TFT LCD Interface Signals

Many microcontrollers are able to directly interface to several different types of LCDs — STN, TFT and HR-TFT — without need for a separate LCD controller chip. Some care should be exercised when specifying a microcontroller (MCU) with an embedded LCD controller to make sure the MCU can handle the data throughput for the display while also handling its computing tasks. The job is complicated somewhat by the diversity of embedded LCD controllers; some are designed to operate a narrow type of display family, and others are more flexible, making it possible to operate several different sizes and resolutions of target panels. We will use a generic MCU for our examples here.

LCD panels unfortunately follow no uniform signal naming convention nor pin arrangement, so different LCD panels from different manufacturers will frequently have different pin arrangements and naming conventions. Pairing a Controller with an LCD will become much easier once the required types of signals can be known and understood. The table shows a TFT LCD signal and pin description from a typical LCD manual. The example used here is the Sharp LQ057Q3DC02.

The interface signals follow three categories: control signals, data signals, and power connections.

• Control Signals: The example in the table shows control signals common to all LCD panels; again, they may differ by type and manufacturer.
• Clock signals (CK): for shifting RGB data into the LCD panel.
• Horizontal Synchronization signals (Hsync): There are ‘N’ number of CK periods per Hsync period where ‘N’ is the number of RGB pixels in a line. This signal marks the point at which the current line ends, and the new line begins.
• Vertical Synchronization signals (Vsync): There are ‘M’ number of Hsync periods per Vsync period where ‘M’ is the number of vertical lines in the display. This signal marks the point at which the current frame ends, and the new frame begins.

CK is the LCD’s clock source, a continuous squarewave. The frequency must be within the panel’s specification to avoid flickering. The Horizontal Synchronization signal is a pulse that is activated when one line of data has been transmitted to the LCD. The Vertical Synchronization signal is a pulse that is activated when one page (or frame) of data has been transmitted to the LCD. The Synchronization signal polarities must be correct, otherwise the image on the LCD will exhibit tearing or flickering.

Other control signals for this TFT LCD are:

• ENAB: Enable signal
• R/L: Right or Left scan direction select
• U/D: Up or Down scan direction select
• V/Q: VGA or QVGA select

The Enable signal enables the LCD; and so must be HIGH to enable the LCD display. Some control signals are specific to certain LCDs. In the sample above, R/L, U/D, V/Q signals must be tied to VCC or GND. They affect the image presentation (such as mirror imaging) and may not be available on all LCD panels. 

The data signals are:

• Blue[5:0]
• Green[5:0]
• Red[5:0]

There are 18 data signals in total. They correspond to the LCD controller outputs from the MCU which we will call LCDVD[17] to LCDVD[0]. These signals are the LCD image data; they are active between the Horizontal Synchronization (Hsync) and Vertical Synchronization (VSync) pulses.

The LCD panel in this example requires VCC (3.3 V or 5 V) and GND.