The HDMI standard has been broadly adopted since its release several years ago. In order to bear the HDMI logo, all HDMI products must pass the HDMI Compliance Test (HDMI CT). To save time and money, customers should pre-test their HDMI products before sending them to Authorized Test Centers (ATC) for certification. Based on the experiences of numerous pre-tests performed at Analog Devices' HDMI CT lab, this article discusses the most common system design issues that cause products to fail the HDMI CT. It also analyzes specific case failures and offers solutions.
ANALYSIS OF COMMON FAILURES OF HDMI CT:
Almost all devices, including TVs and DVD players, failed the test the first time. Most of the failures were system design and PCB layout related. Special features are sometimes required in order to pass the HDMI CT. For instance, HDCP (High Definition Content Protection) may need to be enabled or disabled in source devices. Both IC and system design and manufacturing companies must completely understand the HDMI specification and Compliance Test Specification (HDMI CTS) in order to design compliant HDMI products. Following are the most common failure items and recommendations for resolving the problems.
- EDID Test
The source devices are required to support "Enhanced DDC" specification. This means that the source needs to be able to read the EDID information after 256 bytes by using "segment pointer 0x60". Most of the time only 256 bytes are used for EDID, but HDMI CT requires checking that the system is able to read 4 blocks (128 bytes/block), for a total of 512 bytes. It is very important to meet this requirement for the segment pointer in order to pass the HDMI CT.
- 5V Power of HDMI Transmitter (Tx)
Some customers connected a resistor or diode in series to limit the current of the 5V output. In the HDMI CT, testing 5V power of HDMI Tx requires a 55mA current draw from the +5V power pin during the measurement. This may cause the test to fail, as the power output needs to be between 4.8V and 5.3V to pass. Figure 1 shows that the output drops to 4.45V when a 10-ohm resistor is connected, resulting in the failure of this test.
V = 5 " (100.055) = 4.45V < 4.8v="">
Fig 1 +5V Power
Output drops when a 10-ohm resistor is connected, resulting in test failure