Analyzer/exerciser validates 5-Gbps SuperSpeed USB 3.0
By Dan Strassberg, Contributing Technical Editor -- EDN, August 28, 2008
Although USB (Universal Serial Bus) 3.0 with its 5-Gbps SuperSpeed technology probably won’t reach the market in volume until mid-2010, manufacturers are now developing computers and peripheral devices employing the new bus. To work on these products and their all-important supporting software—especially drivers—engineers need tools for exercising developmental devices and software, analyzing bus performance, ensuring compatibility with earlier versions of USB, and debugging flaws. According to LeCroy Corp, the needed tools weren’t commercially available until its August introduction at the Intel Developers’ Forum, in San Francisco of the Voyager validation system, which LeCroy describes as the first protocol analyzer/exerciser for USB 3.0. Using custom front-end circuits, which the company developed for its 5-Gbps PCIe (peripheral-component-interconnect-express) analyzer/exerciser, the instrument performs simultaneous protocol capture of USB 2.0 and 3.0 signaling. An integrated exerciser option enables the sixth-generation unit to completely test and validate the performance of USB devices, systems, and software.
Engineers often refer to USB 3.0’s SuperSpeed mode as more than 10 times as fast as USB 2.0’s high-speed mode. That characterization may or may not be valid. SuperSpeed operation uses 8-bit/10-bit coding, which reserves 20% of the transmitted bits for error correction, thus allowing the 5-Gbps data stream to transmit a maximum of 4 Gbps of real data. The USB 2.0 high-speed mode, which does not incorporate error correction, can transmit bursts of data at 480 Mbps, or 12% of 4 Gbps. In sustained operation, however, the maximum high-speed-mode data rate is closer to 300 Mbps. Because the SuperSpeed protocol may prove more efficient than the high-speed protocol, USB 3.0 may more than compensate for its error-correction overhead.
Another feature of USB 3.0 is adaptive equalization, which optimizes a link’s output waveshape in response to training sequences that run automatically when you establish a new connection. Unlike buses that mainly find use in backplanes, USB is primarily a cabled bus. Because such buses use cables of unpredictable length and because waveform distortion depends strongly on path length, adaptive equalization is essential to achieving the advertised data rate.
LeCroy regards the situation with USB 3.0 connectors as potentially still fluid. Whereas manufacturers have designed and prototyped connector types that are close to receiving approval from the USB Implementers’ Forum (www.usb.org), modified or added types are still possible. Therefore, the company designed the Voyager system to make it easy for users to change the bus connectors. Users can substitute SMA connectors for USB connectors if the “real” connectors are temporarily unobtainable.
USB 3.0 will be backward-compatible with USB 2.0 but in a different way than USB 2.0 is backward-compatible with USB 1.1. USB 2.0 cables mate and work with USB 1.1 connectors. USB 3.0 devices will have connectors that mate with both USB 2.0 and USB 3.0 cables, but, if you want to use a USB 3.0 device in a system based on USB 2.0, you will need a USB 2.0 cable, which may be an optional accessory to the USB 3.0 device. According to LeCroy, testing of USB hubs will make it nearly essential that USB 3.0 analyzer/exercisers support simultaneous USB 2.0 and 3.0 data capture.
When you research USB 3.0, you are likely to read about implementations that use fiber-optic instead of copper conductors. LeCroy says that such implementations are indeed likely in the future but that initial implementations will use copper cables. US prices for the Voyager system begin in the $20,000 to $30,000 range.
