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SuperSpeed USB: A USB 3.0 Update

Wed, 09/02/2009 - 7:35am
Jon Titus, Senior Technical Editor

Jon TitusSuperSpeed USB connections — coming in 2010 — will zip data between devices at a theoretical rate of 5 Gbps, or more than 10-fold faster than a high-speed USB 2.0 connection. The faster transfers could help embedded-system developers who design video kiosks, TV recorders/players, and test-and-measurement equipment that use large quantities of information.

To find out more about SuperSpeed USB, I talked with Jeff Ravencraft, president and chairman of the USB Implementers Forum (USB-IF) and Rahman Ismail, the chief technology officer for the USB-IF (www.usb.org). The communication specification is formally known as SuperSpeed USB 3.0

"SuperSpeed USB adds two pairs of wires to the USB cable to provide dedicated receive and transmit lanes," noted Ravencraft. "Because a device can transmit and receive simultaneously, you can communicate faster and save power."

"Instead of broadcasting a message to all devices, a SuperSpeed USB host establishes a 'pipe' to communicate with a selected device," added Ravencraft. "The older technique of polling each USB device took extra time and used more power. The new interrupt-driven architecture first polls each device to 'ask' if it needs service. A device might reply that it has no traffic, will enter a sleep mode, and will alert the host when it needs attention."

Implementing SuperSpeed USB will call for some design changes due to the high bit-rate connections and the variability of the connections. The conditions facing designers who use a PCI Express serializer/deserializer (serdes) on a carefully designed PC motherboard pale by comparison to the variety of devices, cables, and connector configurations that developers of SuperSpeed USB equipment encounter. For a discussion of these design problems, see For further reading.

SuperSpeed USB hardware maintains backward compatibility. A SuperSpeed USB cable, for example, contains the original four USB wires along with five new wires — a shield and the two pairs noted earlier. "When you have a USB 2.0 device and either an older USB or a new SuperSpeed USB cable, you can connect the device to a SuperSpeed USB host," explained Ravencraft. "The USB 2.0 device still works but at a USB 2.0 data rate. If you want to communicate between SuperSpeed USB equipment, you must use a SuperSpeed USB cable."

USB software deserves attention, too. "We maintain backward compatibility with existing USB drivers," said Ismail. "So developers can use the same drivers they have now within the SuperSpeed USB framework. The USB-IF has a solid set of class drivers for operating systems such as Windows, Linux, and to some extent Mac OS. We want developers to continue to use and build upon what they already have rather than rewrite drivers and code just for SuperSpeed USB.”

Designing a SuperSpeed USB device or host gets you part way to the goal. Now you must put your product through a certification process. "Typically we start with certification in the Peripheral Interoperability Laboratory, or PIL, that we recently opened," noted Ismail. "Companies can schedule lab time to test their designs and find problems early on. In 2010 we'll move from the PIL to USB-IF workshops where engineers can test designs and products. Toward the end of 2010 we'll recognize certain test labs and transfer authority to them to certify products." When products pass the certification requirements, companies can identify them with the SuperSpeed USB logo. The certification process encompasses everything from silicon, to cables and connectors, to products and IP blocks.

But how do you get an intellectual property (IP) block through the certification lab? "We expect a company that licenses SuperSpeed USB IP to implement it, maybe in an FPGA or complete prototype," explained Ravencraft. "Then we can perform link-level, electrical, protocol, and framework tests."

"We have host hardware and device hardware in the PIL," said Ismail. "We have prototype software stack and we have our test suites. When engineers bring in a device, we test it against the hosts, and when they bring a host, we test it against the lab's devices. Our 'gold tree' will specify certified commercial hosts, and devices engineers can run tests against."

"One USB-IF goal involves having a peripheral developer kit, PDK, that includes a known-good host," said Ravencraft. "USB-IF members can buy the PDK and then focus on building a great device and not have to simultaneously design a host to test it against."

For further reading

Axelson, Jan, "USB Complete: The Developer's Guide," 4th ed., Lakeview Research, 2009. www.amazon.com.

Wilson, Ron, "USB 3.0: A Simple Idea Full of Challenges," EDN, June 11, 2009, pp. 16--19. www.edn.com/article/CA6662625.html

--, "SMSC USB 3.0 Position Paper," SMSC. www.smsc.com/main/catalog/usb30.pdf

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