LVDS and eDP Working

Introduction

In this article, we will develop a better understanding of the concept of display interface and it’s communication link. The working of the LVDS and eDP technology along with the difference that governs their use and application depending upon the requirement of the system. The evolution of technology is taking place and we need to be updated with it to solve the problem easily and conceptually. Now, we will discuss the display technology updates with the LVDS (Low voltage differential signaling) and eDP (embedded Display Port) and it’s interface system.

LVDS Technology

LVDS stands for Low voltage differential signaling. This display technology uses differential data transmission. The differential scheme of data transmission is more beneficial compared to the single-ended scheme. The differential signal is less prone to common-mode noise injection and hence there is very little chance of glitches in the communication link. These signals can be observed using CRO or DSO using dual channel probe. 

This technology rejects the common mode noise and only allows the differential voltage to pass through the channel. So, the original signal is not changed with the noise. The LVDS technology is flexible with the VDD power supply, it can be easily changed to other voltage levels such as 5V, 3.3V, 2.5V, etc compared to the other older PECL (Positive Emitter coupled logic) technology which uses fixed voltage level of 5V. LVDS signaling has a lower level of voltage swing i.e peak-to-peak value.

LVDS vs PECL Voltage level

NOTE

LVDS uses a constant current mode driver to obtain all its features. The value of the current source is approximately 4.5 mA. LVDS was introduced full-fledged in 1994 and it solved the problem of growing data transfer rate at the same time decreasing power supply requirement. When “MB/s data transfer rate” and “mW power” set are required then LVDS technology is well suited for it.

EDP Technology

It is a digital communication interface that utilizes differential signaling to achieve a high bandwidth (BW) bus interface designed to support connection between laptops, PCs and external peripheral display devices. eDP is an embedded version of the display port designed specifically for “All in one” hosted notebooks and machines. The eDP on the PCH can be configured for 2 or 4 lanes. PCH in laptop motherboard is designed as per VESA (Video Electronics Standard Association) display port standards to match the required lane configuration.

Embedded Display Port has major three components:-

* Main Link

* Auxiliary Channel

* Hot Plug Detect Signal

 

Main Link It is a uni-directional link (i.e only one way communication) from laptop motherboard/hosts to the external peripheral devices. It has higher bandwidth for high speed communication without delay or latency in the data transfer which reduces the glitch and provides smooth display system interface. It transports data such as uncompressed video and audio.

EDP main link carries

a) Video Pixel Data.

b) Video Timing Information.

c) Video Format Information.

d) Video Data Error Correction.

Auxiliary Channel – This is a half duplex bi-directional channel used for controlling and linking the display interface with the peripheral devices. This is half duplex means transfer of data takes place at a time from single channel and not simultaneously from the peripheral devices. This is an AC coupled interface.

EDP Aux Channel Carries

a) Display Format Information.

b) Helps in Main link connection setup.

c) Display Status Control like:-

   * BCKLT diming & Frequency control.

   * Dynamic BCKLT & color control.

   * Frame rate control, this will control the video synchronisation.

d) Power management.

e) Error checking of main link data & information.

EDP Display Interface
EDP Communication System with all the three Major Linking Components

HotPlug detect – The HPD signal is used as an optional signal for the acknowledgement of the sink device to the source device. The sink device provides an interrupt to the source device for its presence and linking. Now, we will see the difference between the LVDS and eDP technology. Also, how to check the voltages, data and clock in case there is problem in the display section of laptop motherboard with pin-out configuration and voltage available at the pins to be checked.

 

Conclusion

This article explains the LVDS (Low voltage differential signaling) and EDP (Embedded Display Port) technology used in the display interface system. You will get a better understanding of the working and use of the display interface by going through the above content. In the upcoming article you will understand the difference in the working of the LVDS and EDP according to voltage, Clock and data. Also, pin-out configuration of the two display interface will be dealt in elaborated form with their application. 

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