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Extensible Host Controller Interface
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), the Universal Host Controller Interface (UHCI) and the 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), the Universal Host Controller Interface (UHCI) and the Enhanced Host Controller Interface (EHCI), in several key ways:
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.
The xHCI is a radical break from the previous generations of USB host controller interface architectures (i.e. the Open Host Controller Interface (OHCI), the Universal Host Controller Interface (UHCI), and the Enhanced Host Controller Interface (EHCI)) on many counts. Following are the key goals of the xHCI architecture:
The OHCI and UHCI controllers support only USB 1 speed devices (1.5 Mbit/s and 12 Mbit/s), and the EHCI only supports USB 2 devices (480 Mbit/s).
The xHCI architecture was designed to support all USB speeds, including SuperSpeed (5 Gbit/s) and future speeds, under a single driver stack.
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, 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.
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Extensible Host Controller Interface
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), the Universal Host Controller Interface (UHCI) and the 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), the Universal Host Controller Interface (UHCI) and the Enhanced Host Controller Interface (EHCI), in several key ways:
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.
The xHCI is a radical break from the previous generations of USB host controller interface architectures (i.e. the Open Host Controller Interface (OHCI), the Universal Host Controller Interface (UHCI), and the Enhanced Host Controller Interface (EHCI)) on many counts. Following are the key goals of the xHCI architecture:
The OHCI and UHCI controllers support only USB 1 speed devices (1.5 Mbit/s and 12 Mbit/s), and the EHCI only supports USB 2 devices (480 Mbit/s).
The xHCI architecture was designed to support all USB speeds, including SuperSpeed (5 Gbit/s) and future speeds, under a single driver stack.
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, 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.