Two essential functions that customers look for in HD video systems are fast data processing and high-speed interface. Advances in data process technology are shrinking trace geometries and allowing for more performance and functionality to be included on ever-smaller integrated circuits (ICs). As for high-speed interface, High-Definition Multimedia Interface (HDMI) is the latest standard for devices that handle high-bandwidth, real time data, such as HDTV.
However, the higher the data speed and the more advanced the process technology are, the harder it is to protect the IC adequately from over-voltage stresses caused by ESD (electrostatic discharge) or CDE (cable discharge events). This is because the designated IC becomes more sensitive to the transient threats as the technology advances. As a result, HDMI designs often include additional transient protection to ensure the IC's reliability. Adding external transient protection for HDMI is difficult, however, due to HDMI's sensitivity to signal integrity. Each additional device added to the signal line implies extra parasitic elements that can affect signal transmission.
Introduction to HDMI
HDMI is an uncompressed, all-digital audio/video interface that provides a high-speed link between audio/video source devices such as DVD players, and sink devices such as HDTVs. Since its introduction in 2002, the HDMI standard has undergone various revisions to increase its capabilities. Currently, the most up-to-date HDMI specification is HDMI 1.3, which supersedes its predecessor HDMI 1.2 by doubling the bandwidth and increasing its speed from 1.65 Gbps per differential pair to 3.4 Gbps. HDMI 1.3 also includes unique, new features such as Deep Color, broader color space and the use of the mini connector. These added features enable HDMI 1.3 to present video images with even more color details and to be used with portable electronics.
HDMI compliance test specification
In order for a product to claim compliance to the HDMI standard, it must receive HDMI certification. HDMI certification requires that both the sink and the source device comply with their respective test requirements set forth by the HDMI Compliance Test Specification (CTS). Although HDMI CTS includes many types of tests, due to the fast signal data speed, it is crucial to meet the eye pattern requirement for the sink devices and the TDR requirement for the source devices. In order to meet the eye pattern requirement, the opening of the eye must have a minimum size as specified by the HDMI eye mask for the source device. To meet the TDR requirement, the differential impedance of the HDMI signal line pairs for the sink device must be 100 ohm ±15% with the rise time of the test pulse less than or equal to 200ps. Meeting these requirements is not a trivial task since the opening of the eye and the differential impedance can be easily affected by introduction of even a small amount of capacitance or inductance.
In addition to signal integrity requirements, the externally accessible HDMI ports are becoming more susceptible to transients threats. The threats can come from any charged entity; for example from either a user's direct touch or from hot plugging a charged cable. The internal on-chip ESD protection does not provide sufficient protection needed to keep the sensitive HDMI chip from becoming damaged. Thus, most consumer electronics manufacturers require HDMI ports to meet ESD standard of IEC 61000-4-2. To protect against the over-voltage stress induced by users and ESD testing, external protection is required. With this added concern, HDMI designers need to meet ESD immunity requirement of IEC61000-4-2 while maintaining signal integrity and impedance requirements per the HDMI CTS.
Next: Protecting HDMI from transient threats
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