Universal Serial Bus: Difference between revisions
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== Introduction ==
Despite how attractive USB support is, the 650-page USB 2.0 specification manages to deter even some of the most driven hobbyists (especially if English isn't their primary language). Not only is the USB 2.0 specification long, but it's a prerequisite for the [[XHCI]], [[EHCI]], [[UHCI]], and [[OHCI]] specifications,
=== What this text covers ===
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{{Main|eXtensible Host Controller Interface}}
Like its predecessor USB 2.0, USB 3.0 has only one host controller specification: Intel's '''eXtensible Host Controller Interface'''. Unlike its predecessor EHCI, however, xHCI controllers can and do interface with USB 1.0 and 2.0 devices without the use of companion controllers. Even on early hardware where there was both an EHCI and xHCI controller included (so that OSes which did not yet support xHCI could still use at least some USB devices), ports attached to the EHCI controller could generally be "re-routed" to the xHCI controller, and the EHCI controller disabled entirely.
Also unlike its predecessors, xHCI was designed with some degree of ''forwards compatibility'', so that revisions to the USB specification can be made without designing a new host controller interface (for instance, USB 3.1 and 3.2 add new speeds, with only minor updates to the specification to match them.) Unfortunately, this means that xHCI bears only a passing resemblance to the controllers that came before it, and make it challenging to write drivers for.
== Basic Concepts and Nomenclature ==
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All functions understand the USB protocol, respond to standard operations (e.g, configuration or reset), and describe capabilities to the USB host.
There are
* '''Super-speed''' functions operate at up to 5 Gb/s.
* '''High-speed''' functions operate at up to 480 Mb/s.
* '''Full-speed''' functions operate at up to 12 Mb/s.
* '''Low-speed''' functions operate at up to 1.5 Mb/s.
The original USB specification defined low- and full-speed devices, while USB 2.0 added high-speed devices
===== Hubs =====
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Additionally, host controller drivers are loaded by the PCI subsystem when a corresponding host controller is discovered during PCI enumeration. The host controller driver is thus also responsible for initializing the host controller and perhaps loading the USB Hub Driver and the USB driver. Combined, the USB driver, USB hub driver, and the host controller driver make up a USB subsystem.
=== External Links ===
* [http://www.usb.org/home USB.org]
* [
* [
* [http://www.usb.org/developers/wusb/wusb1_1_20100910.zip Wireless USB Specification Revision 1.1]
* [http://www.kernel.org/ The Linux kernel] (
* [http://www.beyondlogic.org/usbnutshell/usb1.
* [http://www.usb.org/developers/docs/USB_LANGIDs.pdf Currently accepted LANGIDs]
* [http://www.usbmadesimple.co.uk/index.html USB Made Simple]
* [https://www.fysnet.net/the_universal_serial_bus.htm USB: The Universal Serial Bus] is a book on writing device/system drivers for UHCI, OHCI, EHCI, and xHCI with various example devices and available source code.
[[Category:USB]]
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