LYCOS RETRIEVER 
Universal Serial Bus: Devices
built 659 days ago
The device integrates the required USB transceiver with a 3.3V regulator, a Serial Interface Engine (SIE), USB end-point (EP) FIFOs, a versatile 8-bit parallel interface, a clock generator and a MICROWIRE/PLUS� interface. Seven endpoint pipes are supported: one for the mandatory control endpoint and six to support interrupt, bulk and isochronous endpoints. Each endpoint pipe has a dedicated FIFO, 8 bytes for the control endpoint and 64 bytes for the other endpoints. The 8-bit parallel interface supports multiplexed and non-multiplexed style CPU address/data buses. A programmable interrupt output scheme allows device configuration for different interrupt signaling requirements.
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As illustrated in the diagram below, a USB system consists of a host side, a device side, and the physical bus represented by the USB cable. The main responsibility of the host is to provide control interface for data transfers whereas the client provides the end user with various functions. The host side itself consists of a USB adapter or USB Host Controller hardware layer and an upper-level System Software layer. The host controller is responsible for transferring streams of data between the host and the USB devices. The host controller provides services based on parameters provided by the host software when the configuration request is made.
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There are several hardware requirements for devices that are placed on the USB bus. Five volts is the nominal supply voltage on the bus. A device that requires 100mA or less can be powered from the host or any hub, provided that the total available power hasn't already been exhausted by other devices. A device on the bus can draw up to 500mA from it. However, not all USB hosts (especially a battery powered PC) or bus-powered hubs will allow a device to draw more than 100mA from the bus. For this reason, a USB device that draws more than 100mA should, in most cases, be self-powered .
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USB is a data communication standard for a peripheral bus that was developed by Intel and Microsoft. It is a replacement for the plethora of connectors and ports on the back of desktop PCs and provides an interface for computer-telephony devices. Instead of having separate connections for keyboard, mouse, printer, modem, joystick, audio devices, CD-ROMs, digital cameras, and other devices, USB provides a single port for connecting all of the devices. USB eliminates many of the problems associated with the PC, such as the need to open the computer to install adapter cards, change dip switches, and configure IRQs (interrupt requests).
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When a USB device is attached to the bus, an initialization or enumeration process begins during which descriptor requests are made by the host computer. First a Device Descriptor request is sent to determine general device information. A Configuration Descriptor request then follows which returns configuration, interface, and lower level descriptors in the proper order.
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Eventually, 1394 will replace the serial, parallel, and SCSI ports on the back of a computer, but what happens to USB? 1394 and USB ports will both coexist on the computers of the future. Devices which do not require a high speed connection (such as keyboards, mice, keyboards, mice, monitors, joysticks, low-resolution digital cameras, low-Speed CD-ROM drives, and modems) will use USB ports. Devices that do require a high speed connection (such as hard drives, DVD-ROM drives, set-top boxes, digital audio and video, digital cameras, printers, and scanners) will use 1394 ports.
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