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.
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.
Colibri Software, 2010