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USB 3.0 Developers FAQ

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USB Developers FAQ









What is USB 3.0?

USB 3.0 defines a new, 10x faster SuperSpeed bus that operates parallel to the USB 2.0 bus.

Where can I find the specification?

On November 17, 2008, the USB Implementers Forum released the USB 3.0 specification. The current edition includes errata to the original release.


Does USB 3.0 replace USB 2.0?

No. Devices that don't support SuperSpeed should continue to comply with USB 2.0. SuperSpeed devices comply with USB 3.0 when operating at SuperSpeed and comply with USB 2.0 when operating at a lower speed.

The introduction of USB 3.0 thus differs from the change from USB 1.1 to USB 2.0. USB 2 replaced USB 1.1. When USB 2.0 was released, you could ignore the USB 1.1 specification and develop low, full, and high-speed USB 2.0 devices by following the USB 2.0 specification alone.

What devices will benefit from USB 3.0?

The first devices will likely be mass storage. A USB-IF device working group is developing a Mass Storage USB Attached SCSI Protocol (UASP) for efficient transfers at SuperSpeed. (The protocol will also improve efficiency at other USB speeds.) Video will likely be another application to benefit from USB 3.0.

How fast is USB 3.0?

USB 3.0 defines a new SuperSpeed bus with a bus speed of 5 Gbps, which is more than 10x faster than high-speed USB. After encoding and and other overhead, the bus can carry 400 MBps of data. Other features that can increase throughput include:

  • SuperSpeed traffic can travel in both directions at once (dual simplex).
  • Instead of having the host poll IN endpoints, devices initiate notifications when they have something to send and thus free the bus for other traffic.
  • Bulk transfers can use a streaming protocol for faster performance.

What stays the same?

These remain essentially unchanged:

  • Tiered star topology.
  • Four transfer types (control, bulk, interrupt, isochronous).
  • Descriptors. (Some fields are redefined for SuperSpeed, and SuperSpeed adds descriptors.)
  • Device classes.
  • Low, full, and high-speed protocols and cables.

What changes besides the new bus speed?

Other changes with USB 3.0 include:

  • Hubs route downstream traffic only to the receiving device rather than broadcasting.
  • No polling. When the host requests data from a SuperSpeed, non-isochronous endpoint that is is busy or has no data, the endpoint returns Not Ready (NRDY). The host then leaves the endpoint alone until the device sends an Endpoint Ready (ERDY) notification indicating that the endpoint has data to send.
  • New power-saving modes.
  • More bus current available to devices.


When will USB 3.0 host controllers be available?

In May, 2009 NEC Electronics announced the first USB 3.0 host controller, the µPD720200.

When will Windows support USB 3.0?

Support was added with Windows 8 and Windows Server 2012.

When will device controllers be available?

Device controller IP is available from several sources.


Will USB 1.x and USB 2.0 devices work with USB 3.0 hosts?

Yes. A USB 3.0 host has a SuperSpeed bus in parallel with a USB 2.0 bus. In the host, the xHCI controller handles all speeds, eliminating the need for companion host controllers.

Will USB 3.0 devices work with USB 2.0 or 1.x hosts?

Every SuperSpeed device must also support another speed but doesn't have to fully function at that speed. The device can either perform its function at the lower speed or inform the host that the device requires a USB 3.0 host. (The host can then inform the user.) A USB 3.0 Standard-A plug will fit a USB 2.0 Standard-A receptacle.

Will I need to replace host drivers?

A goal of the specification was to require no class changes for SuperSpeed. New drivers may be needed for isochronous transfers.

Can I use USB 2.0 cables with a SuperSpeed host or device?

Yes, but communications will use a slower speed. A USB 3.0 cable contains both SuperSpeed and USB 2.0 wires. USB 2.0 cables fit USB 3.0 receptacles and carry USB 2.0 traffic only.

What changes do I need to make to a USB 2.0 device to comply with USB 3.0?

USB 3.0 supplements, but doesn't replace, USB 2.0. Devices that don't support SuperSpeed can ignore USB 3.0 and just comply with USB 2.0.

Can a low, full, or high-speed device use USB 3.0's higher bus currents?

No. SuperSpeed devices should comply with USB 3.0 when operating at SuperSpeed and comply with USB 2.0 when operating at a lower speed. A high-power device that can operate at both SuperSpeed and high speed can draw 900 mA at SuperSpeed but only 500 mA at high speed.


What is the maximum cable length?

The specification doesn't defines performance standards but not a maximum length for USB 3.0 cables. In practical terms the limit is 3 m with AWG 26 wires.

What if I need to use a 5 m cable?

If the device is low, full, or high speed, you can use a 5 m USB 2.0 cable. At SuperSpeed, use a hub and two cables.

Can two SuperSpeed hosts connect directly to each other?

USB 3.0 defines a new crossover cable with a Standard A plug on each end. The cable does not include VBUS, D+, or D- wires. The SuperSpeed transmit and receive wires are cross-connected to route each output to its corresponding input. The specification says the cable is intended for OS debugging and other host-to-host applications. The cable won't hurt USB 2.0 hosts because the only line that connects on these hosts is GND. USB 2.0 does not have a legal host-to-host cable except for bridge cables that contain two device controllers with a common buffer.


How much bus power can devices draw?

When operating at SuperSpeed, high power devices can draw 900 mA, and low power devices can draw 150 mA. When operating at low, full, or high speed, the USB 2.0's limits apply: high power devices can draw 500 mA, and low power devices can draw 100 mA.

What other new power capabilities does USB 3.0 define?

A USB 3.0 device can have a Powered-B receptacle with two extra contacts that enable a device to provide up to 1A to a device such as a Wireless USB adapter. The adapter thus doesn't need its own power supply. In a wired connection to a host or hub, the extra contacts are unused.


Must USB 3.0 hubs support all speeds?

Yes. A USB 3.0 hub contains a SuperSpeed hub and a USB 2.0 hub that share power and ground lines. The hub enumerates as two devices, a SuperSpeed hub on the SuperSpeed bus and a USB 2.0 hub on the USB 2.0 bus.