Usb System Architecture

USB System Architecture

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Shortcomings of the Original PC I/O Paradigm Limited System Resources Interrupt Lines Shortage I/O address conflicts Non-shareable Interfaces

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USB Paradigm  A single connector type to connect any PC

peripheral  Ability to attach many peripheral devices to the same connector  A method of easing the system resource conflicts  Hot plug support Automatic detection and configuration of peripheral devices

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USB Transmission Rates

 1.5Mb/s – Low Speed Device.

[USB 1.0]

 12Mb/s - Full Speed Device.

[USB 1.1]

 480Mb/s – High Speed Device. [USB 2.0]  4.8Gb/s - Super Speed Device. [USB 3.0] 4

U S B S y s te m in a P C I P la tfo rm

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USB Features

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USB 2.0 Hierarchy

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Interrupt Transfers

Interrupt-driven devices in legacy PC implementations USB keyboard Must be polled periodically to see if the device has data to transfer

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Bulk transfer  Transferring large blocks of data that have no periodic or transfer rate requirement  USB printer  Delivered slowly

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Isochronous transfer

 Transfer requires a constant delivery rate  USB microphone and speaker  To ensure that no frequency distortion results from transferring data across the USB

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Control transfer

 During device configuration  USB Automatic configuration  A special transfer sequence

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Frame-Based Transfers USB is shared by a wide variety of devices, a mix of USB transfer types will likely be performed during each 1ms frame.  Interrupt and Isochronous transfers must occur at fixed intervals, they have a special priority during the execution of each frame (90%)  Control Transfers(10%)  Bulk transfers are allocated the remainder of the available bandwidth 12

Endpoint Descriptor

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Conclusion Each endpoint within a given USB device has particular characteristics that dictate how it must be accessed. The transfer characteristics relate to the requirements of the application. The above discussed four transfer types have been defined by the USB specification, each of which reflects the nature of transfers that may be required by a USB device endpoint 14