TFT LCD Parallel Interface Comparison: MCU vs RGB
July 30, 2021
When choosing a TFT LCD display for your application, display size, contrast, brightness and resolution are key factors. Conversely, picking the right way in how you feed data to the display is just as crucial.
There are multiple interface options available for transmitting data to a TFT LCD display. Parallel interface is one of the commonly used interfaces. Within parallel interfaces, there are two categories, MCU and RGB. These interfaces vary in memory allocation, timing, number of pins and control signals.
The MCU interface has two standards, Intel-8080 and Motorola-6800. When a compact device or simplified main board provides only MCU bus to external system, engineer will need to use TFT LCD display with MCU interface, which has TFT driver with display memory.
These interfaces communicate through read, write and chip-select signals to address registers or display RAM. Depending on color depth (8, 9, 16 or 18-bit), MCU sends RGB signals directly to LCM’s display memory. Following is the timing diagram of a 9-bit 8080 MCU interface, sending RGB 6-6-6 bits/pixel in two write cycles.
MCU interface advantage
MCU interface is simple and low cost. There is no need for either clock signal or synchronization signal.
MCU interface disadvantage
The interface requires LCM embedded display memory, which usually is not very big. Therefore, TFT LCD with MCU interface typically will be small to medium size, and its update speed is relatively slow too, caused by the extra “write” to display buffer. It is not suitable for showing video.
Many newer generation MCUs have embedded TFT controller. It can directly update TFT LCD by providing both RGB sub-pixel data and timing signals through RGB parallel interface.
The number of bits transmitted per clock cycle depends on the setup and programming of TFT LCD display. It varies from 16 to 18 or 24 data pins for each of the RGB signals.
Above is an example of sending RGB signal using 24 data pins. This enables faster data transmission at the cost of more hardware pins.
RGB interface advantage
RGB interface is a mature technology, enjoying low cost and high performance (because of direct writing to LCD) benefits. It is capable of working with medium to large TFT LCD display, for video showing.
RGB interface disadvantage
It requires large pin real-estate (up to 29 pins) and connectors. Another potential issue is with the high-speed edges on RGB data lines, which can create electro magnetic interference.
Choosing the right TFT LCD display for your application requires more than what it needs to look like and its size. Choosing the right interface to the display is very important. Deciding between MCU or RGB interface depends on the amount of data being passed to the display and the refresh rate needed.
Each interface places different demands on MPU and memory. Other aspects like pin availability and processing speed should be part of the equation, too. RGB is good for high performance application, but requires more pin connections, system memory and processing power. Parallel MCU is a great choice for displaying images, as the pictures can be stored in the display’s internal frame buffer. The MCU interface is also easier to command than RGB.