Last week, I attended an all-day Agilent seminar on a variety of interface standards including USB 3.0. As it turns out, there was a lot I didn’t know about USB 3.0. Perhaps you did not know some of these things either.
The USB 3.0 portion of the Agilent seminar was given by Russ McHugh, an Agilent Applications Engineer based here in Silicon Valley. Agilent provides measurement and analysis tools for a variety of interfaces including SuperSpeed USB 3.0.
McHugh started off with the general: USB (all forms) now has an installed base of something north of 10 billion units with a growth rate of approximately 3 billion units per year. That’s a lot of units. SuperSpeed USB 3.0 is a relatively new upgrade to the USB standard, so it’s just now starting to appear in products. In fact, the USB 3.0 specification is so new that the compliance testing spec (version 1.0) wasn’t quite out in time for the latest plug fest. Hopefully, it will be in time for the next one.
McHugh listed some of the salient characteristics of USB 2 and then contrasted these characteristics with USB 3.0:
- NRZI coding, half duplex
- 4 signal lines in the connector (Dp, Dn, Vcc, Gnd)
- 5M max cable length
- 500mA power available
SuperSpeed USB 3.0
- Superset of USB 2
- 8B/10B coding, full simplex transmission
- 8 signal lines (four USB 2 lines, two USB 3.0 differential signaling pairs)
- 3M max cable length
- 900mA power available
- Spread-spectrum clocking option
USB 3.0 ports can support the simultaneous operation of the USB 3.0 differential signaling pairs and the USB 2 signaling, so there actually can be two independent signaling standards operating simultaneously over one cable. Therefore, for full design analysis you need to look at both the USB 2 and USB 3.0 channels simultaneously.
One other interesting facet McHugh discussed was the availability of spread-spectrum clocking for the 5Gbps USB 3.0 differential signaling pairs. If it’s implemented, the clock modulation needs to take the form of a triangle wave with a constrained periodicity. McHugh showed some captured data from Agilent USB 3.0 test equipment that did not really look very triangular and even included some non-monotonic sections in the modulation waveform, reflecting the relative unfamiliarity with all aspects of the new USB 3.0 standard said McHugh.
Note: If you’re designing a USB 3.0 interface into your next SoC, Cadence offers USB 3.0 verification IP.