The Liquid Crystal Display (LCD) driver performs operations such as powering on the LCD and initializing the internal registers of the driver integrated circuits (ICs).
The display driver model developed based on the Hardware Driver Foundation [(HDF)](../driver/driver-hdf-overview.md) provides a basic framework for LCD driver development, improves driver development efficiency, and facilitates porting of the driver across OSs and chip platforms. The figure below shows the HDF-based display driver model.
The display driver model based on the [Hardware Driver Foundation (HDF)](../driver/driver-hdf-overview.md) provides the following functions:
- Provides a basic framework for LCD driver development to improve development efficiency.
- Facilitates driver porting across OSs and chip platforms.
The following figure shows the HDF-based display driver model.
The display driver model consists of the display common driver layer, SoC adapter layer, and LCD panel driver layer. The HDF-based display driver model shields the differences between kernel forms through platform and OSAL APIs so the LCD driver can be easily ported between different OSs and chip platforms. The display driver model connects to the display common Hardware Abstraction Layer (HAL), supports the implementation of the Hardware Device Interface (HDI), and provides various driver capability interfaces for graphics services through the Display-HDI.
The display driver model consists of the display common driver layer, SoC adapter layer, and LCD panel driver layer. The HDF-based display driver model shields the differences between kernel forms through platform and OSAL APIs so that the LCD driver can be easily ported across OSs and chip platforms. The display driver model connects to the display common Hardware Abstraction Layer (HAL), supports the implementation of the Hardware Device Interface (HDI), and provides various driver capability interfaces for graphics services through the Display-HDI.
- Display common driver layer: connects to the display common HAL through the IOService data channel provided by the HDF to receive and process various upper-layer calls in a centralized manner.
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The display driver model leverages the capabilities and APIs provided by the platform to simplify the display driver development and improve the efficiency.
## Available APIs
### Basic Concepts
The LCD interfaces include the Mobile Industry Processor Interface (MIPI) Display Serial Interface (DSI), Transistor-Transistor Logic (TTL) interface, and Low Voltage Differential Signaling (LVDS) interface. The MIPI DSI and TTL interfaces are commonly used. Here is a brief introduction to them.
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The DSI is defined by the MIPI Alliance for the displays used in mobile devices. The MIPI DSI is used to transmit image data, in compliance with the MIPI protocol. Generally, control information is sent to the peer IC in the form of MIPI packets over the MIPI DSI, without the need of additional peripheral interfaces.
The DSI is defined by the MIPI Alliance for the displays used in mobile devices. The MIPI DSI is used to transmit image data, in compliance with the MIPI protocol. Generally, control information is sent to the peer IC in the form of MIPI packets over the MIPI DSI, without the need of additional peripheral interfaces.
- TTL interface
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TTL level signals are generated by TTL devices, which are a major type of digital integrated circuits. TTL devices are manufactured using the bipolar process and feature high speed, low power consumption, and diversified types.
TTL level signals are generated by TTL devices, which are a major type of digital integrated circuits. TTL devices are manufactured using the bipolar process and feature high speed, low power consumption, and diversified types.
The TTL interface is used to transmit data in parallel mode under control signals. It transmits data signals, clock signals, and control signals (such as line synchronization signals, frame synchronization signals, and data validity signals). For the LCD with the TTL, additional peripheral interfaces, such as the Serial Peripheral Interface (SPI) and Inter-Integrated Circuit (I2C), are required for the read and write of the internal registers.
The TTL interface is used to transmit data in parallel mode under control signals. It transmits data signals, clock signals, and control signals (such as line synchronization signals, frame synchronization signals, and data validity signals). For the LCD with the TTL, additional peripheral interfaces, such as the Serial Peripheral Interface (SPI) and Inter-Integrated Circuit (I2C), are required for the read and write of the internal registers.
### Constraints
In addition to the IC model, you must consider the LCD peripheral circuit design, LCD interface unit of the baseband chip, backlight IC control, and upper-layer software programs. All these factors affect the development of the LCD driver.
## How to Develop
## Development Guidelines
The HDF-based display driver model provides a unified driver model for LCDs regardless of the OS (LiteOS or Linux OS) and chip platform (Hi35xx, Hi38xx, or V3S). The development procedure is as follows:
### When to Use
Before applying your device with OpenHarmony system, you need to perform LCD driver adaption. The HDF-based display driver model provides a unified driver model for LCD adaptation regardless of the OS (LiteOS or Linux OS) and chip platform (Hi35xx, Hi38xx, or V3S).
| static int32_t LcdResetOn(void) | Sets the status of the reset pin.|
| int32_t SampleEntryInit(struct HdfDeviceObject *object) | Initializes the entry function of the device driver. |
### How to Develop
1. Add the device configuration related to the LCD driver.
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3. Add the LCD panel driver and register the panel driver functions in the driver entry function **Init**. The functions provide capabilities for:
- Powering on/off the LCD device
Based on the LCD hardware connection, use the GPIO APIs provided by the platform to perform operations on the LCD pins, such as the reset pin and IOVCC pin. For details about the power-on sequence, see the SPEC provided by the LCD supplier.
- Sending the initialization sequence
Based on the LCD hardware interfaces, use the I2C, SPI, and MIPI interfaces provided by the platform to download the LCD initialization sequence. For details, see the SPEC provided by the LCD supplier.
4. Implement other HDF APIs as required, for example, **Release()**.
5. Use the HDF to create other device nodes for implementing service logic or debugging as required.
## Development Example
Add the device configuration.
```
/* Configuration of the devices related to the display driver */
display :: host {
hostName = "display_host";
/* Configuration of the HDF display driver */
device_hdf_disp :: device {
device0 :: deviceNode {
policy = 2;
priority = 200;
permission = 0660;
moduleName = "HDF_DISP";
serviceName = "hdf_disp";
}
}
/* Configuration of the driver device at the SoC adapter layer */
device_hi35xx_disp :: device {
device0 :: deviceNode {
policy = 0;
priority = 199;
moduleName = "HI351XX_DISP";
}
}
/* Configuration of the LCD driver */
device_lcd :: device {
device0 :: deviceNode {
policy = 0;
priority = 100;
preload = 0;
moduleName = "LCD_Sample";
}
device1 :: deviceNode {
policy = 0;
priority = 100;
preload = 2;
moduleName = "LCD_SampleXX";
}
}
}
```
Adapt the driver to the chip at the SoC adapter layer. The following example shows how to adapt the MIPI device to the Hi35xx series chips at the SoC adapter layer:
