High-speed HDMI interface design for high-definition audio and video systems

　After becoming a standard interface for consumer electronics, HDMI has also been widely used in other fields such as car displays. For design engineers who were not originally professional in the field of consumer audio and video, designing high-frequency HDMI interfaces requires a learning and practice process. This article uses the ADV7842/ADV7511 reference design solution as an example to introduce the design and basic requirements of HDMI input and output interfaces, the requirements of HDMI Compliance Test, as well as common problems and recommended solutions.

　　In the past two years, the application of HDMI has been greatly popularized. It has gone far beyond the scope of traditional TVs, projectors, game consoles and A/V receivers and has become an essential interface for high-definition digital cameras and home video cameras. Its application in smart phones and car displays is also booming. Design engineers must have a basic understanding of the design requirements and compatibility tests (HDMI Compliance Test) of HDMI input and output interfaces, so that they can consider them in advance when designing circuits and PCBs to avoid duplication of design.

　　HDMI I/O terminal and power supply design

　　ADV7842 is a video I/O chip that integrates analog (CVBS, RGB, and YPbPr) and HDMI receiver (HDMI Rx) inputs. ADV7511 is an HDMI transmitter chip. The following uses the HDMI I/O reference circuit made with these two chips to discuss the most common problems and recommended solutions in high-speed and high-precision video PCB design. Figure 1 is the block diagram of the ADV7842-ADV7511 AV reference circuit, and Figure 2 is the actual PCB of the ADV7842-ADV7511 AV reference circuit.

　　

　　Figure 1. ADV7842-ADV7511 AV reference circuit block diagram

　　

　　Figure 2 Actual PCB of ADV7842-ADV7511 AV reference circuit

　　HDMI input

　　The length from the HDMI connector to the HDMI Rx data input pin on the PCB should be minimized, so the HDMI Rx chip should be placed as close to the HDMI connector as possible, as shown in Figure 2. Too long a connection line will be affected by other external sources and generate noise. Short connections also help control differential impedance. The differential impedance of the HDMI TMDS transmission line should be controlled at around 100Ω. The differential impedance is closely related to the board material, board thickness, stacking structure, line width, and line spacing. It is recommended that users negotiate relevant parameters with the board manufacturer and take them into consideration during design to avoid changing the board or adjusting the design again.

　　HDMI Receiver Power Supply

　　In order to achieve good design performance (especially when transmitting high-definition signals at high speed), the design and layout of the HDMI Rx power supply are very important. Issues to note in typical HDMI Rx power supply design:

　　• Analog Line Power: HDMI Rx Analog Line Section

　　–When designing, pay attention to isolating important power supplies from other 1.8V power supplies with magnetic beads.

　　– The power supply needs to be equipped with bypass capacitors (ADV7842 uses 10nf and 100nf). The closer the capacitor is to the power pin, the better.

　　– Place a large decoupling capacitor (e.g. 10uF) close to the current source.

　　•Digital Line Power: HDMI Rx Digital Line Section

　　– When designing, pay attention to separating important power supplies from other

　　The 3.3V power supply is isolated by ferrite beads.

　　– The power supply needs to be equipped with bypass capacitors (ADV7842 uses 10nf and 100nf). The closer the capacitor is to the power pin, the better.

　　• When designing the power supply PCB layout, please pay special attention to the following points:

　　– It is recommended to use a single ground plane.

　　– When impedance matching allows, consider using the top or bottom layer as the power and ground layer.

　　– Consider separating the power and ground planes with as thin a dielectric layer as possible, provided impedance matching allows.

　　– In the PCB power layer: Avoid connecting the power layer with a single line on other layers. If it cannot be avoided, add appropriate bypass capacitors. When routing high-current power supply, consider the voltage drop on it. If a power supply cannot be connected with a power layer, use the thickest possible trace to connect it to minimize the trace impedance. Fill the unused area on the power layer with the ground layer as much as possible. Coupling of power supplies on different layers (such as the digital power layer above the analog power supply) should be avoided as much as possible. The return path of the current should be ensured to be low impedance.

　　• Notes when using bypass capacitors:

　　– Bypass capacitors should be placed as close to the pins as possible.

　　– The wires connecting the pins should be as short and wide as possible.

　　– The line connecting to ground should also be as short and wide as possible.

　　– Use more than one via per pad when possible but the spacing between vias should be at least as deep as the via.

HDMI Test

　　There are many issues that need to be paid attention to in HDMI testing. According to the requirements of the HDMI CT (Compliance Test) spec, all HDMI receiving devices need to provide EDID information. In the past, most customers would choose to plug in an EEPROM on the DDC bus to store EDID information. In order to further reduce the user's BOM (bill of material) cost, the ADV7842 integrates an internal RAM to store EDID information. After power-on, the user can write the EDID information. Using the SPI Master interface provided by the ADV7842, the user can also plug in an SPI EEPROM, so that the user can permanently burn the EDID information into the SPI EEPROM. The ADV7842 can read the information in the SPI EEPROM into the internal RAM. In this way, the burden on the software can be reduced. Of course, these functions have corresponding registers to provide enable control. Users can turn off these functions and use the traditional plug-in method to be compatible with past designs.

　　With hardware support, of course the content of EDID must also comply with the requirements of HDMI CT Spec. Based on ADI's experience in providing free HDMI pre-testing to customers for many years, some items related to EDID testing often fail during the HDMI pre-testing process. In fact, if you are familiar with the relevant requirements of the specification, it is relatively easy to pass these items. For example, the relevant specification requires that the "Monitor Range Limit Header" and "Monitor Name Header" must be provided in the first 128 bytes of EDID, but if the content of these two items is less than 18 bytes each, it is necessary to end with 0x0A and fill the remaining bytes with 0x20. Another problem often encountered in EDID testing is that customers cannot make the SVD (Short Video Descriptor) module in EDID consistent with the submitted CDF (Capabilities Declaration Form), and the format supported in CDF is not supported in the SVD module of EDID, or vice versa.

　　Table 1 ADV7842 3D video format test list

　　

　　HDMI Compatibility Test

　　The HDMI Compliance Test has test requirements for supported video formats that are worth noting. ADV7842 has further improved the robustness of the product and provides support for more video formats, such as the 3D video signal format added to the new HDMI Spec. Here, we need to mention an HDMI CT test issue for your reference. In the test of video format support, all video formats that users declare to be supported will be tested, and the support capabilities of devices supporting 50Hz for 50Hz±0.5% (i.e. 49.75Hz and 50.25Hz), and devices supporting 59.94Hz or 60Hz for 59.94%~0.5% (i.e. 59.64Hz) and 60＋0.5% (i.e. 60.3Hz) field frequency changes will be tested. ADV7842 can fully support this range of field frequency changes, but users need to pay attention to these requirements for video format tolerance in software design.

　　Although 3D timing is clearly defined in the HDMI1.4 standard, it is very important to conduct comprehensive testing of the video format of HDMI products during design in order to ensure product compatibility in the market. The previous paragraph mentioned the support for 3D timing. ADV7842 has done a very complete test on 3D timing. Table 1 shows all the commonly used and must-support 3D timings that ADV7842 has tested. The yellow highlights are the 3D timings that must be supported.

　　The above is a general discussion of the common problems and solutions in HDMI I/O design and testing. The faster the video signal, the more prominent the problems discussed above. Therefore, when choosing HDMI I/O chips, especially receivers (HDMI Rx), products that have been strictly tested by HDMI CT should be selected to avoid compatibility issues.