Android platform Display driver Development brief: LCD Fundamentals chapter "one"

Platform Information:
Kernel: linux3.4.39
System: android4.4
Platform: s5p4418 (Cortex A9)

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From playing the dot-matrix screen at school, to the LCD under the work Android system, they are like a line, conform, simple principle constitutes a complex structure:

Let's comb it out:

Play 51 using the dot matrix

Play 51 with the lcd1602

Play 51, 32 with the TFT color screen

For the principle of dot matrix compared to most friends are very familiar with, then LCD1602, is also a collection of points, using smart classmates, still can use small tools in 1602 to achieve a lot of small graphics;

For the TFT, students put the point has three primary color composition, and then the logic and lattice screen is a principle. Let's take a look at the principle of color screen.

First, the structure of TFT-LCD
1, the structure of TFT-LCD as shown

Reflector: Reflector CCF lamps: Cold Light Tube Ligh Pipe: Light Guide Plate Extraction Pattern: Diffuser: Scattered light plate, the effect of astigmatism, so that more evenly spread	This part is mainly the light source part, CCFL or LED backlight light source, other reflectors, scattered light plate, the role of these optical distribution more evenly
Rear Polarizer: Rear polarizing Plate Front polarizer: Front polarizing Plate	The role of polarizing film to turn natural light into an aurora borealis
Selected subpixels: Sub-pixel Tft: Liquid Crystal: Liquid crystal Color Filters: Colour filter	This part is the core part of the LCD, select the light source of the conduction, blocking, color in this part of the control.
Back glass: Rear glass base Plate Front Glass: Front glass base plate	Play a protective role

the functions of the TFT-LCD structure
(1), backlight module: Provide the source of light;
(2), Upper and Lower polarizer, TFT Glass substrate, liquid crystal: The formation of polarized light, control the passage or not;
(3), Color filter: To provide a TFT LCD red, green, blue (three primary colors of light) source;
(4), Ito transparent conductive layer: to provide transparent conductive path;
(5), Photo Spacer: Provide a fixed height to color filters and TFT Glass substrate. As the space to pour into the liquid crystal, and as a support for the upper and lower layers of glass.
2. Side view of TFT-LCD structure

First, TFT-LCD display principle
1, the LCD backlight
Backlight is the light source of the monitor, there are two kinds of backlight commonly used in LCD: CCFL backlight, LED backlight
(1), CCFL cold cathode fluorescent lamp abbreviation for CCFL, the Chinese name for the cold Aurora lamp, with high power, high brightness, low energy consumption and other advantages, widely used in display, lighting and other fields.

(2), LED backlight
Relative to the ccfl,led has low power consumption, light source uniformity, long life, small size advantages, price will be expensive, but now the flat-panel with the TFT-LCD seems to be led backlight, the last break a piece by the way to see the next.

(3), LED and LED backlight
The so-called LED display, in fact, is "LED backlight LCD display"; Now the popular liquid crystal display, belongs to "CCFL backlight LCD display". So the two are still liquid crystal display, but the backlight is not the same. Do not see the LED display is mistaken for the next-generation technology display, in fact, the latest technology is called OLED. So don't be fooled when you buy a TV.
2. Introduction of LCD
(1), the shape of liquid crystal crystal
The liquid crystal used in TFT-LCD is tn (Twist nematic) type liquid crystal, and the liquid crystal molecule is oval shape.

(2), the characteristics of liquid crystal
TN-type liquid crystal is generally followed by the long axis of the thread, the long axis between each other parallel arrangement. When the surface of the groove is contacted, the liquid crystal molecules are arranged in the groove in the direction of the groove.

(3), liquid crystal vertical distribution
When the liquid crystal is contained in the middle of two grooved surfaces, and the direction of the grooves is perpendicular to each other, the liquid crystal molecules are arranged as follows:
Upper surface molecule: along a direction;
Lower surface molecule: along the b direction;
A molecule in the middle of the upper and lower surface: the effect of rotation. So the liquid crystal molecule produces a 90 degree rotation between the two grooved surfaces.

(4) Light and liquid crystal molecules produce deflection effect

(5), liquid crystal under the voltage to do the uniform distribution
When voltage is applied between the upper and lower surfaces, the liquid crystal molecules are arranged in the direction of the electric field, forming an upright arrangement. The incident light is not affected by the liquid crystal molecule, and the straight line shoots down the surface.

3, the characteristics of the Polarizer
Effect: The non-polar lights (general light) are filtered into the aurora borealis. When the non-polar light passes through a polarization in a direction, the ray is filtered into a linear aurora that is parallel to the a direction.
: Linear auroras continue to move through the second polarizing film, passing through the light.
: The light is completely blocked by the second pass.

The optical effect produced by the combination of a polarizer, a grooved surface, and a liquid crystal, as shown in

(1), when the upper and lower polarizer perpendicular to each other, if the voltage is not applied, the light can pass

(2), when the voltage is applied, the light is completely blocked

When the electric field changes through the transistor, causing the liquid crystal molecules to deflect, so as to change the polarity of the light, and then use the polarizer to determine the brightness and Pixel of the picture elements. This allows for light to dim the control, if you want to display color, we are talking about color filters behind.

4. Color filter Principle Color fliters
(1) Structure of c/f
These squares can be seen carefully by a display with a lower pixel than before. A simple way to put a drop of water on the display, you can see the red, green, blue, three-color points.

(2), c/f Pixel Array Common arrangement
As shown in the mosaic, straight bar, triangular form, the four-picture pigment.

(3), different colors of the display
Let's see if we want to show the corresponding color, we can control the corresponding pixel electrode. As shown in the following:

C/F color unit, corresponding to the TFT control unit, you can complete our pixel color control. The TFT Array equivalent circuit is as shown:

Three, TFT-LCD imaging principle
We explained in front of the liquid crystal transmission principle, polarizing film principle, color filter working principle, which he understood as a pixel control unit, and then we to organize the overall display of the TFT-LCD principle.
(1), SCAN IC transmission signal;
Complete the image signal input;
(2), DRIVER IC transmission image control signal;
Complete the TFT unit control;
(3), when a sub-pixel conduction, the sub-pixel due to the inability of light to appear black;
This part of the completion pixel is bright or dark.
(4), if the sub-pixel is not conductive, because the light through the CF and display color . With the effect of light synthesis, the display can produce color effects. As shown in the following:

Now look back we started that structure chart is not clear: the light source part of the first light through the polarizer into the Aurora-->TFT Subpixels Unit Control whether the liquid crystal Unit is light, color---through the front of the polarizer to display the color image. In fact, the image is a different color of light, we see the light is actually the aurora borealis.

This picture looks like a clear description:

Four, the LCD commonly used knowledge points
1. Residual Shadow
Residual shadow refers to the screen after the first screen will not immediately disappear, but slowly disappear phenomenon, the residual shadow and reaction time is not the same thing, the residual shadow may take two or three seconds to completely disappear, and the liquid crystal reaction time is more than 10 to dozens of milliseconds. A well-designed LCD display, even if the reaction time is 15+35ms, it is impossible for users to see the shadow.
The mechanism of the residual shadow is somewhat complex, usually the same picture shows that the liquid crystal in the case of an electric ion adsorption in the upper and lower glass at both ends of the formation of built-in electrical field, the screen after the switch these ions are not released immediately, so that the liquid crystal molecules do not immediately turn to the angle caused by. Another possibility is that the paint electrode is poorly designed, which makes the liquid crystal molecules in a state-switching disorder, which may also be seen in the presence of a residual image.

2, the bad point (dot defect)
The so-called bad point, refers to the LCD monitor can not control the constant light or constant dark points, the cause of the bad is the LCD panel production due to a variety of factors caused by the defects may be particles fall in the panel, may be the damage of electrostatic damage panel, may be the process of poor control and so on. The bad points are divided into two types: highlights and dark spots. Generally speaking, the bright spot will be more unacceptable than the dark point, so many manufacturers will ensure that no bright spots, but it seems to be less guaranteed no dark point, some panel manufacturers will be in the shipment before the bright spot into a dark point.
Panel manufacturers will have a bad point of the panel price to sell, usually no bad point of a grade, three points within the B grade, six points within the C grade.
The market now a lot of companies do flat, with IPAD2, IPAD3, Minipad screen, and then in the promotion of the product when the Apple said things, compared with Apple, the domestic companies are really funny. In fact, they use the screen is the production process of Apple test pass the screen, such as Apple only with a + screen, a-and a-level of the screen, all into the market, the domestic flat is garbage, although I have been doing these things, the environment is not good.

3. Mura
Mura was originally a Japanese word, meaning uneven, speckled, with Japan's liquid crystal display throughout the world to flourish. Mura refers to the display of uneven brightness caused by various traces of the phenomenon, the simplest way to judge is to switch to the black screen in the darkroom and other low-level screen, and then from a variety of different angles to see hard, the problem of the display is more easily seen.

4, color saturation (color gamut)
Color saturation refers to the brightness of the display, the display is composed of red green blue three color light to combine into any color light, if the RGB three primary colors more vivid, then the display can represent a wider range of colors.

5. Brightness
Brightness is the degree to which the display is bright under a white screen, in cd/m^2, or NIT. Brightness is an important factor that directly affects the quality of the screen. In the laboratory we often say a word: "A bright cover three ugly." A bright display may look beautiful even if there are other disadvantages, such as poor color saturation or color yellowing.
Brightness and lighting related, lamp life, especially the earlier CCFL backlight, time will be yellow, this if the home has older display can obviously feel.

6. Perspective
Because of the natural physical characteristics of the liquid crystal display, the quality of the screen will change when users look at it from different angles. When viewed diagonally, when compared to a positive view, turn to the perspective of the display when the quality of the screen has changed to an unacceptable critical angle.

7, color temperature (color temperature):
The colour temperature is used to describe the white color of the display, not limited to the LCD, all the monitors are universal, when the color of the display and the temperature of the blackbody to a certain absolute temperature of the light emitted from the same time, the color temperature is called the display is equal to the temperature. For example, when the white design of the display is close, the color of light emitted by the blackbody at a temperature of 6500K (near a sunny morning sun light), called the display's color temperature of 6500K.
The lower the color temperature is more yellow, the higher the color temperature will be more blue, a high-temperature display in the show picture when the whole picture will be blue.

8. Gamma Curve:
Gamma curve refers to the relation curve of different gray scale and luminance. The curve is called gamma curve when the x-axis, the brightness, and the y-axis of the gray scale is zero to 255. Gamma curve is usually not a straight line, because the human eye has different effects on different intensities of brightness, such as low-brightness identification (a little bit of brightness change has a feeling).
Five, the LCD debugging process commonly used to download pictures
Download Link: LCD commonly used test pictures, which are colored bands, color bars and other pictures. Used to debug the effect.

Everyone in the use of not only the size of a different color screen, but also appeared their interface is different, then the driver is different, the next collection of different color screen interface mode;

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Android platform Display driver Development brief: LCD Fundamentals chapter "one"