Why does the RGB interface of a small LCD screen require initialization code via SPI?

    First, the RGB interface typically uses parallel transmission, requiring a large number of data lines (such as 8-bit, 16-bit, or 18-bit), as well as control signals such as HSYNC, VSYNC, and DE. This results in a relatively large number of pins. For small screens, the package may not allow for this number of pins, or the main controller chip may lack sufficient GPIO. This is where SPI comes in handy, requiring only a few lines (SCK, MOSI, CS, DC, RST, etc.), saving pins.

    Next, the initialization process typically requires sending a series of commands and parameters, such as setting resolution, display mode, timing parameters, and gamma correction. This initialization data volume is small, but precise timing control is required. While the SPI communication rate may not be as high as the high-speed transmission stage of the RGB interface, it is sufficient for initialization configuration, and SPI control is simpler and easier to implement.

    Furthermore, the driver IC for a small screen may be designed to support multiple interfaces, including SPI and RGB. However, in resource-constrained situations (such as low-cost MCUs), SPI is easier to integrate because it doesn't require dealing with high-speed parallel timing issues. Parallel interfaces require considerations such as data alignment and clock synchronization, while SPI can be handled by a hardware SPI controller or simple software emulation, reducing development complexity. Initialization configuration requirements may also need to be considered. The RGB interface is primarily used for image data transmission, while initialization configuration requires command and control information, which is typically handled through a dedicated control interface (such as SPI or an 8-bit/16-bit parallel control interface). Even if the display has an RGB data interface, initialization configuration may still require a separate control bus. SPI is a common choice due to its efficiency for small data volumes and low pin counts.

    Some display driver ICs require specific wake-up or configuration sequences during startup. These sequences may only be sent via SPI or other serial interfaces. Parallel interfaces have stricter timing requirements, while SPI offers greater flexibility, allowing software to precisely control the order and timing of each command.

Generally speaking, the main reasons may include: reducing the number of pins to accommodate a small package size; reducing the need for host control resources (GPIO, parallel interface); simplifying hardware design (PCB layout, level conversion); the small amount of initial configuration data, which is sufficient and easy to implement with SPI; and the driver IC design supporting serial interface configuration.

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