Tuesday, April 07, 2009

USB3 Super Speed 5Gbps (10x faster than USB2.0

USB3.0 Logo with SuperSpeed

USB (universal serial bus) is one of the most sucessful device that can live up today because of its portablity and low cost. Recently, USB3 specification (Rev 1.0) had been released by www.usb.org in Nov 2008. It is a exciting news which I have been waiting, because I was working with USB design when I was working in Intel. This new USB3 transfer rate is 5Gbps, which is around 10x faster than USB2.0 (480Mbps), and it is dubbed with Super Speed to distinguish with High-Speed in USB2.0. In other words, bulky file sizes like video can be transferred in short period. Besides this, it is also backward compatible to USB2.0 (480Mbps High-Speed) and USB1.1 (12Mbps Full-Speed and 1.5Mbps Low-Speed). In other advantage is that it supports bidirectional transfer as shown in the USB3 stylish logo with two opposite arrows. It employs dual-simplex way to achieve bidirectional transfer.

Pat Gelsinger (Intel’s Senior Vice-President) displayed USB3.0 (source: apcmag.com)

USB3.0 using optical link

Initially USB3.0 used optical link which can easily achieve good signal quality. However, due to higher production cost, it was dropped off. It uses normal electrical cables with one UTP (unshielded twisted pair) and 2 sets of SDP (shielded differential pair).

USB3.0 cable

In order to support bidirectional transfer by dual-simplex structure, USB connector comprises 5 additional pins. They are Rx differential plus and minus pins, Tx differential plus and minus pins, and ground drain pins sitting at the middle of of Tx and Rx links as shown in the diagram below. This ground drain pin serve as current return path to Tx and Rx links and provides sheilding effect between Tx and Rx. Of course, due to backward compatible, it also contains the old version pins which are D+, D-, Power and Ground pins. They are 9 pins in total. (therefore backward compatible is kinda costly)

USB3.0 type A connector

Apart from that, USB3 no longer implements polling technique as in USB2.0. Polling techniques is like the USB host controller is always asking or checking for data at the down stream (mainly because USB is host initiative). It generates some traffic, therefore power consumption cannot be reduced althought in idle mode. USB3 uses asynchronous traffic flow resulting in power saving in idle mode.

In the physical layer, it uses 8B10B coding and AC coupling as in PCIe. Some people said USB3.0 just like a mini PCIe version. There is a clock and data recovery specification to be complied. Besides this, it requires to support spread spectrum clocking SSC due to EMI concerns.

As in the electrical signalling, Tx and Rx employ current-mode driver with deemphasis requirement. The Tx has two modes, they are full-swing mode and low-power mode. In the low-power mode, the Tx can allow to send low swing to reduce power consumption.

For more information, you could download the specification from www.usb.org

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