An introduction to USB

In 1994, an alliance of four industrial partners (Compaq, Intel, Microsoft, and NEC) started specifying the USB protocol. The original goal of the protocol was to connect the PC to the telephone and to provide I/O interfaces that were easy to expand and reconfigure. In January 1996, the first version of the USB specification was released, and a subsequent revision (version 1.1) was released in September 1998. The specification allowed 127 devices to be connected together at the same time, with the total communication bandwidth limited to 12 Mbps. Later on, three more members (Hewlett-Packard, Lucent, and Philips) joined the alliance. In April 2000, version 2.0 of the USB specification, which supports transfer rates up to 480 Mbps, was released. Today, USB plays a key role in high-speed (video, imaging, storage) and full-speed (audio, broadband, microphone) data-transfer applications. It also connects a variety of low-speed devices (keyboards, mice, game peripherals, virtual reality peripherals) to the PC.

The USB protocol is strictly hierarchical. In any USB system there is only a single host, and the USB interface to the host computer is referred to as the host controller. There are two standards for host controllers -- the Open Host Controller Interface (OHCI, by Compaq) and the Universal Host Controller Interface (UHCI, by Intel). Both standards provide the same capabilities and work with all USB devices; the hardware implementation of a UHCI is simpler, but requires a more complex device driver (and thus puts more load onto the CPU).

The USB physical interconnect is a tiered star topology, with up to seven tiers. A hub is at the center of each star, and the USB host is considered the root hub. Each wired segment is a point-to-point connection between a hub and USB device; the latter can be either another hub that provides additional attachment points to the system, or a device of some sort that provides functional capabilities. The host uses a master/subordinate protocol to communicate with the USB devices. This approach solves the problem of packet collision but also prevents the attached devices from establishing direct communication with each other.

All the data transfers are initiated by the host controller. Data directed from the host to a device is called downstream or out transfer; data directed from a device to the host is called upstream or in transfer. Data transfer occurs between the host and a particular endpoint on the USB device, and the data link between the host and the endpoint is called a pipe. A given USB device may have many endpoints, and the number of data pipes between the host and the device is the same as the number of endpoints on the device. A pipe may be uni-directional or bi-directional, and the data flow in one pipe is independent of the data flow in any other pipes.

Communication on the USB network can use any one of four different data transfer types:

* Control transfers: These are short data packets for device control and configuration, particularly at attach time.
* Bulk transfers: These are data packets in relatively large quantities. Devices like scanners or SCSI adapters use this transfer type.
* Interrupt transfers: These are data packets that are polled periodically. The host controller will automatically post an interrupt at a specified interval.
* Isochronous transfers: These are data streams in real time with higher requirements for bandwidth than for reliability. Audio and video devices generally use this transfer type.

Like a serial port, each USB port on a computer is assigned a unique identification number (port ID) by the USB controller. When a USB device is attached to a USB port, this unique port ID is assigned to the device and the device descriptor is read by the USB controller The device descriptor includes information that applies globally to the device, as well as information on the configuration of the device. A configuration defines the functionality and I/O behavior of a USB device. A USB device may have one or more configurations, which are described by their corresponding configuration descriptors. Each configuration has one or more interfaces, which can be considered as a physical communication channel; each interface has zero or more endpoints, which can be either data providers or data consumers, or both. Interfaces are described by interface descriptors, and endpoints are described by end-point descriptors. Furthermore, a USB device might also have string descriptors to provide additional information such as vendor name, device name, or serial numbers.

Post Source

http://www.ibm.com/developerworks/linux/library/j-usb.html