Introduction to USB

USB (Universal Serial Bus) is an inter-system interface methodology used to establish communication between devices and a host controller (usually personal computers). USB has replaced many varieties of serial and parallel ports. USB has been using to connect computer peripherals such as mice, keyboards, digital cameras, printers, personal media players, flash drives, and external hard drives. For many of those devices, USB has become the standard connection method. USB was designed for personal computers, but it has become commonplace on other devices such as smart phones, PDAs and video game consoles, and as a power cord between a device and an AC adapter plugged into a wall plug for charging. Market surveys estimated that more than 6 billion USB 2.0 devices are sold in the last 3 years.

USB 3.0 is the next major revision of popular Universal Serial Bus. As of 2010, USB SuperSpeed (USB 3.0) superseded its popular previous version USB High Speed (USB 2.0). By end of 2012, USB 3.0 devices will replace all previous USB devices in the consumer electronics market.

Why USB 3.0? Isn’t USB 2.0 good enough?

For many applications, USB 2.0 provides sufficient bandwidth for a variety of devices and hubs to be connected to one host computer. However, with today’s ever increasing demands placed on data transfers with high-definition video content, terabyte storage devices, high megapixel count digital cameras, multi-gigabyte mobile phones and portable media players, 480Mbps (60 MB/s) is not really fast anymore. Furthermore, no USB 2.0 connection could ever come close to the 480 Mbps(60 MB/s) of theoretical maximum throughput. After protocol overhead and host system data bus availability, typical high speed USB operates at the actual real-world maximum speed around 240 Mbps (30 MB/s). Similarly, USB 3.0 connections will never achieve 4.8 Gbps (600 MB/s), but even half of that in practice is almost a 10 times improvement over USB 2.0.

USB 3.0 Features

  • USB 3.0 features SuperSpeed bus, which provides 4.8 Gbit/s data transfer, and USB 3.0 is expected to achieve 3.2 Gbit/s (0.4 GByte/s or 400 MByte/s) in practice.
  • USB 3.0 supports full duplex data transfer. Unlike previous versions where data can only be piped in one direction at a time, USB 3.0 can read and write data simultaneously. This is achieved by adding two new lanes dedicated to transmit SuperSpeed data and another pair for receiving it, bringing the total number of connections from four on USB 2.0 (power, ground and two for sending/receiving non-SuperSpeed data) to nine counting the 3.0 ground contact.
  • Cables and connectors are upgraded to support full duplex data transfer with USB 2.0 devices.
  • USB 3.0 SuperSpeed supports host-directed protocol.
  • USB 3.0 extends the bulk transfer type in SuperSpeed with Streams.
  • 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 150 mA (+50% over minimum using USB 2.0). An unconfigured device can still draw only 1 unit load, but a configured device can draw up to 6 unit loads (900 mA, an 80% increase over USB 2.0 at a registered maximum of 500 mA). Minimum device operating voltage is dropped from 4.4 V to 4 V.
  • 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.
  • Technology is similar to a single channel (1x) of PCI Express 2.0 (5-Gbit/s). It uses 8B/10B encoding, linear feedback shift register (LFSR) scrambling for data and spread spectrum. It forces receivers to use low frequency periodic signaling (LFPS), dynamic equalization, and training sequences to ensure fast signal locking.
  • USB 3.0 SuperSpeed bus is backward compatible to USB 1.0 (Low Speed USB), USB 1.1 (Full Speed USB), and USB 2.0 (High Speed USB).

SuperSpeed USB 3.0 Summary

  • USB 3.0 is fast. USB 3.0 supports a theoretical maximum of 4.8Gb/s where as USB 2.0 supports a theoretical maximum of 480Mb/s.
  • USB 3.0 is Host driven. USB 3.0 host controller drives the data transfer, while USB 2.0.
  • USB 3.0 is Bi-directional. Unlike previous versions where data can only be piped in one direction at a time, USB 3.0 can read and write data simultaneously.
  • USB 3.0 is more power Efficient. USB 3.0 supports increased maximum bus power and increased device current draw to better accommodate power-hungry devices
  • USB 3.0 is Backwards Compatible. USB 3.0 SuperSpeed bus is backward compatible to USB 1.0 (Low Speed USB), USB 1.1 (Full Speed USB), and USB 2.0 (High Speed USB).