Understanding RGB Interface in Embedded Systems

In the realm of embedded systems and single-board computers (SBCs), display interfaces play a vital role in delivering visual information to end users. Among the many options available, the RGB interface remains one of the most popular and widely used solutions, particularly for applications that require cost-effective, high-quality graphical outputs without the complexity of high-speed serial transmission.

This article explores the RGB interface in embedded hardware platforms—its fundamentals, signal structure, applications, advantages, limitations, and how it compares with other common interfaces such as LVDS, MIPI, and HDMI.

What is an RGB Interface?

An RGB (Red-Green-Blue) interface is a parallel video transmission standard that sends image data in separate color channels along multiple lines. It is one of the most direct methods for communicating pixel data from a processor or display controller to an LCD panel. Each frame of video is transmitted line by line and pixel by pixel, using dedicated lines for the red, green, and blue color components.

Unlike modern serial interfaces like HDMI or MIPI DSI, RGB does not encode data, nor does it use differential signaling. It transmits raw pixel values in real-time and synchronizes the stream using timing signals.

How RGB Works

The RGB interface transmits video signals using multiple parallel lines:

• Color data lines: Separate lines for R (typically 5–8 bits), G (5–8 bits), and B (5–8 bits).
  
  

• Control signals:
  
  

  • HSYNC (Horizontal Sync): Marks the start of each line.
    
    

  • VSYNC (Vertical Sync): Marks the start of each frame.
    
    

  • DE (Data Enable): Indicates valid pixel data.
    
    

  • PCLK (Pixel Clock): Synchronizes data sampling.
    
    

A typical 24-bit RGB interface would use:

• 8 bits for Red
  
  

• 8 bits for Green
  
  

• 8 bits for Blue
  
  Totaling 24 data lines, in addition to sync and clock signals.
  
  

Data is sampled on the rising (or falling) edge of the pixel clock, which is typically in the range of 1 MHz to 65 MHz, depending on resolution and refresh rate.

Applications of RGB Interface

The RGB interface is commonly found in systems where:

• The display panel is located close to the controller (e.g., within 10–20 cm)
  
  

• High bandwidth and low latency are required
  
  

• EMI (Electromagnetic Interference) is not a major concern
  
  

• Simplicity and cost are more important than signal integrity over long distances
  
  

Common use cases include:

1. Industrial Human-Machine Interfaces (HMIs)
   
   Industrial panels that require simple, stable connections often choose RGB for short-range, embedded setups.
   
   

2. Automotive Displays
   
   Some dashboard displays and infotainment systems still rely on RGB for low-cost embedded solutions.
   
   

3. Home Automation Panels
   
   Smart home wall panels using Linux or Android SBCs often use RGB to drive their capacitive TFT LCDs.
   
   

4. Medical Equipment
   
   Embedded systems in portable or handheld diagnostic devices frequently use RGB displays due to their ease of integration.
   
   

5. Educational and Maker Projects
   
   Many development boards like STM32, Rockchip, and Allwinner SBCs offer RGB outputs for use with low-cost TFT displays.
   
   

Key Advantages of RGB Interface

1. Low Latency
   
   Since pixel data is transmitted in real-time with no encoding, RGB delivers ultra-low latency—ideal for responsive applications.
   
   

2. Simple Hardware Integration
   
   No need for complex serializers, receivers, or PHYs. The interface connects directly to the LCD panel’s driver IC.
   
   

3. Cost Effective
   
   Because it doesn’t require advanced signal processing or differential drivers, RGB reduces the overall BOM cost.
   
   

4. Open and Mature Standard
   
   Supported by nearly all display controllers and MCUs/SOCs with built-in TFT controllers.
   
   

5. Flexibility with Panel Selection
   
   Thousands of display models support RGB input, including TN, IPS, and TFT types ranging from 2” to 10.1”.
   
   

Limitations of RGB Interface

Despite its simplicity, RGB comes with several limitations that engineers must consider:

1. Short Transmission Distance
   
   Due to parallel signaling and lack of differential drivers, RGB is susceptible to noise and signal degradation over longer cables.
   
   

2. High Pin Count
   
   A 24-bit RGB interface may require over 30 lines (including sync and clock), which complicates routing on PCBs and increases connector size.
   
   

3. EMI and Signal Integrity
   
   High-frequency parallel signals over multiple lines can cause EMI issues, especially in densely packed designs.
   
   

4. Not Hot-Pluggable
   
   RGB displays usually cannot be hot-swapped and often require proper power sequencing to avoid screen damage.

RGB is ideal when:

• You want to reduce system complexity
  
  

• The display and processor are mounted on the same PCB
  
  

• You are designing for fixed embedded environments
  
  

RGB Interface in Rockchip and Allwinner SBCs

SBCs based on Rockchip (e.g., PX30, RK3566) or Allwinner (e.g., A64, V3S, R528) often include RGB display controllers as part of their video output subsystem. This allows direct connection to 24-bit RGB TFT LCDs without additional bridge ICs.

For example:

• PX30: Features 24-bit RGB output supporting resolutions up to 1920x1080
  
  

• Allwinner A64: Includes a DE2.0 engine with native RGB interface
  
  

Developers can configure the RGB interface using device tree overlays and kernel drivers under Linux or Android BSPs.

Example configuration in device tree:

panel {

    compatible = "rocktech,rk070cu01";

    backlight = <&backlight>;

    port {

        panel_input: endpoint {

            remote-endpoint = <&lcd_out>;

        };

    };

};

lcd_out: endpoint {

    remote-endpoint = <&panel_input>;

    bus-width = <24>;

};

Tips for Designing with RGB Interface

1. Minimize Trace Length
   
   Keep RGB signal traces as short and straight as possible to reduce skew and signal degradation.
   
   

2. Add Series Resistors
   
   Adding 22–33Ω resistors on RGB signal lines can help dampen reflections and reduce EMI.
   
   

3. Use Ground Guard Traces
   
   For critical lines like HSYNC, VSYNC, and PCLK, adding ground shielding can improve integrity.
   
   

4. Ensure Proper Timing
   
   RGB displays require specific sync pulse widths and porches (front/back). Carefully consult the LCD panel’s datasheet.
   
   

5. Power-On Sequence
   
   Many RGB displays require a strict power-on sequence to avoid flickering or damage. Use PMIC or GPIO control for sequencing.
   
   

When to Avoid RGB

While RGB is a solid choice for simple designs, you may want to avoid it if:

• Your panel is over 30 cm away from the SBC
  
  

• You need to reduce EMI for compliance reasons (e.g., FCC/CE)
  
  

• You want to support multiple display types with one interface
  
  

• Your PCB space or connectors are limited
  
  

In such cases, LVDS or MIPI DSI might offer better performance and scalability.

Conclusion

The RGB interface continues to be a reliable and cost-effective solution for driving TFT displays in embedded systems. Its simplicity and real-time performance make it ideal for many industrial, medical, and consumer applications. However, designers must carefully consider its limitations in terms of distance, pin count, and EMI.

For modern embedded products built around Android/Linux SBCs—particularly with Rockchip and Allwinner SoCs—RGB remains a practical and widely supported choice. By following best practices in hardware design and software configuration, engineers can achieve stable, vibrant, and responsive displays using this classic parallel interface.

Further Reading on Display Interfaces

If you’re exploring various display options for embedded systems, we recommend reading our Display Interface overview to compare RGB with LVDS, MIPI, HDMI, and eDP technologies. This will help you choose the most suitable interface for your specific application scenario.