what is EDP cable?
EDP (Embedded, Embedded Displayport) , which is a kind of internal digital interface based on DisplayPort architecture and protocol. EDP is gradually replacing the old low-voltage differential signal (LVDS) transmission interface, especially on panels with FHD (1,920 x 1,080 or 1,920 x 1,200) or resolution beyond FHD. You can easily find EDP applications in a variety of products with embedded display panels, including All-in-One pcs, laptop or tablets.
The EDP has three basic architectures. The following is a brief description of the basic architecture of DisplayPort, including the DisplayPort data transfer Main Link, AUX Channel, and Link Training.
1) main channel for video and audio data transmission
The data coding protocol adopts 8B/10B coding, through which only a Differential Signal Pair is needed to transmit data and clock simultaneously. The connector consists of four pairs of differential signal lines, or four main channels, which are used to transmit image data. Depending on the amount of data displayed, one, two or four Lane channels can be used to transmit data
EDP defines three different transmission rates, each of which can choose to use 1.62 Gbit/s, 2.7 Gbit/s or 5.4 Gbit/s. Because DisplayPort uses 8b/10b encoding, which adds a bit more data to the encoding, the maximum data transfer rate that DisplayPort can actually support is:
(4-path) x (5.4 Gbit/s per path) x (8/10 Coding Overhead) = 17.28 Gbit/s
The AUX channel is used to transmit settings and instructions
The DisplayPort connector also contains a separate two-way transmission auxiliary channel called Aux Channel or AUX for short, again using two differential signal lines with a single direction rate of only about 1 Mbit/s, used to transmit settings and control instructions, and more on the use of EDP later. It also includes reading Extended display identification data information (EDID) to ensure that the correct image format is transmitted (other interfaces such as LVDS, VGA, DVI and HDMI are transmitted through I2C) ; Read the contents of DisplayPort items supported by the display, such as how many main channels, transfer rates and other items, set various display configuration registers, and read the display status register.
3) the connection process helps to enhance the reliability of the main channel
Connection is the process of establishing a connection between the DisplayPort Transmitter and the Receiver before the data is transmitted. The sending end and the receiving end communicate with each other through AUX to determine whether the connection is successful. The connection can increase the reliability of the main channel and reduce data errors, and it can compensate the difference of electric property caused by different length and kinds of cables, especially the difference of signal routing on the system board of the transmitting end and the receiving end.
What are the components of EDP interface signal
The ADP interface signal consists of three parts: Main Link, Auxch and HPD, as shown in the following figure.
There are three Main components: Main Link, AUX CH, and HPD
The Main Link represents the Main channel for the transmission of all types of video and audio data, and AUX CH represents the auxiliary channel for the transmission of data with low bandwidth requirements, as well as Link management and device control signals HPD represents the hotplug detection channel.
Main Link consists of 1-4 pairs of data lines, each pair of data lines are a pair of difference lines. 1) adopting AC coupling technology, the transmitter and the receiver have different common-mode voltages, so the interface can be made smaller; 2) the current transmission rate of each line is: 1.62/2.7/5.4 Gbps; 3) each data line is data line, no clock line, reduce EMI; 4) using ANXI8B/10B coding, improve data transmission accuracy.
For an LCD screen, the Main Link requires several pairs of data lines, depending on the resolution of the screen and the number of color bits.
The signals transmitted in the channel include video pixel signal, video timing signal, video format signal, bit/pixel and color space signal and error correction signal of video signal, anxi 8b/10B coding is used to improve the accuracy of data transmission. Data transmission uses AC coupling technology, sending and receiving end have different common-mode voltage, so can make the interface smaller.
TIP: ANSI8B/10B coding is to divide a group of 8-bit data into two groups, a group of 3-bit, a group of 5-bit, and then after coding, a group of 4-bit, a group of 6-bit binary data.
AUX CH: used to transmit data for small bandwidth requirements, link management, and device control
It is a bi-directional half-duplex transmission channel, whose signal is transmitted by ac-coupled differential transmission, and whose signal is encoded by MANCHESTERLL, with the transmission rate of lMbps and the transmission distance of 15m. Low latency, less than 500 μs per transfer task. In addition, the channel is connected with EDID and DPCD memory, and read and write by bus. EDID for extended display identification data, used to store display parameters, and DPCD for EDP interface configuration data, connected to link management layer for link configuration.
HPD is a one-way channel, which is used to detect whether the e-layer equipment and the lower equipment are connected, and then realize the connection and interrupt of the line.
How to choose the number of Lane?
How Main Link selects the number of Lane
The number of Lane required is based on the screen resolution and color depth.
NOTE: 1Lane can transmit 1.62 g * 8/10 = 1.296 Gbps if the data rate is 1.62 Gbps and only applies to one lane, while 1Lane can transmit 2.7 g * 8/10 = 2.16 Gbps if the data rate is 2.7 g * 8/10 = 2.16 Gbps
Bit rate requirement = pixel of clock rate * pixel depth (BPP) ;
In EDP, the Pixel of clock rate is fixed and can be obtained by table lookup.
For example, a 1080P@60hz LCD screen,
The minimum bandwidth required for transmission is 148.5 * 24 = 3.564 Gbps,
so a minimum of 2 lane = 4.32 Gbps > 3.564 Gbps is required