Analysis of Common Failures in HDMI Compatibility Testing

Analysis of Common Failures in HDMI Compatibility Testing

Almost all devices, including TVs and DVD players, do not pass certification testing the first time. Most of these failures are related to system design and PCB layout. In order to pass HDMI compliance testing, special features are sometimes required. For example, it may be necessary to enable or disable the HDCP (High Definition Content Protection) function in the source device. In order to design products that comply with the HDMI standard, IC and system design companies and manufacturers must fully understand the HDMI specification and the Compliance Test Specification (HDMI CTS). The following lists the most common failures and their corresponding recommended solutions.

EDID Test

The source device must support the "Enhanced DDC" specification, which means that the source device must be able to use the "segment pointer 0x60" to read the EDID information after 256 bytes. In most cases, only 256 bytes are used for EDID, but the HDMI compliance test requires checking whether the system can read 4 blocks (128 bytes/block), that is, a total of 512 bytes. In order to pass the HDMI compliance test, the segment pointer must meet this requirement.

5V power supply for HDMI transmitter (Tx)

Some customers use a series resistor or diode to limit the current of the 5V output. In the HDMI compliance test, the 5V power supply test of HDMI Tx requires 55mA current to be drawn from the +5V power pin. This may cause the test to fail because the power supply output must be between 4.8V and 5.3V to pass. Figure 1 shows that when a 10Ω resistor is connected, the output drops to 4.45V, thus failing the test.

Figure 1 When a 10Ω resistor is connected to the +5V power supply, the output drops, causing the test to fail.

HDMI Tx DDC/CEC Line Test

The Consumer Electronics Control (CEC) line (pin 13 of the HDMI connector) must be properly connected. If the system is not designed to support CEC functionality, the designer can leave this line floating. Sometimes, people want to connect the CEC line to the general I/O of the video signal processing (VIP) chip to support possible future expansion. In this case, the designer must ensure that the connection meets the HDMI compliance test guidelines, including keeping the maximum capacitance of the DDC line less than 100pF.

Video formats that HDMI source devices must support

The HDMI specification requires that all HDMI source devices must support one of the following formats: 640480p@59.94/60Hz, 720480p@59.94/60Hz¼, or 720576p@50Hz. When designing HDMI source devices, you must sometimes be aware of another requirement: if any YPbPr or other uncompressed digital port on the source device can support the following formats, then the HDMI port on the device must also support them.

1280 720p @59.94/60Hz

1920 1080i @59.94/60Hz

720 480p @59.94/60Hz

1280 720p @50Hz

1920 1080i @50Hz

720 576p @50Hz

Compatibility between HDMI source devices (HDMI Tx) and DVI sink devices (DVI Rx)

The HDMI specification requires that all HDMI source devices be compatible with DVI 1.0 compliant sink devices. When an HDMI source device is connected to a DVI sink device, the following requirements must be met:

• The video format sent is RGB

• Video guard band is not sent

• Do not send Data Island

When the source device detects that a sink device is plugged in, it should assume that the sink device is a DVI device. At the same time, the source device will check whether the EDID of the sink device contains the CEA extension, and whether the CEA extension contains a VSDB (Vendor Specific Data Block) with a valid length. If both are included, the source device will assume that the connected sink device is an HDMI device.

EDID Test for HDMI Receiver (Rx)

The pass rate for this test is very low. Therefore, designers should familiarize themselves with the EDID requirements of the latest HDMI specification. Here are some common mistakes that lead to failure.

I. In the first 128 bytes of EDID, the "Monitor Distance Limit Header" and "Monitor Name Header" must be provided, both of which are 18 bytes. If the content of either is shorter than 18 bytes, it must be padded with 0x20 and terminated with 0xA0.

II. The "Short Video Descriptor (SVD)" in EDID does not match the video formats supported by the "Capability Declaration Form (CDF)". Any format declared in the CDF must be listed in the SVD of the EDID.

To simplify system design, some designers directly connect the +5V voltage of the HDMI input to the HPD pin of the HDMI Rx through a 1K resistor. In this type of design, when the system is in standby mode or the AC power is disconnected, the EDID EEPROM cannot be read and the HPD pin voltage may remain high, which will cause the HDMI compliance test to fail because the HDMI compliance test specification requires that the EDID must be accessible and readable when the HPD pin voltage is high, even if the AC power is disconnected or the system is placed in standby mode. A reference design that solves this problem will be proposed in the following section.

HDMI Rx TMDS Differential Impedance

A large number of devices fail this test, although most system designers are aware of the requirements for input differential impedance. In most cases, the reason why the system fails this test is that the parasitic capacitance of the input ESD protection device or EMI choke is too large. Most high-speed signal ESD protection device manufacturers provide customers with system PCB layout reference designs with impedance characteristics. For this test, the system PCB layout will affect the test results. If it fails, the user has to redesign the PCB, which will affect the production schedule and the time to market of the product.

Please note that in HDMI CTS V.1.3, the criteria for passing this test are different from HDMI CTS V.1.2. As with previous CTS, the impedance requirement for through connections is still 100Ω±15%, but a single excursion of maximum/minimum values ​​of 100Ω±25% is allowed, but the duration is shorter than 250ps.

HDMI Rx DDC/CEC Line Capacitance and Voltage

This test has the highest failure rate due to the high capacitance of the MOSFET used for level shifting. To avoid failure, it is recommended to use MOSFETs with Ciss and Coss less than 10pF on the DDC lines.

HDMI Rx CEC cable connectivity

When an HDMI system has multiple HDMI inputs, for non-independent CEC lines, the HDMI specification requires that the CEC lines from all HDMI inputs (if any) can be interconnected with one HDMI output (if any). The CTS requirement for this test is less than 5Ω. Even for systems that do not support CEC, the input CEC lines are required to be interconnected.

HDMI Rx HPD output voltage

Some customers use the circuit shown in Figure 2 to utilize the 5V voltage provided by the HDMI connector and use VCC to provide the required 5V voltage for HPD. This violates the specification requirement that when the HDMI 5V input is 0V, the HPD voltage should be higher than 0V but lower than 0.4V. The simplest way to solve this problem is to connect a 1K resistor in series to the HPD pin. The simple switch circuit shown in Figure 3 can also solve the problem. Its function is to control the HPD pin and notify the source device when the downstream sink device is ready, or decide whether the source device should restart certain functions such as High Definition Content Protection (HDCP) authentication. This will greatly improve system compatibility.

Figure 2 HPD output voltage

Using the 5V voltage from the HDMI connector and VCC to provide the 5V voltage required for HPD violates the HDMI compliance test requirements.

Figure 3 HPD output voltage

A switch connecting a 1 K resistor in series with the HPD pin can solve this problem.

HDMI Rx video format support

A common failure of this test is that some systems fail to meet the requirement that all HDMI sink devices must be able to receive 640 480p@59.94/60Hz. Systems designed for 60 Hz video must be able to support 720 480p @ 59.94/60Hz input formats, and systems designed for 50 Hz video must be able to support 720 576p@50Hz input formats. Many customers have overlooked the requirement to support 640 480p video formats in source devices tested in the ADI lab.

Less tolerance for video format timing variations is another common reason for test failures. This test covers all supported video formats. Systems supporting 50Hz must be able to tolerate timing variations between 49.75Hz and 50.25Hz (50Hz 0.5%). Systems supporting 59.94Hz or 60Hz must be able to tolerate timing variations between 59.64Hz and 60.3Hz ((59.94Hz "0.5% to 60Hz +0.5%).

New test requirements for wireless repeaters

In the last 6 to 9 months, wireless repeater devices have grown rapidly. HDMI Licensing LLC has released testing guidelines for these wireless devices. If the wireless repeater supports High-bandwidth Digital Content Protection (HDCP), the device must be tested as an HDMI repeater. If the device does not support HDCP, the device can be tested as a repeater or as a standalone source and sink. If the wireless device is a CEC root device, the "Physical Address" test can be omitted.

Conclusion

As mentioned above, in order to ensure that the product complies with the HDMI specification, passing the HDMI compatibility test is a basic and important step, which also helps to improve the interoperability between products of different manufacturers. Providing solutions for the most common HDMI compatibility test failures can not only provide high-quality HDMI interface products for consumer electronics customers, but also help them design highly compatible systems.