Principle of hot swapping and detection of HDMI/DVI display (HPD)

Source: Internet
Author: User

HDMI (19pin)/DVI (16 pin) features hot swapping detection (HPD), which serves as the basis for the host system to determine whether the HDMI/DVI sends tmds signals. HPD is a detection signal sent from the display to the Computer Host. The hot swapping detection function is to detect this event through the HPD pin of HDMI/DVI when the display and other digital displays are connected to or disconnected from the computer host through the DVI interface, and make a response. In terms of transmission signals, HDMI is completely compatible with DVI, but the interface encapsulation is different. The following uses HDMI as an example to describe the principle and implementation method of HPD. It is an interface between hdmi and DVI.

Interface between HDMI (left) and DVI (right)

1. The display is online through HDMI
When the computer host detects that the display is connected to the computer host through the HPD pin of the HDMI interface, the graphics display system (graphics card) in the host sends a signal, the computer is required to read the edid data (Extended display identification data) stored in the DDC memory through the DDC (DDC I2C bus) in the display data channel in the HDMI interface ), if it is detected that the display's working mode range is compatible with the video card, the host system can activate the tmds signal transmission circuit (digital video signal transmission circuit) of the video card ).

2. Disconnect the HDMI connection of the monitor.
When the computer host detects that the HDMI interface of the display is disconnected from the computer host through the HPD pin, the graphic display system (graphics card) in the host sends a signal, notifies the computer's operating system to interrupt the work of The tmds signal sending circuit (installed on the video card) of the video card.

3. Requirements for HPD signals on computer hosts
When the video card on the Computer Host detects that the HPD pin voltage of the DVI interface is greater than 2 V, it is determined that the display is connected to the host through the DVI interface: When the HPD pin voltage is detected to be less than 0.8v, the DVI connection between the monitor and the host is disconnected.

4. Implementation of HPD Signal

Display HPD and PWR peripheral Diagram

For example, when a computer is connected to a display through an HDMI interface, the host uses the HDMI 18th foot (pwr_con_pin18) to add the + 5 V voltage to the DDC memory (edid data storage) of the display) power the DDC memory to ensure that the computer host can read edid data through the HDMI interface even if the monitor is not turned on.
After the host is started, 5v_sys is generated and the power is supplied to the monitor through 18th feet. After the monitor receives 5 V voltage, the HDMI port 19th HPD is converted to a high level through an internal circuit, and Q1 is driven to enable ce conduction, as a result, hpd_gpu is also converted to high level. When the host (graphics card controller) detects that HPD is high, it determines that the monitor is connected to the host through HDMI, and read the edid data in the display through the 15th and 16-pin DDC channels (I2C) of the HDMI interface, and enable the tmds signal sending circuit in the host graphics card to work.
When the HDMI connection between the monitor and the host is disconnected, the HDP signal on the host side is low, And the tmds signal sending circuit in the host video card stops working.

1. DDC is a bus standard for the display to communicate with the computer host. Its full name is: Display Data chnnel. Its basic function is to display the electronic file information of the display, such as the field frequency range that can be received, manufacturer, production date, product serial number, product model, standard display mode and its parameters, supported DDC standard category, and edid version information. The advanced DDC standard bus also allows computer hosts to directly adjust basic parameters of the display, such as brightness, contrast, line field amplitude, Line Field Center Position, color temperature parameters, and so on.

2. edid Data Standard: edid (Extended display identification data standard) is the standard data information that the display transmits to the computer host through DDC. It has been released to the third version, that is, edid version 3, the previous versions include edid Version 1.0, Revision 0, edid version 1, Revision 1, edid version 2, Revision 0, edid version 2, and Revision 1. Edid is divided into 128 bytes and 256 bytes based on the amount of data information. In the future, there may be a new version of edid with more data information.

3. tmds is short for minimizing the transmission difference signal.
Silicon image began to promote its ownership standard to the display industry in the form of panel connections, digital visual interfaces (DVI) and High-Definition Multimedia interfaces (HDMI)-minimizing the transmission of differential signals (tmds ). In this case, the transmitter end is mixed with a more advanced encoding that reduces EMI on copper wires.AlgorithmSo that the acceptor has robust clock recovery performance.

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