LCD common interface principle

TTL

1. Overview of TTL interface

TTL is transistor-transistor logic, and TTL level signals are generated by TTL devices. TTL devices are a major category of digital integrated circuits. They are manufactured using bipolar technology and have the characteristics of high speed, low power consumption, and variety.

The TTL interface is an interface for data transmission in parallel. When using this interface, it is not necessary to use a dedicated interface circuit on the driver board side and the LCD panel side of the liquid crystal display, but the TTL data signal output by the main control chip of the driver board through the cable Directly transmitted to the input interface of the LCD panel. Due to the high signal voltage of the TTL interface, many connections, and long transmission cables, the circuit's anti-interference ability is relatively poor and it is prone to electromagnetic interference (EMI). In practical applications, TTL interface circuits are mostly used to drive small size (below 15in) or low resolution LCD panels. The highest TTL pixel clock is only 28MHz. TTL is the only signal that TFT-LCD can recognize when the signal is in. The early digital processing chips are all TTL, that is, RGB is directly output to TFT-LCD.

2. Signal type of TTL interface

The TTL output interface of the driver board generally contains three types of signals: RGB data signals, clock signals and control signals.

(1) RGB data signal includes single-channel TTL, single-channel 6bit TTL output interface, single-channel 8bit TTL output interface, dual-channel TTL (dual channel is two sets of RGB data, divided into odd channel, even channel, and some clock It is also divided into OCLK/ECLK, and some share one). Dual-channel 6bit TTL output interface (also known as 36-bit or 36bitTTL interface), dual-channel 8bit TTL output interface (also known as 48-bit or 48bitTTL interface).

(2) Clock signal

Refers to the pixel clock signal, which is the basis for transmitting data and reading data signals. When using odd/even pixel dual-channel transmission of RGB data, different output interfaces use pixel clock methods differently. Some output interfaces for odd/even pixel dual data share a pixel clock signal, and some output interfaces for odd/even two channels respectively set odd pixel data clock and even pixel clock signals to meet the needs of different LCD panels.

(3) Control signal

The control signal includes a data enable signal (or effective display data strobe signal) DE, a horizontal synchronization signal HS, and a vertical synchronization signal VS.

Two, LVDS

1. LVDS interface overview

LVDS, or Low Voltage Differential Signaling, is a low voltage differential signal technology interface. A digital video signal transmission method developed to overcome the shortcomings of high power consumption and large EMI electromagnetic interference when transmitting broadband high bit rate data in TTL level mode. The LVDS output interface uses a very low voltage swing (about 350mV) to transmit data through two PCB traces or a pair of balanced cables differentially, that is, low-voltage differential signal transmission. Adopting LVDS output interface, can make the signal transmit at the speed of several hundred Mbit/s on the differential PCB line or balanced cable, because of adopting the low voltage and low current driving mode, therefore, realize low noise and low power consumption.

2. Composition of LVDS interface circuit

  In the liquid crystal display, the LVDS interface circuit includes two parts, namely the LVDS output interface circuit on the main board side (LVDS transmitter) and the LVDS input interface circuit on the liquid crystal panel side (LVDS receiver). The LVDS transmitter converts the TTL signal into an LVDS signal, and then transmits the signal to the LVDS decoder IC of the LVDS receiver on the side of the LCD panel through the flexible cable (flat cable) between the driver board and the LCD panel. The LVDS receiver then serializes the The signal is converted into a TTL level parallel signal and sent to the LCD screen timing control and row and column drive circuit.