How to connect graphics controller to MCU and LCD?

This article explains a connection of the graphics controller, such as Epson S1D13781, to the TFT LCD and MCU. An example  of the embedded graphics system is Colibri Graphics Epson S1D13781 Development  Platform (http://www.colibrigraphics.com).

 

From the top view, the embedded graphics system  consists of: MCU or processor, graphics controller with a frame buffer, and LCD.  Sometimes, the graphics controller with the frame buffer and LCD may be integrated  in one module. The MCU writes pixel information into the frame buffer. The  graphics controller continuously reads the pixel information from the frame  buffer and forms a high-speed video stream for the LCD.

 

 

 

Usually, the graphics controller is connected to  the MCU via a parallel data bus. There are two main types of parallel data  interfaces: Intel 80 and Motorola 68. The difference is the read/write signaling  method. Intel 80 has separate read and write strobes while Motorola 68 has an  enabling strobe and a read/write selection signal.


 

Motorola  68 and Intel 80 Signaling Methods

 

 

Typically, the data width of the parallel interface  can be 8- or 16-bit. For 8-bit variants, the first address bus line, A0,  defines the byte for the current read or write operation. If the data bus width is  16 bits, the first address bus line, A0, selects the 16-bit words.  To operate on just one byte within the 16-bit word, the 16-bit interface has two  additional signals: BSL (byte select low) and BSH (byte select high). Sometimes it  is possible to connect an 8-bit bus to a 16-bit device. The following  diagram shows the data and address signals translation.

 

8-Bit  Parallel Data Bus to 16-bit Parallel Data Bus Connection

 

 

To reduce the number of address lines, the address multiplexing can be used. In this case, the data transfer on the  parallel bus consists of several stages. At first, the master device transmits the  address on the data bus with special address latch strobes. The slave device  uses these address latch strobes to store the address in its internal latches.  After this, the data are transmitted.

 

Address Multiplexing

 

 

A graphics controller is connected to the color TFT  LCD via RGB interface. There are different types of RGB interfaces, but usually,  it has a parallel RGB data bus, data clock, data enable strobe, and vertical  and horizontal synchronization signals. An RGB glass interface can use  different data clock polarity (data may be latched on rising or falling clock  edges). Sometimes, the glass can be synchronized with an enable signal (without vertical and horizontal signals) and sometimes the enable strobe is not required. Most often, the data width is 18 bits (6 red: 6 green: 6 blue)  or 24 bits (8 red: 8 green: 8 blue).  If the number of LCD data inputs is more than graphics controller data outputs, then superfluous LCD  least-significant data inputs for each color must be connected to the most significant  data signals. In this case, the glass can output maximum bright and minimum  black intensity.

 


18-Bit  Graphics Controller LCD Data Bus to 24-Bit LCD Data Bus Connection

 

 

 

TFT display may require several power sources.  Usually, these are digital logic (VDD = +3.3V), analog circuitry (usually VAA = +5V),  TFT transistors gates control (usually VGH = +15V and VGL = -10V), and  backlight power. The TFT LCD should be powered correctly. Otherwise, the liquid  crystal can be degraded during on/off cycles. The recommended power sequence is synchronization signals and digital logic power (+3.3V), analog power  (+5V), gate voltage low (-10V), and gate voltage high (+15V). Often, the LCD  module has built-in power regulators to get all voltages from one +3.3V source.

 

In some cases, the LCD module must be programmed  before it can be used.  Usually, the programming is performed with a 3-wire SPI interface (without slave output line). The number of bits in one  transfer can differ from the standard 8- or 16-bit transfers supported by most MCUs.  It can be 9- or 24-bit, for example. Also, the additional line may be required  for data or command selection.

 

There are TFT LCDs without VCOM (common electrode)  level generation. This analog signal must be formed from a digital liquid  crystal polarity signal (ID or REV) with an external analog level shifter.

 

Anton Alkhim,

Colibri Software, 2010
http://www.colibrigraphics.com