Extensible Host Controller Interface: Difference between revisions

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The '''eXtensible Host Controller Interface''' ('''xHCI''') is a technical specification that provides a detailed framework for the functioning of a computer's [[host controller]] for [[Universal Serial Bus]] (USB). Known alternately as the [[USB 3.0]] host controller specification, xHCI is designed to be backward compatible, supporting a wide range of USB devices from older USB 1.x to the more recent USB 3.x versions.

Distinct from its predecessors, the [[Open Host Controller Interface]] (OHCI) and, the [[Universal Host Controller Interface]] (UHCI) and the [[Host controller interface (USB, Firewire)#EHCI|Enhanced Host Controller Interface]] (EHCI), xHCI offers several technological improvements. Specifically, it is designed to handle multiple data transfer speeds (low, full, high, and SuperSpeed) within a single unified standard. This makes it more efficient in managing computational and power resources, a feature particularly beneficial for mobile devices with limited power capabilities like tablets and smartphones. Additionally, xHCI simplifies the architecture needed to support a mixture of low-speed and high-speed devices, which streamlines the development of drivers and system software.

xHCI marks a significant improvement over its predecessors, the Open Host Controller Interface (OHCI) and, the Universal Host Controller Interface (UHCI) and the Enhanced Host Controller Interface (EHCI), in several key ways:

# '''Multi-Speed Support''': Unlike OHCI, UHCI and UHCIEHCI, which were limited to specific USB speeds, xHCI is capable of managing multiple data transfer speeds—low, full, high, and SuperSpeed—under a single standard. This eliminates the need for multiple host controllers or complex switching mechanisms when dealing with various types of USB devices, thereby improving efficiency.

# '''Power Management''': xHCI includes advanced power management features that allow for selective suspension of USB devices and more granular power distribution. This is especially beneficial for mobile devices with limited battery life, such as tablets and smartphoneslaptops, as it helps to maximize power utilization and extend battery life.

# '''Streamlined Architecture''': xHCI's architecture is designed to be simpler and more straightforward, reducing the complexity of driver development. In older architectures like OHCI, UHCI and UHCIEHCI, supporting a mix of low-speed and high-speed devices required complicated algorithms and multiple transaction translators. xHCI simplifies this by integrating these functions into the host controller itself, thus easing the burden on system software and driver developers.

By enhancing support for multiple speeds, optimizing power management, and simplifying the underlying architecture, xHCI serves as a more efficient and unified standard for USB host controllers.

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When USB was originally developed in 1995, it was targeted at desktop platforms to stem the proliferation of connectors that were appearing on PCs, e.g. [[PS/2 connector|PS/2]], [[serial port]], [[parallel port]], [[game port]], etc., and host power consumption was not an important consideration at the time. Since then, mobile platforms have become the platform of choice, and their batteries have made power consumption a key consideration. The architectures of the legacy USB host controllers (OHCI, UHCI, and EHCI) were very similar in that the "schedule" for the transactions to be performed on the USB were built by software in host memory, and the host controller hardware would continuously read the schedules to determine what transactions needed to be driven on the USB, and when, even if no data was moved. Additionally, in the case of reads from the device, the device was polled each schedule interval, even if there was no data to read.

* The xHCI eliminates host memory based USB transaction schedules, enabling zero host memory activity when there is no USB data movement.

* The xHCI reduces the need for periodic device polling by allowing a USB 3.0 or later device to notify the host controller when it has data available to read, and moves the management of polling USB 2.0 and 1.1 devices that use interrupt transactions from the CPU-driven USB driver to the USB host controller. EHCI, OHCI, and UHCI host controllers would automatically handle polling for the CPU if there are no changes that need to be made and if no device has any interrupts to send but they all rely on the CPU to set the schedule up for the controllers.<ref>{{cite web|url=http://ftp.netbsd.org/pub/NetBSD/misc/blymn/uhci11d.pdf |title=UHCI11D.DOC |website=Ftp.netbsd.org |date= |accessdate=2017-01-09}}</ref><ref>{{cite web |url=http://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/ehci-specification-for-usb.pdf |title=Archived copy |accessdate=2014-07-02 |url-status=dead |archiveurl=https://web.archive.org/web/20150810175253/http://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/ehci-specification-for-usb.pdf |archivedate=2015-08-10 }}</ref><ref>[ftp://ftp.compaq.com/pub/supportinformation/papers/hcir1_0a.pdf] {{dead link|date=JanuaryMay 20172025|bot=medic}}{{cbignore|bot=medic}} </ref> If any USB device using interrupt transactions does have data to send, then an xHCI host controller will send an interrupt to notify the CPU that there is a USB interrupt transaction that needs handling. Since the CPU no longer has to manage the polling of the USB bus, it can spend more time in low power states.

* The xHCI does not require that implementations provide support for all advanced USB 2 and 3 power management features, including USB 2 LPM, USB 3 U1 and U2 states, HERD, LTM, Function Wake, etc.; but these features are required to realize all of the advantages of xHCI.

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{{wikibooks|Serial Programming:USB Technical Manual|USB connectors}}

* [http://www.usb.org/ USB official website (USB Implementers Forum, Inc.)]

* [ftp://ftp.compaq.com/pub/supportinformation/papers/hcir1_0a.pdf Open Host Controller Interface (OHCI)]{{dead link|date=September 2017May 2025|bot=InternetArchiveBot medic}}{{cbignore|fix-attemptedbot=yes medic}}

* [http://www.usbman.com/WebDrivers/usbpdffiles/UHCI%20Intel%20Specification.pdf Intel Universal Host Controller Interface (UHCI)] [https://web.archive.org/web/20100326195950/http://www.usbman.com/WebDrivers/usbpdffiles/UHCI%20Intel%20Specification.pdf Archived there]

* [http://www.intel.com/technology/usb/download/ehci-r10.pdf Intel Enhanced Host Controller Interface (EHCI)]