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Introduction

As an “Instant, No Hassle Connection”, USB connects more than computers and peripherals. It has the power to connect you with a whole new world of PC experiences. Today, it has been widely applied to not only the PC and PC peripheral, but also embedded systems such as in consumer electronics, and the communications segment. The USB standards including USB 1.1, USB 2.0, USB On-The-Go, and the newest entrant, Wireless USB (WUSB) were released for applications in these various segments. In the specification of these standards, mechanical, electrical and protocol features are specified and detailed for the USB chip designer, but are not adequately addressed for the USB product or application designer. This paper provides an overview of all the USB standards. It is intended to help designers quickly see which USB standard is available for their design. It also provides circuit design guidelines to help designers implement their physical layer design with the current USB chips on the market.

USB Background

The serial RS-232 port and parallel port dominated the "wired" communications world for a number of years before USB was widely applied to the PC and PC peripheral. Today, USB has proliferated to the point that it is not easy to find a product with a serial port or parallel port on the market. More importantly, USB is continuing its success. In fact, it has been growing at a very fast rate – one that no one could have imagined ten years ago.

USB enjoys broad market acceptance today. One of the reasons is that there are a number of disadvantages for new technology when it comes to the RS-232 port and parallel port, Some of these disadvantages include a speed limitation (the RS-232 port is 115,000 bps and the parallel port is about 150 kbytes per second; neither of which is adequate for today's technology), the problematic size of the DB9/DB25 connector, and the inability of the power supply to support new technologies.

Another key reason for USB’s success is that it is very flexible and has been continuously improved and perfected. Developed to simplify PC peripheral connections, USB allows expandability of the PC's capabilities via an external port. It supports up to 127 physical devices and has an error handling/fault recovery mechanism that is built into the protocol. It supports not only a flexible data transfer rate from 1.5 Mb/s, 12 Mb/s to 480 Mb/s for different applications, but also flexible roles from host, device to dual-role device, and a flexible connection from wired mini-port, standard port connection to wireless connection. Since USB 1.0 was released in January 1996, different s revisions of the USB standard have been released, the later versions are all backward compatible. These include:

· Universal Serial Bus Specification Revision 1.0, the first edition, was released in January 1996. It supported 1.5 Mb/s (low speed) and 12 Mb/s (full speed) transfer rates.

· Universal Serial Bus Specification Revision 1.1 was released in September 1998. This edition fixed many of the problems in release 1.0.

· Universal Serial Bus Specification Revision 2.0 was released on April 27, 2000 and increased the maximum transfer speed by a factor of 40 up to 480 Mb/s. USB 2.0 is backward compatible with USB 1.x. The USB 2.0 specification supersedes the USB 1.1 specification. These USB 1.x peripherals continue to operate with no change in Hi-Speed USB systems. The higher bandwidth of Hi-Speed USB permits PC peripherals with more functionality, including higher resolution video conferencing cameras, next-generation scanners and printers, fast storage units, and faster broadband Internet connections. It makes today's user applications more productive, such as taking the time to download a "roll" of digital photos from a few minutes on Original USB down to a few seconds on Hi-Speed USB.

· On-The-Go Supplement to the USB 2.0 Specification, Revision 1.0, released on Dec 18, 2001. Revision 1.2 of the USB On- The-Go Supplement was released on April 4, 2006. The On-The-Go Supplement addresses the need for mobile interconnectivity by allowing a USB peripheral to have the following enhancements: 1. Limited host capability to communicate with selected other USB peripherals, 2. A small USB connector to fit the mobile form factor. 3. Low power features to preserve battery life

· Wireless USB Specification Revision 1.0 was released on May 12, 2005. Certified Wireless USB from the USB-IF is the new wireless extension to USB that combines the speed and security of wired technology with the ease-of-use of wireless technology. It offers a solution for high bandwidth, low cost, low power consumption and physical size requirements of nextgeneration consumer electronic devices. Certified Wireless USB performance is targeted at 480 Mbps at 3 meters and 110 Mbps at 10 meters.

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