tailieunhanh - USB Architecture

A USB system consists of a host computer, one or more USB devices, and a physical bus. The host consists of two layers: an upper software layer, which includes USB device drivers, and a host controller hardware layer, also known as an adapter layer. The main responsibility of the host computer is to control data transfers to and from USB devices. USB devices are peripherals that use the USB electrical and data format specifications to communicate with the host computer. The physical bus is the set of USB cables that links the controller with the peripherals | USB Architecture A USB system consists of a host computer one or more USB devices and a physical bus. The host consists of two layers an upper software layer which includes USB device drivers and a host controller hardware layer also known as an adapter layer. The main responsibility of the host computer is to control data transfers to and from USB devices. USB devices are peripherals that use the USB electrical and data format specifications to communicate with the host computer. The physical bus is the set of USB cables that links the controller with the peripherals. Bus-powered and Self-powered USB Devices Windows CE and later provide full support for bus-powered and self-powered USB devices. When a user connects a self-powered or bus-powered device to a Windows CE based platform the USB system software automatically accepts or rejects the device based on the power requirements of the device. The power model is the same for both bus-powered and self-powered devices. When a USB device is attached to a Windows CE based platform the HCD module sets the initial power configuration. During the device attachment processing phase the HCD module reads the power requirements of the USB device configurations from the device configuration descriptor structures. In this way the HCD module can choose an appropriate power configuration for the device. Some devices may provide several configurations with different power requirements. OEMs who port an HCD module to their hardware can implement policies to choose the appropriate power configurations from those provided by the USB devices. For example Windows CE based platforms have a registry setting that specifies the maximum total current draw allowable for USB devices connected to the host computer. If enabling a device would exceed this power threshold the device is not configured unless the device has an alternate configuration with acceptable power requirements. OEMs can customize the platform-specific portions of the