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What's Good About USB 3.0? You Tell Me

Comments(2)Filed under: PCB design, USB 3.0, PHY

I read a recent article (June 11, 2009) in EDN magazine - "USB 3.0: A simple Idea Full of Challenges" by Ron Wilson.

In a nutshell, Ron says "Super-speed USB (Universal Serial Bus) 3.0 sounds like a great idea. Just start with widely used, fast, and bulletproof USB 2.0 and graft in the PHY (physical-layer) interface from another common and reliable standard, PCIe (peripheral-component-interconnect express) Generation 2. Put two differential pairs into the USB connector to carry the high-speed serial signals from the Generation 2 PHY, and you have a rugged, flexible, inexpensive interface that can operate at 5 Gbps over consumer-priced cables and connectors with interfaces cheap enough to drop into a flash drive."

"The idea promises to unleash new ways of using PCs with mobile devices and with storage. With application-level throughput approaching 400 Mbytes/sec and the ability to simply plug anything from a flash drive to 3m of USB cable into a host, usrs could link PCs and netbooks, quickly dump the contents of huge flash drives, or easily transfer HD (high-definition) video between devices. They could even create their own external storage networks (Figure 1). This promise of speed and flexibility, however, carries the seeds of a difficult challenge for chip, board, and system designers."

Some of the challenges encountered are:

  • PCB material used and real estate needed on the PCB for connector pairs and routes.
  • With speeds of 5 Gbps, cabling certification becomes an important consideration (and increases the manufacturing costs).
  • The PHY (physical-layer) is quite variable due to the types of devices (e.g. thumb drive, cable, etc.) that are plugged into the port.
  • The interface must be low enough in power to allow cable-powered operation. "The USB 3.0 standard will allow a device to draw as much as 900 mA during operation, but it must draw no more than 150 mA before configuration. That limitation itself demands a well-studied power-management strategy at the chip level."

As for the timeframe to market "USB 3.0 will start out expensive, but, by 2011, it will probably be standard in netbook computers and handheld consumer products, such as cameras, media players, and flash drives."

"One of the greatest differences between PCIe Gen 2 PHYs and USB 3.0 PHYs will be in the receiver-equalization circuit. Many designers expect the quality of this block to be a major differentiator in the market, for PHY-IP (intellectual-property), chips, and the systems that use them. The equalizer must be both powerful in its action and adaptive. Otherwise, a PHY would be unable to handle the range of channel conditions that USB can throw at it. As an adaptive equalizer, this circuit will require a training sequence to lock onto. Yet, the equalizer must be low in power and compact to meet the needs of consumer-product applications. Those challenges are formidable."

"If a design team has really understood PCIe Gen 2, then they can adapt about 90% of their work to USB 3.0, says Scott Kim, manager of business development at Texas Instruments. Yes, the equalizer will need beefing up, but much of the circuitry will remain the same. For example, getting 5-Gbps performance from the PHY requires a careful trade-off between deep pipelining to meet throughput requirements and limited latency to meet bus timing."

I'm curious about any experience you have with designing around the USB 3.0 protocol and what Cadence products/flows you're using to get past the design challenges.

Jerry "GenPart" Grzenia

Comments(2)

By Bob Moodley on July 15, 2009
Should be great once the power and equalizer issues are resolved but I never did like the crude nature of the usb port. USB 3.0 should have an  optical data link akin to the spdif ports on DVD players and perhaps only the power supplied via electrical contacts if there isn't a practical way of avoiding it. This should improve reliabilty and fault tolerance in addition to being more' 21st century'.

By Jerry GenPart on July 15, 2009
Hi Bob,
Thanks for the comments! Perhaps your design technique should become a USB 3.1 protocol - kidding, but not really. I wonder how difficult adjusting/tuning the implementation of the protocol is from a standards committee perspective? How can a designer propose improvements?

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