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To allow for voltage drops, the voltage at the host port, hub port, and device are specified to be at least 4.75 V, 4.4 V, and 4.35 V respectively by USB 2.0 for low-power devices, [a] but must be at least 4.75 V at all locations for high-power [b] devices (however, high-power devices are required to operate as a low-powered device so that they ...
USB 3.0 ports may implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 W, or, in the case of USB 3.1, the USB Power Delivery Specification for charging the host device up to 100 W. [15]
A USB device can be located by the bus:device address, or by the hub:port address. [ 16 ] Also, after the descriptors is retrieved, the host performs another control transfer exchange, but instead to set the address of the USB device to a new ADDRx.
The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port. High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. There may be one translator per hub or per port.
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.
The USB port is fully compatible with USB 5 Gbit/s (USB 3.0), USB 480 Mbit/s (USB 2.0) and USB 12 Mbit/s (1.1); USB 3.0 devices are compatible, but will operate at USB 2.0 speed if internal USB 3.0 connector is not connected.
USB 3.0 SuperSpeed – host controller (xHCI) hardware support, no software overhead for out-of-order commands; USB 2.0 High-speed – enables command queuing in USB 2.0 drives; Streams were added to the USB 3.0 SuperSpeed protocol for supporting UAS out-of-order completions USB 3.0 host controller (xHCI) provides hardware support for streams
The USB OTG and Embedded Host Supplement to the USB 2.0 specification introduced three new communication protocols: . Attach Detection Protocol (ADP): Allows an OTG device, embedded host or USB device to determine attachment status in the absence of power on the USB bus, enabling both insertion-based behavior and the capability to display attachment status.