Introduce the drive control method of embedded LCD in detail

There are many kinds of LCD interfaces, and the classification is very fine. It mainly depends on the driving mode and control mode of the LCD. At present, the color LCD on the mobile phone generally has several connection modes: MCU mode, RGB mode, SPI mode, VSYNC mode, MDDI mode, DSI mode. MCU mode (also written as MPU mode). Only TFT modules have RGB interface.

But there are more applications in MUC mode and RGB mode. The differences are as follows:

1. MCU interface: it will decode commands, generate timing signals by timing generator, and drive COM and SEG drivers.

RGB interface: When writing LCD register setting, it is no different from MCU interface. The difference is only in how the image is written.

2. When using the MCU mode, since the data can be stored in the IC’s internal GRAM first and then written to the screen, the LCD in this mode can be directly connected to the MEMORY bus.

It is different when using RGB mode, it has no internal RAM, HSYNC, VSYNC, ENABLE, CS, RESET, RS can be directly connected to the GPIO port of MEMORY, and use the GPIO port to simulate waveforms.

3. MPU interface mode: Display data is written to DDRAM, which is often used for still picture Display.

RGB interface method: The display data is not written to DDRAM, but directly written to the screen, which is fast, and is often used to display video or animation.

The main differences between the MCU interface and the RGB interface are:

MCU interface mode: display data is written into DDRAM, often used for still picture display.

RGB interface method: The display data is not written to DDRAM, but directly written to the screen, which is fast, and is often used to display video or animation.

MCU mode

Because it is mainly used in the field of single-chip microcomputers, it gets its name. Later, it is widely used in low-end mobile phones, and its main feature is that it is cheap. The standard term for the MCU-LCD interface is the 8080 bus standard proposed by Intel, so I80 is used to refer to the MCU-LCD screen in many documents. Mainly can be divided into 8080 mode and 6800 mode, the difference between the two is mainly timing. There are 8 bits, 9 bits, 16 bits, 18 bits, 24 bits for data bit transfer. The connection is divided into: CS/, RS (register selection), RD/, WR/, and then the data line. The advantages are: the control is simple and convenient, no clock and synchronization signals are required. The disadvantage is: it consumes GRAM, so it is difficult to achieve a large screen (above 3.8). For LCM with MCU interface, its internal chip is called LCD driver. The main function is to transform the data/commands sent by the host computer into the RGB data of each pixel, so that it can be displayed on the screen. This process does not require point, line, frame clocks.

The Driver IC of the LCD with the MCU interface is equipped with GRAM. As a co-processor of the MCU, the Driver IC accepts the Command/Data sent by the MCU and can work relatively independently. For the LCM (LCD Module) of the MCU interface, the internal chip is called the LCD driver. The main function is to transform the data/commands sent by the host computer into the RGB data of each pixel, so that it can be displayed on the screen. This process does not require point, line, frame clocks.

M6800 mode

M6800 mode supports selectable bus widths of 8/9/16/18-bit (default is 8-bit). The actual design idea is the same as that of I80. The main difference is that the bus control read and write signals in this mode are combined in On one pin (/WR), while adding a latch signal (E), the data bit transfer is 8-bit, 9-bit, 16-bit and 18-bit.

I8080 mode

The I80 mode connection is divided into: CS/, RS (register selection), RD/, WR/, and then the data line. The advantages are: the control is simple and convenient, no clock and synchronization signals are required. The disadvantage is that it consumes GRAM, so it is difficult to achieve a large screen (above QVGA).

The standard name of the MCU interface is I80, and there are 5 control pins:

CS chip select signal

RS (set to 1 for write data, set to 0 for write command)

/WR (0 means write data) data command distinguish signal

/RD (0 means read data)

RESET reset LCD (reset with fixed command sequence 0 1 0)

VSYNC mode

In fact, this mode is to add a VSYNC signal to the MCU mode, which is applied to the update of the moving picture, which is very different from the above two interfaces. This mode supports the function of direct animation display, and it provides a solution to realize animation display with minimal changes to the MCU interface. In this mode, the internal display operation is synchronized with the external VSYNC signal. Animation display at a higher rate than internal operations can be achieved. However, due to the different operation modes, this mode has a limit on the speed, that is, the write speed to the internal SRAM must be greater than the speed of the display read internal SRAM.

RGB mode

The large screen adopts more modes, and the data bit transmission also has 6, 16 and 18, 24 bits. The connections are generally: VSYNC, HSYNC, DOTCLK, CS, RESET, some also need RS, and the rest is the data line. Its advantages and disadvantages are just the opposite of MCU mode.

The main difference between the MCU-LCD screen and the RGB-LCD screen is the location of the video memory. The video memory of RGB-LCD is acted by system memory, so its size is only limited by the size of system memory, so RGB-LCD can be made larger, such as 4.3″ can only be considered entry-level now, and 7″ in MID , 10″ screens have begun to be widely used. At the beginning of the design of MCU-LCD, it was only necessary to consider that the memory of the microcontroller was small, so the video memory was built into the LCD module. Then the software updated the video memory through special display commands, so the MCU screen Often it cannot be done very much. At the same time, the display update speed is slower than that of RGB-LCD. The display data transmission mode is also different. The RGB screen only needs to organize the data in the video memory. After starting the display, the LCD-DMA will automatically transfer the data in the video memory through the RGB The interface is sent to the LCM. The MCU screen needs to send a drawing command to modify the RAM inside the MCU (that is, the RAM of the MCU screen cannot be directly written). Therefore, the RGB display speed is obviously faster than that of the MCU, and in terms of video playback, the MCU-LCD also slower.

For the LCM of the RGB interface, the host directly outputs the RGB data of each pixel, and does not need to be converted (except for GAMMA correction, etc.). For this interface, an LCD controller is required in the host part to generate RGB data and Dot, line and frame sync signals.

Color TFT LCD screen mainly has two kinds of interfaces: TTL interface (RGB color interface), LVDS interface (RGB color is packaged into differential signal transmission). The TTL interface is mainly used for small-size TFT screens under 12.1 inches, and the LVDS interface is mainly used for large-size TFT screens above 8 inches. The TTL interface has many lines and the transmission distance is short; the LVDS interface has a long transmission distance and a small number of lines. The large screen adopts more modes, the control pins are VSYNC, HSYNC, VDEN, VCLK, S3C2440 supports up to 24 data pins, and the data pin is VD[23-0].

The image data sent by the CPU or graphics card is a TTL signal (0-5V, 0-3.3V, 0-2.5V, or 0-1.8V), and the LCD itself also receives a TTL signal, because TTL signals are transmitted at high rates over long distances. At the time, the performance was poor and the anti-interference ability was relatively poor. Later, a variety of transmission modes were proposed, such as LVDS, TDMS, GVIF, P&D, DVI and DFP. They actually just encode the TTL signal sent by the CPU or graphics card into various signals for transmission, and decode the received signal on the LCD side to obtain the TTL signal.

But no matter what transmission mode is used, the essential TTL signal is the same.

Note: TTL/LVDS are two signal transmission modes, TTL is a mode in which high level means 1, and low level means 0. LVDS is two corresponding waveforms, positive and negative. The difference between the two waveforms is used to indicate that the current 1 or 0

SPI mode

There are 3 wires and 4 wires, and the connection is CS/, SLK, SDI, SDO four wires. There are few connections but the software control is more complicated.

MDDI mode (MobileDisplayDigitalInterface)

The interface MDDI proposed by Qualcomm in 2004 can improve the reliability of mobile phones and reduce power consumption by reducing wiring, which will replace the SPI mode and become a high-speed serial interface in the mobile field. The connection is mainly host_data, host_strobe, client_data, client_strobe, power, GND several lines.

DSI mode

This mode is a serial bidirectional high-speed command transmission mode, with D0P, D0N, D1P, D1N, CLKP, CLKN.