USB Speeds and Specifications

• USB 1.0: Released in January 1996.   

Specified data rates of 1.5 Mbit/s (Low-Speed) and 12 Mbit/s (Full-Speed). Does not allow for extension cables or pass-through monitors (due to timing and power limitations). Few such devices actually made it to market.

• USB 1.1: Released in September 1998. Fixed problems identified in 1.0, mostly relating to hubs. Earliest revision to be widely adopted.

USB 2.0

USB 2.0: Released in April 2000.

Added higher maximum speed of 480 Mbit/s (now called Hi-Speed). Further modifications to the USB specification have been done via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:

• Mini-B Connector ECN: Released in October 2000. Specifications for Mini-B plug and receptacle. These should not be confused with Micro-B plug and receptacle.

• Errata as of December 2000: Released in December 2000.

• Pull-up/Pull-down Resistors ECN: Released in May 2002.

• Errata as of May 2002: Released in May 2002.

• Interface Associations ECN: Released in May 2003. New standard descriptor was added that allows multiple interfaces to be associated with a single device function.

• Rounded Chamfer ECN: Released in October 2003.  A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.

• Unicode ECN: Released in February 2005. This ECN specifies that strings are encoded using UTF-16LE. USB 2.0 did specify that Unicode is to be used but it did not specify the encoding.

• Inter-Chip USB Supplement: Released in March 2006.

• On-The-Go Supplement 1.3: Released in December 2006

USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.

• Battery Charging Specification 1.0: Released in March 2007.

Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision which allows devices to temporarily draw 100 mA current after they have been attached. If a USB device is connected to dedicated charger, maximum current drawn by the device may be as high as 1.8A. (Note that this document is not distributed with USB 2.0 specification package.)

• Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.

• Link Power Management Addendum ECN: Released in July 2007.

This adds a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.

• High-Speed Inter-Chip USB Electrical Specification Revision 1.0: Released in September 2007.

On September 18, 2007, Pat Gelsinger demonstrated USB 3.0 at the Intel Developer Forum. The USB 3.0 Promoter Group announced on November 17, 2008, that version 1.0 of the specification has been completed and is transitioned to the USB Implementers Forum (USB-IF), the managing body of USB specifications. This move effectively opens the spec to hardware developers for implementation in future products.

• A new major feature is the SuperSpeed bus, which provides a fourth transfer mode at 5 Gbit/s. The raw throughput is 500 MByte/s, and the specification considers it reasonable to achieve 400 MByte/s or more after protocol overhead.

• USB 3.0 receptacles are compatible with USB 2.0 device plugs for the respective physical form factors. However, only USB 3.0 Standard-B receptacles can accept USB 3.0 Standard-B device plugs.

• When operating in SuperSpeed mode, dual-simplex signaling occurs over 2 differential pairs separate from the non-SuperSpeed differential pair. This results in USB 3.0 cables containing 2 wires for power and ground, 2 wires for non-SuperSpeed data, and 4 wires for SuperSpeed data.

• SuperSpeed establishes a communications pipe between the host and each device, in a host-directed protocol. In contrast, USB 2.0 broadcasts packet traffic to all devices.

• USB 3.0 extends the bulk transfer type in SuperSpeed with Streams. This extension allows a host and device to create and transfer multiple streams of data through a single bulk pipe.

• New power management features include support of idle, sleep and suspend states, as well as Link-, Device-, and Function-level power management.

• The bus power spec has been increased so that a unit load is 150mA (+50% over USB 2.0). An unconfigured device can still draw only 1 unit load, but a configured device can draw up to 6 unit loads (900mA, an 80% increase over USB 2.0). Minimum device operating voltage is dropped from 4.4V to 4V.

• USB 3.0 does not define cable assembly lengths, except that it can be of any length as long as it meets all the requirements defined in the specification. However, electronicdesign.com estimates cables will be limited to 3 m at SuperSpeed.

• Technology is similar to a single channel (1x) of PCI Express 2.0 (5-Gbit/s). It uses 8B10B encoding, linear feedback shift register (LFSR) scrambling for data, spread spectrum. It forces receivers to use low frequency periodic signaling (LFPS), dynamic equalization, and training sequences to ensure fast signal locking.

Consumer products are expected to become available in 2010. Commercial controllers are expected to enter into volume production no later than the first quarter of 2010.

Windows 7 drivers are under development but no public releases have been made available as of April 2009. The Linux Kernel supports USB 3.0 as of version 2.6.30.