/dstn type is similar. The difference is that the TN Type Liquid Crystal Molecules rotate the incident light 90 degrees, while the STN can rotate the incident light 180 ~ 270 degrees. The pure tn display only has two kinds of Display (or black and white), and cannot produce color changes. Tn LCD uses "Direct Drive" to display a large number of pixels, and the image comparison is small, resulting in slow response, the angle of view is only below + 30 degrees (that is, the viewing angle is about 60 degrees), and the display quality is also poor. Therefore, tn LCD is mainly used for displaying simple numbers and text, such: workbooks and calculators. The emergence of the STN improves the narrow angle of view and increases the ratio. The display quality is higher than that of the TN by increasing the number of scanning lines in the "multi-task drive" mode. With the use of color filters, any pixel in the monochrome display matrix is divided into three sub-pixels, which display the primary colors of red, green, and blue through the color filter, then, the color approaching the full color mode can be displayed through the reconciliation of the three primary colors. Because the color contrast of the screen displayed by the STN is still up to 30: 1 (the smaller the contrast, the more unclear the screen), the response speed is 150 ms (milliseconds), which is acceptable as the interface for general operation display, however, the speed of playing a movie is not enough. As there are still many shortcomings in the STN, the subsequent dstn will be displayed through the dual-scan mode. Because the dstn adopts the dual-scan technology, the display effect is greatly improved compared with that of the STN. The response speed of dstn can reach 100 ms. However, because they are all "passive drives", the recovery process of each pixel is slow when the electric field repeatedly changes the voltage, when the screen changes rapidly, for example, displaying the broadcast of a tennis match, the so-called "tail" phenomenon will occur. Especially when a tennis player hits the ball, you can see the phenomenon of "ball tail" on the screen. However, dstn is still used as a display device because of its low price, low power consumption, and PDAs. Both tft tn and stn LCD Use field voltage driving mode. If the display size is increased, the response time of the center part to the electrode change will be extended, and the display speed will not keep up. To solve this problem, the active matrix driver was proposed. The structure of the active tft LCD is complex, including: backlight tube, Light Guide Board, partial light board, light filter board, glass substrate, distribution membrane, liquid crystal material and thin mode transistor (3-2 ). In a tft LCD, a small net line is drawn on the conductive glass, and the electrode is a matrix switch arranged by a thin film transistor. It is equipped with a control gate at the intersection of each line, the control gate of each display point works with the driving signal. The transistor matrix on the electrode enables or disables the voltage of Liquid Crystal Molecules Based on the display signal, and converts the crystal molecular axis to a bright or dark contrast, this avoids the dependence of the display on the electrical field effect, and converts it to the rate at which the transistor is enabled and disabled as a decision step. Therefore, the display quality of TFT-LCD is better than that of TN/STN, the screen display can reach 150: 1 or above, and the reaction speed approaches 30 ms or even faster. At the same time, it can display all colors or even true colors. The product is suitable for PDA, notebook computers, LCD monitors, vehicle navigation systems, digital cameras, and LCD projectors. Figure 3-2 The following table compares the respective characteristics of TN, STN, and TFT. The comparison table lists the operators of TN, STN, and tft LCD, reverse 90 degrees reverse 180 ~ 270-degree liquid crystal molecules, black and white characteristics of 90 degrees of torsion, low contrast of monochrome (20:1) black and white, color (0.26 million colors) low contrast, better than TN (40: 1) color (16.67 million colors) high contrast, better than STN (300: 1) full colorization no comparable to CRT full color animation display no comparable to CRT Angle of View 30 degrees below 40 degrees below 80 degrees below panel size 1 ~ 3 inch 1 ~ 12-inch 6 ~ 17-inch or above Application Scope electronic table, Calculator electronic dictionary, mobile phone color notebook computer, projector, ultra-thin plane color TV

Iv. Development and Future of LCD
The success of tft LCD is that each pixel is followed by a Transistor Switch for control integration, so that the entire tft LCD looks like a large integrated circuit. Tft LCD must be very small, so that people can only see the screen and can not tell the painting, so the production process of tft LCD is quite accurate. In the past, because the technology was not yet mature, it was inevitable that some nodes in a large tft LCD could not be connected or connected incorrectly, leading to the failure to display correct elements. These are collectively referred to as "bad points ", common "red, blue, and green dots" cannot be controlled by themselves, and "Black, white dots" cannot be used. At present, the high-precision technology is enough to overcome the probability that tft LCD will produce "bad points" in the production process, some "bad points" can also be hidden through "Dark points" (human eyes are not sensitive to dark painting.
Because the TFT-LCD successfully solves the shortcomings of CRT, the joint so that its application scope to add a wide range! At the same time, some unexpected problems have also occurred. For example, if the tft LCD display in the sun is not good, you need to rely on the hood or transparent design to reduce the occurrence of reflection, so that you can see it clearly. In addition, special coating technology can be used to reduce background light leakage, increase screen darkness, increase contrast, and reduce glare in daily bright working environments.

V. Detailed description of the built-in LCD controller of S3C2410
An LCD screen not only needs an LCD driver, but also an LCD controller. Generally, the LCD driver is made together with the LCD glass substrate in the form of cof/COG, while the LCD controller has an external circuit. The LCD controller has been integrated into the S3C2410, so it is easy to control various types of LCD screens, such as: STN and tftscreen. As tftscreen will be the mainstream application in the future, the next step is to focus on tftscreen control.
Features of the S3C2410 LCD controller:
STN Screen
-Three scan modes are supported: 4-bit single scan, 4-bit double scan, and 8-bit single scan.
-Supports monochrome, 4-level grayscale, and 16-level grayscale screens
-Supports 256-color and 4096-color STN screens (cstn)
-Supports resolution of 640*480, 320*240, 160*160, and other types of LCD
-Color palette display modes of monochrome, Level 4 gray, and 256 colors are supported.
-Supports 64 K and 16 M color non-palette display modes
-Supports resolution of 640*480,320*240 and other LCD specifications
For tftscreen control, in addition to sending video materials (VD []) to it, the following signals are essential:
Vsync (vframe): frame synchronization signal
Hsync (vline): synchronous line signal
Vclk: digital-like clock signal
Vden (VM): valid data mark Signal
Figure 3-3 shows the logic of the LCD controller in S3C2410:

Figure 3-3
Regbank is a register group of the LCD controller. It is used to set parameters of the LCD controller. Lcdcdma is a DMA channel dedicated to the LCD controller. It obtains video data from the system bus and sends it to the LCD screen through vidprcs from VD. At the same time, timegen and lpc3600 are responsible for generating the control sequence required for the LCD screen, such as vsync, hsync, vclk, and vden, and then sending the video MUX to the LCD screen.
Tftscreen Timing Analysis
Figure 3-4 is a typical time series of the tftscreen. Among them, vsync is the frame synchronization signal. Every time vsync sends one pulse, it means that the new one-screen video data is sent. Hsync is the line synchronization signal, and each hsync pulse indicates that the new line of video data starts to be sent. Vden is used to indicate the validity of video data. vclk is used to lock the image clock of video data.
In addition, there must be a return time for both frame synchronization and row synchronization headers and tails. For example, for vsync, the first return time is (vspw + 1) + (vbpd + 1 ), the back-to-sweep time is (vfpd + 1), and hsync is similar. The timing requirement is that it takes time for the CRT display to deflection due to the electron gun, but later it became an actual industrial standard, and the tftscreen appeared later in order to be compatible with the CRT in time series, this control sequence is also used.

Figure 3-4
YFARM9-EDU-1 uses Samsung company 1 3.5 inch tft LCD screen, resolution is 240*320, for the timing requirements of the screen.
Figure 3-5

By comparing Figure 3-4 and figure 3-5, we can see that:
Vspw + 1 = 2-> vspw = 1
Vbpd + 1 = 2-> vbpd = 1
Linval + 1 = 320-> linval = 319
Vfpd + 1 = 3-> vfpd = 2

Heat map + 1 = 4-> heat map = 3
Hbpd + 1 = 7-> hbpw = 6
Hozval + 1 = 240-> hozval = 239
Hfpd + 1 = 31-> hfpd = 30
The preceding parameters, except for linval and hozval, are directly related to the screen resolution. The preceding parameters should be used as a reference during actual operations and should not be too biased.
Detailed description of main registers of LCD Controller
(1) lcdcon1

Linecnt: The current row scan counter value, indicating the number of rows currently scanned
Clkval: determines the vclk division ratio. The output vclk of the LCD controller is directly obtained from the operating frequency of the system bus (AHB. As a 240*320 tftscreen, make sure that the obtained vclk is within 5 ~ Between 10 MHz
Mmode: VM signal trigger mode (valid only for the STN screen and meaningless for the tftscreen)
Pnrmode: select the current display mode. For tftscreen, select [11], that is, tft LCD panel.
Bppmode: select the color mode. For true color display, select 16 BPP (64 K color) to meet the requirements.
ENVID: Enable LCD signal output
The meanings of vbpd, lineval, vfpd, and vspw have been shown in the previous sequence diagram.
The meanings of hbpd, hozval, and hfpd have been shown in the previous sequence diagram.

The meaning of the heat map has been shown in the previous sequence diagram. I will not go into details here.
Mval is only valid for the STN screen and meaningless for the tftscreen.

The meaning of the heat map has been shown in the previous sequence diagram. I will not go into details here.
Mval is only valid for the STN screen and meaningless for the tftscreen.

Vstatus: indicates the scanning status of the current vsync signal.
Hstatus: The current hsync signal scanning status, indicating the scanning phase of the current hsync synchronization signal
Bpp24bl: Specifies the order of video data in the display buffer when the 24bpp display mode is set (that is, whether the low position is valid or the high position is valid ). This setting is meaningless for 64 K color display mode of 16bpp.
Frm565: For 16bpp display mode, there are 2 in the form, one is RGB = 5: 5: 5: 1, and the other is. The latter mode is most commonly used. It indicates that red (r) occupies 5 bits and green (g) occupies 6 bits in 16-bit RGB data of 64 K colors, blue (B) occupies 5 bits
Invvclk, invline, invframe, and invvd: As shown in the preceding sequence diagram, we know that the timing output by the cpu LCD controller is a positive pulse by default, while the LCD requires vsync (vframe) and vline (hsync) therefore, invline and invframe must be set to "1", that is, select reverse output. The functions of invvden, invpwren, and invlend are similar to those in the previous section.
Pwren enables LCD Power Supply Control. In the output signal of the cpu LCD controller, there is a power supply enabling pin LCD _pwren, which is used as the switch signal of the LCD screen power supply.
The enlend is invalid for common tftscreens and can be ignored.

Bswp and hwswp are in bytes or half words (half-word. Since different guis manage framebuffer (display buffer) differently, you need to adjust bswp and hwswp to adapt to Gui when necessary.


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