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Understanding design implications of USB Type-C

Posted: 29 May 2015     Print Version  Bookmark and Share

Keywords:USB  USB Type-C  configuration channel  biphase mark coding  downstream facing port 

To do this, USB Type-C defines a mechanism to allow the power delivery budget to be negotiated and the role of the power delivery to be defined and switched. This new mechanism is to communicate the control messages defined by the power delivery standard over the CC pin. The communication requires a special biphase mark coding (BMC) encoding, and that requires special circuitry and logic to handle the protocol (comparable to differential Manchester coding with 10Mbps Ethernet).

"If you want to claim your USB solution natively supports the Type-C connector, from the IP support perspective, you need the logic to support detection of cable orientation on the CC pin, the logic to support bus routing selection accordingly, and the logic to support Type-C power advertisement and BMC-based power delivery messages over the CC pin," Qi said.

Utilitarian cabling possibilities
The standard also defines an alternate mode for other functions to be negotiated over the cable, such as audio and display (Voila! Your two smart phones ports—USB + audio—now just became one). It also has the capability to handle debugging duties. Many alternate-mode functions are defined by the liaison work between other standard bodies and USB-IF. The vendor-defined message (specified in USB power delivery) over the CC pin is used to discover and establish the alternate mode function over Type-C cable.

Given the opportunity and the new standard, designers will be busily crafting logic to handle this for the time being, as USB Type-C is yet to be natively integrated into SoCs. Most are designing either an external controller or discrete component to handle the USB Type-C support functionality with a vanilla USB host port, according to Qi. But in the new future, commercial IP with Type-C capability will be available. And multi-protocol IP might be particularly valued in designs by designers conscious of their device form factor.

"From an IP standpoint, the implication of Type-C is it's no longer just USB," Qi said. "There's a single cable that has to serve multiple functions on the customer's end device. Whatever function the customer wants to pass through that's the only cable they're going to get. They have to think about power through the cable, other functionalities that might come through, whether it's audio, debug, high-resolution display. These are the issues they're going to think through."

About the author
Brian Fuller, a former editor-in-chief of EE Times, is now editor-in-chief with Cadence in San Jose.

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