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Maximizing multi-vendor compatibility

Posted: 17 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:HDMI? video? consumer electronics? home entertainment connection?

By Del Jones and Hisaki Fujiwara
Analog Devices Inc.

While the public may not be familiar with Consumer Electronic Control (CEC), many may have heard of some of the leading consumer electronics industry makers' brand names related to it. Among them are Sony's "Bravia Link," Panasonic's "Viera Link" and Sharp's "Aquos Link." Their names may differ, but all support the CEC functionality described in the HDMI standard.

CEC basics
CEC is a single-wire, bidirectional interface intended to facilitate the control of any device on an HDMI network, as seen in Figure 1, with the remote control unit or on-device control buttons of any other device connected to the network. Defined as an optional feature in the HDMI specification, it is based on the AV Link function defined in the European Syndicat des Constructeurs d'Appareils Radiorcepteurs et Tlviseurs (SCART) specification.

Many of these end-user features require sending multiple messages over the CEC bus such as "active source" and "routing change," which support the CEC feature "routing control." This allows a device to play and become the active source by switching the TV's source input. If the TV is displaying another source at the time this command is used, it may place the other source into "stand-by" mode, depending on the implementation.

Physical structure
The CEC interface consists of a single-wire bus that connects all devices in an HDMI-enabled system. Figure 2 illustrates a typical application. Any device in a CEC-enabled system can initiate a CEC command. The initiator sends the message structure and data on the common wire. Each device on the network must set acknowledge (ACK) bits when they receive a CEC message.

CEC messages are sent in frames that include a start bit and data bits. Data bits can be informational (like logical addresses and CEC commands), or they can be control bits (end-of-message [EOM] or ACK).

A high-to-low transition followed by a low-to-high transition that adheres to the timing indicates a start condition. The data that follows the start condition must adhere to the timing requirements for logic '0' and logic '1'

CEC frames are made of data blocks that consist of 10bits each. Eight bits carry information, while the last two are the EOM bit and ACK bit. In the header block, the information bits contain the initiator's logical address (4 most significant bits) and the destination logical address (4 least significant bits). A logic '0' in the EOM bit position indicates more data follows. A logic '1' indicates the end of the message.

A single CEC message is made of a start command, a CEC header and one or more data blocks. A CEC feature is constructed from multiple CEC messages.

CEC interoperability is a concern that should be at the forefront of any system designer's mind from the beginning of the design project to the end. Three levels of testing can help ensure compatibility with other CEC-enabled devices.

HDMI-compliance testing is the first type of testing that needs to be completed. The HDMI Compliance Test Specification (CTS) is a supplement to the HDMI specification that provides detailed procedures for testing a device and must pass before becoming HDMI-certified. Several CTS tests are dedicated to the testing of CEC. Any device that supports CEC must pass these tests as part of the HDMI compliance test to be HDMI-certified. The second type of testing made available to CEC-enabled devices is the CEA 861/HDCP PlugFest.

Figure 1: Shown is a typical all-HDMI home theater.

Compatibility issues
The adoption rate of CEC ramps up, and so are concerns about CEC interoperability. The main issues behind CEC compatibility problems are the use of proprietary commands and differing interpretations of the CEC specification.

While the definition of many basic CEC features is clear, flexibility that allows custom CEC features and messages is part of the specification. Each vendor defines its own vendor specific messages. These messages are valid only when the same vendor produces both the message sender and the receiver. This leads to interoperability problems, since these messages are typically proprietary.

Due to backlash from the distribution channel and end users, there is increasing pressure on CEC equipment vendors to resolve these interoperability issues. System designers should undergo designing and testing against the CEC compliance test specification. This should always be the priority for all CEC-enabled equipment makers to start before claiming to be HDMI-compliant with their CEC- enabled system.

A rigorous interoperability testing with multiple brands and models of CEC-enabled equipment must be provided. Designers and CEC solutions suppliers can use this testing not only to improve the CEC-enabled devices, but also can help increase the system's ability to tolerate the variances in "real-world" environments.

Figure 2: This is the CEC interface connecting all devices in an HDMI system diagram.

Hardware networking
The physical connection to the HDMI network is straightforward. Per the CEC specification, the CEC pin of the HDMI connector should be pulled up to a 3.3V supply via 27K? resistor. However, the pull-up should be disconnected when the device (CE equipment) is powered off.

The recommendation for the PCB layout is to keep the distance between the HDMI and CEC components and the HDMI connector to a minimum. It is also recommended that the routing of noise-emitting digital circuitry and the signal routing to the HDMI connector be separated. Remember that the differential HDMI signals should be 100? differential impedance ?15?.

An HDMI device with an integrated CEC controller, such as ADV7520NK, reduces the barrier to entry for the system designer. Meanwhile, the enhanced PHY used in CEC hardware automatically generates the required low-to-high and high-to-low signal timing that is required to send CEC messages and measures the low pulse time and high pulse time to receive CEC messages. This automated bit signaling function eliminates the real-time response requirement and allows a system microprocessor to operate CEC in a polling mode against an interrupt-driven mode eliminating the need for a dedicated CEC MCU.

Implementation differences in a board or system can lead to various timing delays and capacitive loading conditions that can affect the timing of the CEC interface. An integrated CEC interface must have the ability to adjust the minimum and maximum pulse time to accommodate a variety of board and system designs. This flexibility provides a high level of tolerance to CEC-enabled products that may not be perfectly compliant to the HDMI specification.

Computerized response
The integrated CEC interface should also include an automated message retransmission mode. This allows the system designer to set the number of times the CEC hardware will try to retransmit each outbound message.

If the CEC hardware detects an unexpected condition while sending a message, it should automatically terminate the session and restart the same message from the beginning. If the condition persists, the CEC hardware will continue retransmission up to the limit set by the designer. This retransmission system should be automated and will not require the intervention of the system's MCU.

Reduced power
Rising concerns about energy conservation and an increase in the prevalence of battery-powered video devices drive increasing focus on low power use. A smartly designed integrated CEC solution can help to reduce system power consumption through a multilevel power down mode.

About the authors
Del Jones
is an applications engineer manager and Hisaki Fujiwara is a senior applications engineer at Analog Devices Inc.'s Advanced TV Segment.

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