give me some details of USB.?

Question

what are its functions?

Answer 1

USB.port on your computer USB.device plugs into usb port
cam/// printer joystick and so on

Answer 2

USB is Universal Serial Bus. It is a evalotion in computer technolody. USB 2.0 provides 12 MBPS speed. A single port can attached at same time to 2000 devices ! It have quick connect and Plug And Play support. No need of install drivers. Many devices like printer, scanner, digicam, handycam, cell phones. USB Disk Drives, Pen drives, Ipods, MP3 players, and so on . To get more information about USB click on following link:
http://en.wikipedia.org/wiki/USB

Answer 3

USB universal serial bus connects several devices to a computer. It is an interface, advanced interface through which many devices can communicates with the computer

Answer 4

USB 2.0 max data transfer speed is 480Mbps and can supply up to 500mAmps

Answer 5

I think bacti has eloquently described USB and all you need to know about it.

Answer 6

Universal Serial Bus (USB) is a serial bus standard to interface devices. It was originally designed for computers, but its popularity has prompted it to also become commonplace on video game consoles, PDAs, portable DVD and media players, cellphones; and even devices such as televisions, home stereo equipment (e.g., digital audio players), car stereos and portable memory devices.

USB connects several devices to a host controller through a chain of hubs. In USB terminology devices are referred to as functions, because each individual physical device may actually host several functions, such as a router that is a Secure Digital Card reader at the same time. The hubs are special purpose devices that are not officially considered functions. There always exists one hub known as the root hub, which is attached directly to the host controller.

These devices/functions (and hubs) have associated pipes (logical channels). The pipes are synonymous to byte streams such as in the pipelines of Unix. Pipes are connections from the host controller to a logical entity on the device named an endpoint The term endpoint is also occasionally used to refer to the entire pipe.

These endpoints (and their respective pipes) are numbered 0-15 in each direction, so a device/function can have up to 32 active pipes, 16 into the host controller and 16 out of the controller.

Each endpoint can transfer data in one direction only, either into or out of the device/function, so each pipe is uni-directional. Endpoint 0 is however reserved for the bus management in both directions and thus takes up two of the 32 endpoints — all USB devices are required to implement endpoint 0, so there is always an inward and an outward pipe numbered 0 on any given device.

In these pipes, data is transferred in packets of varying length. Each pipe has a maximum packet length, typically 2n bytes, so a USB packet will often contain something on the order of 8, 16, 32, 64, 128, 256 up to 512 bytes.

The pipes are also divided into four different categories by way of their transfer type:

* control transfers - typically used for short, simple commands to the device, and a status response, used e.g. by the bus control pipe number 0
* isochronous transfers - at some guaranteed speed (often but not necessarily as fast as possible) but with possible data loss, e.g. realtime audio or video
* interrupt transfers - devices that need guaranteed quick responses (bounded latency), e.g. pointing devices and keyboards
* bulk transfers - large sporadic transfers using all remaining available bandwidth (but with no guarantees on bandwidth or latency), e.g. file transfers

When a device (function) or hub is attached to the host controller through any hub on the bus, it is given a unique 7 bit address on the bus by the host controller.
USB Enumeration Trace
USB Enumeration Trace

The host controller then polls the bus for traffic, usually in a round-robin fashion, so no device can transfer any data on the bus without explicit request from the host controller. The interrupt transfers on corresponding endpoints do not actually interrupt any traffic on the bus: they are just scheduled to be queried more often and in between any other large transfers, thus "interrupt traffic" on a USB bus is really only high-priority traffic.
USB device descriptors are hierarchical and quite complex. This UML diagram tries to give an entity relation between the different descriptors: the lower left device descriptor is highest in the hierarchy, this has configuration descriptors, which have interface descriptors, which have interface settings which in turn hold the actual endpoints.
USB device descriptors are hierarchical and quite complex. This UML diagram tries to give an entity relation between the different descriptors: the lower left device descriptor is highest in the hierarchy, this has configuration descriptors, which have interface descriptors, which have interface settings which in turn hold the actual endpoints.

To access an endpoint, a hierarchical configuration must be obtained. The device connected to the bus has one (and only one) device descriptor which in turn has one or more configuration descriptors. These configurations often correspond to states, e.g. active vs. low power mode. Each configuration descriptor in turn has one or more interface descriptors, which describe certain aspects of the device, so that it may be used for different purposes: for example, a camera may have both audio and video interfaces. These interface descriptors in turn have one default interface setting and possibly more alternate interface settings which in turn have endpoint descriptors, as outlined above. An endpoint may however be reused among several interfaces and alternate interface settings.

[edit] Host controllers

The hardware that contains the host controller and the root hub has an interface geared toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer. In practice, these are hardware registers (ports) in the computer.

At version 1.0 and 1.1 there were two competing HCD implementations. Compaq's Open Host Controller Interface (OHCI) was adopted as the standard by the USB-IF. However, Intel subsequently created a specification they called the Universal Host Controller Interface (UHCI) and insisted other implementers pay to license and implement UHCI. VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. The main difference between OHCI and UHCI is the fact that UHCI is more software-driven than OHCI is, making UHCI slightly more processor-intensive but cheaper to implement (excluding the license fees). The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations which increased cost. During the design phase of USB 2.0 the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Extended Host Controller Interface (EHCI). Only EHCI can support hi-speed transfers. Each EHCI controller contains four virtual HCD implementations to support Full Speed and Low Speed devices. The virtual HCD on Intel and VIA EHCI controllers are UHCI. All other vendors use virtual OHCI controllers.

On Microsoft Windows platforms, one can tell whether a USB port is version 2.0 by opening the Device Manager and checking for the word "Enhanced" in its description; only USB 2.0 drivers will contain the word "Enhanced." On Linux systems, the lsusb -v command will list all USB devices, and USB controllers will be named OHCI, UHCI or EHCI respectively, which is also the case in the Mac OS X system profiler. On BSD systems, dmesg will show the detailed information hierarchy.

[edit] Device classes

Devices that attach to the bus can be full-custom devices requiring a full-custom device driver to be used, or may belong to a device class. These classes define an expected behavior in terms of device and interface descriptors so that the same device driver may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device.

Device classes are decided upon by the Device Working Group of the USB Implementers Forum. If the class is to be set for the entire device, the number is assigned to the bDeviceClass field of the device descriptor, and if it is to be set for a single interface on a device, it is assigned to the bInterfaceClass field of the interface descriptor. Both of these are a single byte each, so a maximum of 254 different device classes are possible (values 0x00 and 0xFF are reserved). If bDeviceClass is set to 0x00, the operating system will look at bInterfaceClass of each interface to determine the device class. Each class also optionally supports a SubClass and Protocol subdefinition. These can be used as the main device classes are continuously revised.
Mass Storage Capture
Mass Storage Capture

The most used device classes (grouped by assigned class ID) are:

0x00
Reserved value - used in the device descriptor to signify that the interface descriptor holds the device class identifier for each interface.
0x01
USB audio device class, sound card-like devices.
0x02
USB communications device class used for modems, network cards, ISDN connections, Fax.
0x03
USB human interface device class ("HID"), keyboards, mice, etc.
0x06
Still image capture device class, identical to the Picture Transfer Protocol as used across USB
0x07
USB printer device class, printer-like devices.
0x08
USB mass storage device class used for flash drives, portable hard drives, memory card readers, digital cameras, digital audio players etc. This device class presents the device as a block device (almost always used to store a file system).
0x09
USB hubs.
0x0E
USB video device class, webcam-like devices, motion image capture devices.
0xE0
Wireless controllers, for example Bluetooth dongles.
0xFF
Custom device class - used to establish that a device or interface does not support any standard device class and requires custom drivers.