Basic Principles of HDMI / DVI In recent years, the field of video transmission has undergone a fundamental transformation from analog to digital. VGA and component video — analog video connection methods — are being replaced by HDMI ™ and DVI ™. The requirements of these two digital video transmission standards are almost identical, and they simultaneously process a set of high-frequency and low-frequency signals. The DVI specification was finalized in 1999. At the time of this writing, version 1.3 of the HDMI specification is under development. Both standards use TMDS® (Minimum Transition Differential Signaling) to transmit the high frequency (video) portion of the data. The TMDS signal uses four differential pairs to transmit R, G, B and clock, occupying 8 pins of a 19-pin connector. HDMI and DVI are designed as "plug and play", that is, when the monitor (receiver) and the video source are connected together, they find a way to work together with the best performance. Most new TMDS HDTV chips include two complete sets of TMDS (high frequency) inputs, but cannot process LoF (low frequency) signals.

 

    To implement the "plug and play" function in HDMI and DVI systems, the source (usually a computer, DVD player or game console) and the receiver (usually a monitor or receiver) must be connected. HDMI and DVI borrow the open standards of VESA (Video Electronics Standards Association), using DDC (Digital Display Channel), a new signal called HPD (Hot Plug Detect), and a channel that can provide 50mA from the source to the receiver Standard 5V signal. In the standard VESA method, the source addresses EDID (Extended Display Identification Data) EPROM. The EPROM device contains the brand, type number, and supported resolution modes of the receiving device. The source and receiver must have at least one display mode in order for the two to work together. Figure 1 shows a schematic diagram of connecting the source and sink EDID EPROMs via an HDMI / DVI connector.

 

<IMG border = 1 hspace = 0 alt = detailed analysis--the basic principles of HDMI / DVI src = "http://www.pjtime.com/img_UpArticle/2012-07/2012730172151148788.gif">

 

Figure 1. Schematic showing the EDID EPROM of the HDMI / DVI connector connecting the source and sink.

 

    Figure 1 shows the TMDS signals connected as four differential pairs, + 5V, HPD, and DDC signals. The DDC signal is connected to EDID. The EDID power is provided internally by the receiver. This figure illustrates the general connection mode of the source and sink. The source and receiver communicate via an I²C-compatible DDC line. The I²C specification introduced in 1980 was the + 5V specification. A typical EDID EPROM such as 24LC22 contains a 2kb EPROM for storing the required information and can operate from 2.5V to 5.5V. When operating on a + 3.3V power supply, typical low-cost EDID EPROMs do not have a + 5V withstand voltage. Therefore, the EDID EPROM device must work from a + 5V power supply or have + 5V protection externally.

 

    In order to switch between two HDMI / DVI sources, the designer must handle two different signals: the TMDS high frequency signal and the aforementioned low frequency signal. Some new HDMI processors already include two sets of inputs that can process high-frequency TMDS signals, but cannot handle low-frequency signals with high voltages. Maxim's MAX4929E offers maximum flexibility in processing these low-frequency signals.

 

    Switch between low-frequency video sources

 

    With the MAX4929E, all signals connected to external connectors are protected to ± 15kV HBM (Human Body Model). This high level of ESD protection usually eliminates the need for additional protection on each pin. The MAX4929E allows access to two sets of DDC signals, and the device selects one of the inputs. This source switching can achieve multiple functions: provide ESD protection for the signal, only select one source at the same time and provide a logic level clamp to protect the EDID EPROM terminal from voltages higher than its power supply. The MAX4929E consumes very little current, and the + 5V provided by the source device can meet its power requirements.

 

    The MAX4929E has no special power-up sequence requirements, so designers do not have to worry about what happens when one source device is turned on and other source devices are turned off. The switching function between source devices is necessary for the following reasons: the switching function puts the receiving end under the control of which connectable device; it also prevents the second cable from loading the I²C bus. The latter advantage solves two problems encountered when two source devices are connected to a load at the same time:

 

    Possible host conflicts

 

    Additional capacitive load (200pf / m distributed capacitance per cable)

 

    If a second 2m cable is connected to the main cable, it will exceed the 700pf maximum distributed capacitance specification. By using switches, the I²C driver "sees" only one load at a time. In this way, even two 3m cables do not violate the 700pf specification.

 

    In most systems, the MCU controls various operations. The MCU must determine whether the input is valid and return a TTL-compatible HPD signal after the EDID handshake. The following features of the MAX4929E (Figure 2) solve three problems:

 

    ESD protection at HPD output

 

    Allows the MCU to determine if the selected HDMI input is connected

 

    Provides logic level translation from low voltage MCU to 5V TTL compatible signals

 

<IMG border = 1 hspace = 0 alt = detailed analysis--the basic principles of HDMI / DVI src = "http://www.pjtime.com/img_UpArticle/2012-07/2012730172155128412.gif">

 

Figure 2. The schematic shows a typical circuit connection for the MAX4929E. This device provides the switching, logic level matching, and ESD protection required to implement a complete 2: 1 HDMI or DVI switch.

 

    ESD protection has been discussed earlier. If the HPR signal is logic high (nominal value + 5V), the logic high level will appear on the pin connected to the MCU, and the logic level of the pin is guaranteed to be compatible with the MCU level, because the MAX4929E The reference pin is connected to the power supply of the MCU. If a + 5V signal appears on the selected input, a logic-level compatible signal will be available on the MCU. After receiving this signal, the MCU then sends out the HPD signal. The MAX4929E "leads" the HPD signal to the appropriate HDMI device and generates a fully TTL-compatible signal on the connected device. The MAX4929E receives a logic 1 or logic 0 from a low-voltage MCU and generates an in-phase logic 0 or 1 (TTL compatible level) for the HPD. Using its input, this signal is guaranteed to be logic-level compatible, as its reference is connected to the same power supply as the MCU.

 

    In addition to working with TMDS devices containing two high-frequency inputs, the MAX4929E can also form a chipset with the MAX4886HDMI / DVI video switch to integrate and integrate two sets of TMDS into a single device. The MAX4886 / MAX4929E chipset provides a second set of inputs for single-input devices.

 

    in conclusion

 

    The MAX4929E controls the switching of all low-frequency signals in a 2: 1 HDMI / DVI switch. Provides a high level of ESD protection for all external leads. The MAX4929E is used in conjunction with an EDID EPROM. Its input accepts + 5V signal levels and clamps the output to + 3.3V to match EDID. In addition, the MAX4929E isolates the capacitance of a cable, so the DDC output has only one set of DDC connections at a time. The MAX4929E has a third input voltage that can be connected to the same power supply for the system MCU. This feature ensures that the SEL, HPR, and HPD signals are compatible with the logic levels of the MCU. The MAX4929E handles all the switching, logic-level matching, and ESD protection functions needed to implement a 2: 1 HDMI or DVI switch. The MAX4929E also enhances the MAX4886 to implement complete single-input device switching.