Analysis of Common Failures in HDMI compliance testing

Since the release of the HDMI standard a few years ago, it has been widely used. To stick the HDMI logo, all HDMI products must pass the HDMI compliance test (hdmi ct ). To save time and money, consumers should test their HDMI products in advance before sending them to the authorized test center (ATC) for authentication. Based on the numerous pre-testing experience in the hdmi ct lab of simulator, this article discusses the most common system design problems that cause products to fail to pass the hdmi ct. It also analyzes various failures in specific situations and provides solutions.

Analysis of Common Failures in HDMI compliance testing

Almost all devices, including televisions and DVD players, cannot pass the test for the first time. Most failures are related to system design and PCB layout cabling. To use hdmi ct, special functions are sometimes required. For example, high-resolution Content Protection (HDCP) may need to be enabled or disabled on the source device. To design HDMI-compliant products, IC and system design and manufacturing companies must fully master the HDMI specifications. The following are the most common failures and recommended solutions to solve these problems.

1. edid Test

The source device is required to support the "enhanced DDC" Specification. This means that the source end needs to be able to read edid information after 256 bytes using the segment pointer 0x60. In most cases, only 256 bytes are used for edid. However, hdmi ct needs to check whether the system can read four modules (126 bytes per module), totaling 512 bytes. To use hdmi ct, this is very important for segment pointers.

2. 5 V power supply for HDMI Transmitter

Some customers connect a resistor or diode in series to limit the current of 5 V output. In hdmi ct, the 5 V power supply for the test hdmi tx needs to absorb 55mA of the current from the + 5 V power supply during measurement. This may cause the test to fail because the power output must be between 4.8v and 5.3v. Figure 1 shows that when a 10 euro resistor is connected, the output drops to 4.45 V, leading to this test failure.

V = 5 "(10-0.055) = 4.45 v <4.8 V

Figure 1: When a 10 euro resistor is connected, the output drops to 4.45 V, leading to the failure of this test.

3. hdmi tx ddc/CEC Line Test

It is critical to correctly connect the consumer electronic control (CEC) line (13th pin of the HDMI connector ). If the system is designed to not support the CEC function, the design engineer can leave the cables blank. Sometimes people want to connect the CEC line to the common purpose I/O of the video signal processing (VSP) chip, so as to allow subsequent extensions that may be needed. At that time, the design engineer had to be sure that the connection met the hdmi ct guidelines, including keeping the maximum DDC line capacitor less than 100pf.

4. The required video format must support HDMI source devices.

HDMI specifications require that all HDMI sources must support one of the following formats: 640-480p@59.94/60Hz, 720-480p@59.94/60Hz or 720-576p @ 50Hz. When designing an HDMI source, another requirement is sometimes ignored, that is, if any ypbpr or other non-Compressed digital ports on the source device can support the following formats, the HDMI ports on the same source device can also support them:

1280-720 p @ 59.94/60Hz

1920-1080i @ 59.94/60Hz

720-480 P @ 59.94/60Hz

1280-720 p @ 50Hz

1920-1080i @ 50Hz

720-576 P @ 50Hz

5. Compatibility Between HDMI Tx and DVI receivers (dvi rx)

HDMI specifications require that all HDMI sources be compatible with the receiving end devices that are compatible with DVI 1.0. When an HDMI source is connected to the DVI receiver, it must meet the following requirements:

The sent video is in RGB format;

No video protection band is sent;

No data island is sent;

When the source device detects the insertion at the receiving end, it should assume that the receiving end is a DVI device. At the same time, the source end will check whether the edid of the receiving end contains the CEA extension and whether the CEA extension contains a data block (vsdb) of a specific supplier with a valid length ). If both conditions are true, the source end determines that the inserted receiver is an HDMI device.

6. Test the edid Of The HDMI receiver (RX)

The failure rate of this test is quite high. Because of this, design engineers should familiarize themselves with the edid requirements of the latest HDMI specifications. Common Errors that cause failures are listed below:

I. In the edid header 128 bytes, the "Monitor range restriction Header" and "monitor name header" must be provided ". Both are 18 bytes. If either of these two bytes is less than 18 bytes, it must be filled with 0x20 and ended with 0xa0.

Ii. Conflict between the supported video formats in edid "short video descriptor" and "performance declarative form (CDF. Any format announced in CDF must also be listed in the edid SVD.
To simplify the system design, some design engineers connect the + 5 V voltage of the HDMI input to the HPD pin of the hdmi rx through a 1 K resistor. In this design, the edid EEPROM cannot be read and the HPD pin voltage may remain high when the system enters standby mode or when the AC power supply is removed. This causes an hdmi ct failure because the HDMI compliance test specification requires that the edid must be accessible and can be read when the HPD pin voltage is high, even when the AC power supply is removed and the system enters standby mode. A sample reference design circuit provided in the subsequent sections solves this problem.

7. hdmi rx tmds differential Impedance

A large number of devices cannot pass this test, even if many system design engineers are aware of the need to adopt an input differential impedance. Most of the time, the reason the system fails this test is because of the large Parasitic Capacitance from the input ESD protection device or EMI throttling. Most manufacturers of High-speed signal ESD protection devices provide their customers with reference designs for System PCB layout cabling with impedance characteristics. This is one of the tests that affect the testing results of the system's PCB layout and wiring. If the test fails, the customer needs to redesign the PCB. This will affect the production progress and time to market.

Note: In hdmi cts V.1.3, the rule for passing this test is different from that for hdmi cts v.1.2. As in the past CTS, the impedance of the pass-through connection is still 100 Ω +-15%. However, A single drift is allowed to be 100 Ω +-25% and the duration is less than 250 ps.

8. hdmi rx ddc/CEC wire capacity and Voltage

This is one of the most frequent failed tests because the power-Encoder used for voltage level transfer has a large capacitance. To avoid failure, we recommend that you use the CISS and Coss less than 10pf on DDC.

9. hdmi rx cec connection

When the HDMI system has multiple HDMI inputs, The HDMI specifications are required-for an independent CEC cable-the CEC cable from all HDMI inputs and a single HDMI input (if any) connected. The CTS requirements for this test are less than 5 euro. Even if the system does not support CEC, it is still required to connect to the input CEC line.

10. hdmi rx hpd output voltage 
Some customers use the circuit shown in 2 to use 5 V from the HDMI connector and use VCC to supply the 5 V voltage required by HPD. This violates the requirement that when the hdmi5v input is 0 V, the HPD voltage should be higher than 0 V and lower than 0.4 V. The simplest way to solve this problem is to concatenate A 1 K resistor on the HPD pin. The simple switch circuit shown in 3 works well. Its function is to control the HPD pin and inform the source end when the row Rx is ready or if the source end should re-start some actions, such as high definition content protection (HDCP) proof. This capability greatly improves system compatibility.

Figure 2: HPD output voltage.

Use 5 V and VCC from the HDMI connector to supply the required 5 V voltage for HPD, which violates the requirements of hdmi ct.

Figure 3: HPD output voltage.

Solve this problem by concatenating A 1 K resistor with the HPD pin.
11. hdmi rx video format support
　　11. Support for hdmi rx video formats 
A common failure in this test project is that some systems do not meet the requirement that "All HDMI Receivers must be able to receive 640-480p@59.94/60Hz format. The system designed for 60Hz videos must support 720-480p@59.94/60Hz input formats, while the system designed for 50Hz videos must also support 720-576p @ 50Hz input formats. Many customers have not noticed the requirements for supporting the 640-480p video format for the source devices tested in the ADI lab.
Another common failure in this test is the small tolerance for timing changes in the video format. This test should cover all supported video formats. A 50Hz system is required to allow time series changes from 49.75 to 50.25Hz (50 +-0.5% Hz. Systems supporting 59.94 or 60Hz must allow time series changes between 59.94 "0.5% Hz (59.64Hz) and 60 + 0.5% Hz (60.3Hz.
In the past 6-9 months, Wireless Repeater devices have grown rapidly. HDMI licensingllc released a test guide for such radio devices. If the radio reseller supports high-bandwidth Digital Content Protection (HDCP), various devices must be tested as HDMI referer. If the devices do not support HDCP, they can be tested as Referer or as independent sources and receivers. If the radio device is an CEC root device, the physical address test can be abandoned.
　Summary 
As mentioned above, hdmict is a fundamental and important step to ensure that the product complies with the requirements of HDMI specifications. It also greatly helps improve the interoperability of various products from different manufacturers. Providing solutions for the most common failures of hdmict not only provides high-quality HDMI interfaces to consumer electronics, but also helps them design highly compatible systems.