Embedded Linux bare Metal Development (15)--LCD

Embedded Linux Bare Metal Development (15)--LCD First,LCD Introduction

the LCD (Liquid Crystal Display) is LCD display abbreviation. The LCD is constructed in two parallel Glass substrates placed in the LCD box, the lower substrate glass set TFT(thin film transistor), on the substrate glass set color filter, Through the signal and voltage change on the TFT to control the direction of rotation of the liquid crystal molecules, so as to control the polarization of each pixel to shoot or not to achieve the display purpose.

1.LCD type

depending on the backlight, the LCD can be divided into CCFL and the LED two kinds.

A, CCFL

refers to using CCFL (cold cathode fluorescent lamp tube) as a backlit light source of the liquid crystal display (LCD). The advantage of CCFL is that the color performance is good, the shortage is high power consumption.

B, LED

refers to using LED (Light emitting diode) is a liquid crystal display (LCD) that is used as a backlight light source, usually in the sense that wled (white Light LED).

LED is the advantage of small size, low power consumption, so with LED as backlight, can be both light and thin at the same time to achieve higher brightness. The main problem is that the color performance is worse than CCFL, so the professional graphic LCD still uses the traditional CCFL as the backlight light source.

2. Technical Data

A. Contrast Ratio

the control IC used in LCD manufacturing, Optical Filter and directional membrane accessories, and the contrast of the panel, for the average user, the contrast can reach 350:1 is sufficient, but in the professional field such contrast does not meet the needs of users. Compared to the CRT display easily up to 500:1 or higher contrast, only high-end liquid crystal display can achieve such a degree. Samsung, Asus,LG and other first-line brands on the market today's LCD monitors can reach the level of 1000:1 contrast .

B, brightness

An LCD is a substance between solid and liquid, which cannot be illuminated by itself, and requires additional light sources. Therefore, the number of tubes is related to the brightness of the LCD display. The first LCD monitor only up and down two lamps, popular type of the lowest is also four lights, high-end is six lights. Four lamp design is divided into three types of display: One is four sides have a lamp, but the disadvantage is that the middle will appear dark, the solution is from top to bottom four lamp flat arrangement of the way, the last one is "U" type of the display form, in fact, the two lamps in disguise produced two lamp. Six tube design is the actual use of three lamps, manufacturers will be three lamps are bent into a "U" type, and then placed in parallel to achieve the effect of six lamp.

C, Signal

Response time refers to the speed at which a liquid crystal display responds to an input signal, that is, the reaction time (brightness from 10%-->90% or 90%-->10%) of the liquid crystal from installation or light to dark, usually in milliseconds (ms). There is a phenomenon of "visual residue" in human eyes, and a short-term impression is formed in the high-speed motion picture. Animation, film and so on until now the latest game is the application of the principle of visual residue, so that a series of gradient images in the eyes of the rapid continuous display, then form a dynamic image. People can accept the screen display speed is generally 24 per second, which is the movie 24 frames per second of the speed of the origin, if the display speed below this standard, people will obviously feel the picture of the pause and discomfort. According to this indicator, the time required to display each picture is less than 40ms. In this way, for the liquid crystal display, the response time of 40ms has become a barrier, higher than 40ms display will appear obvious picture flicker phenomenon, people feel dizzy. If you want to make the image not flash, it is best to achieve a speed of 60 frames per second.

D, visual angle

    lcd backlight fix

Visual angle is divided into parallel and vertical visual angle, horizontal angle is the vertical axis of the liquid crystal Center, left and right to move, you can clearly see the angle range of the image. The vertical angle is centered on the parallel center axis of the display, moving up and down, and the angle range of the image can be clearly seen. Visual angle in "degrees", the more commonly used form of labeling is directly marked the total horizontal, vertical range, such as: 150/120 degrees, the current lowest visual angle of 120/100 degrees (horizontal/vertical), below this value is not acceptable, it is best to reach 150/120 degrees above.

3. features

The LCD features are as follows:

Low Power Micropower

Small and delicate appearance, only 6.5~8mm thickness

Passive display type (no glare, no eye irritation, no eye strain)

Display large amount of information (because pixels can be made very small)

Easy to color (can be reproduced very accurately on the chromatographic spectrum)

No electromagnetic radiation (for human safety, information security)

Long life (very long life, but LCD backlight life is limited, can be replaced)

Second, LCD Interface Technology

interface of LCD based on the driving mode and control mode of LCD have a : MCU mode, RGB mode, SPI mode, VSync mode, Mddi mode, DSI mode. There are more applications with MCU mode and RGB mode. The main differences between the MCU interface and the RGB interface are:

A, MCU Interface mode: Display data write Ddram, often used for still picture display.

B, RGB Interface Mode: Display data is not written to Ddram, direct writing screen, fast, often used to display video or animation.

RGB mode is large screen with more mode, data bit transmission has 6 bits, 16 bits and 18 bits, 24 bits of the points. Wiring generally has: vsync,hsync,dotclk,cs,reset, some also need RS, the rest is the data line. RGB-LCD's memory is played by the system's RAM, so its size is limited only by the size of the system memory . RGB screen only need memory to organize good data. When the display is started, the LCD-DMA automatically sends the data in the video memory to the LCM via the RGB interface. The MCU screen will need to send a PIP command to modify the internal MCU RAM (that is, the MCU can not directly write the screen RAM). So the RGB display speed is significantly faster than the MCU, and video playback, MCU-LCD is also relatively slow.

commonly used interfaces of TFT-LCD have a TTL (RGB), LVDS, EDP, MIPI .

1. TTL(RGB)

TTL (Transistor transistor logic) is transistor-transistor logic, and TTL level signals are generated by TTL devices. TTL interface is the interface of parallel transmission of data, the use of such an interface, do not need to be in the LCD driver board end and the LCD panel end using a dedicated interface circuit, but by the driver board Master chip output TTL data signal transmitted directly to the LCD panel of the sender interface. TTL interface signal voltage is high, the connection is many, the transmission cable is long, thus the circuit's anti-jamming ability is inferior, and is prone to produce the electromagnetic interference (EMI). In practical applications, the TTL interface circuit is used to drive small-size (below 15in) or low-resolution LCD panels. The TTL maximum pixel clock is only 28MHz.

TTL is the signal when TFT-LCD only recognizable signal, the early digital processing chips are TTL, that is, RGB direct output to TFT-LCD.

RGB Interface:

VD[23:0]:24 root data cable, transfer image data with Love

HSYNC: Horizontal sync signal,

VSYNC: Vertical sync signal,

VCLK: Pixel clock,

Vden:

LEND: Line End Flag

2. LVDS

The LVDS, or low Voltage differential signaling, is an interface for differential signaling technology. A digital video signal transmission method is developed to overcome the disadvantage of large power consumption and EMI electromagnetic interference when transmitting wideband high bitrate data at TTL level. The LVDS output interface utilizes a very low voltage swing (approx. 350mV) to transmit data via differential transmission on two PCB traces or a pair of balanced cables, i.e. low-voltage differential signal transmission. The LVDS output interface allows the signal to be transmitted at a rate of hundreds of Mbit/s on a differential PCB line or balanced cable, resulting in low noise and low power dissipation due to low-voltage and low-current drive modes.

Third, LCD display image process

The LCD-TFT image display process is as follows:

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1, pixel

pixels are image Elements refers to the basic original pigment and its Grayscale the basic code. Pixels are only dimensions of resolution, not quality. Pixels are the basic units that make up digital imagery , and typically represent the size of the image resolution in pixels per inch PPI (pixels per inches). A complete image can be displayed by controlling the display of a certain color per pixel.

2. Image Scan loading

The LCD image loads the entire image by loading each pixel from left to right, top to bottom.

3. LCD Driver, controller

The LCD controller is the internal peripheral of the SOC for generating digital signals, communicating with the LCD driver in accordance with a certain communication sequence, and controlling the LCD driver operation.

The LCD driver is integrated in the LCD panel and is driven by an analog electrical signal to drive the LCD panel's liquid crystal molecules to spin.

Four,LCD parameters 1.RGB interface Timing

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HSPW: Horizontal sync signal pulse width

HBPD: Horizontal sync signal Front shoulder

HFPD: Horizontal sync signal rear shoulder

VSPW: Vertical sync signal pulse width

VBPD: Vertical sync signal Front shoulder

VFPD: Vertical sync signal back shoulder

Row data format: hspw+hbpd+ valid information Data +HFPD (one Vden cycle)

Frame data format: vspw+vbpd+ frame valid information +VFPD

Specific parameter values consult the LCD technical documentation.

2. LCD Display parameters

Pixel pixel: Image element

Pixel spacing pitch: The distance between the centers of a neighboring pixel

Resolution RESOLUTION:LCD the number of pixels in the screen landscape and portrait, and the resolution is independent of the screen size.

Clarity: The clarity of each detail shadow and its boundary on the image

Pixel depth BPP (bits per pixel): How many bits of a pixel are saved

Five,LCD programming Practice 1, circuit schematic view

Check LCD document LCD Timing section to get the timing parameters of LCD

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#define HSPW (Ten)

#define HBPD (40-1)

#define HFPD (To)

#define VSPW (7)

#define VBPD (at)

#define VFPD (a)

#define ROW (480)

#define COL (+)

#define Hozval (COL-1)

#define Lineval (ROW-1)

Review Core Board circuit schematic diagram LCD related section, the corresponding Gpio pin is GPF0-GPF3

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Check the backplane circuit schematic LCD section, the backlight open pin is XpwmTOUT1 , corresponding Gpio is GPD0.

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2. Project

The project is modeled with SDRAM initialization, mainly in the LCD screen portrait, horizontal line, vertical line, slash, Circle.

source see annex.

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Embedded Linux bare Metal Development (15)--LCD