| 摘要 |
USB 2.0 supports communication in low-speed (LS), full speed (FS) and high speed (HS). In the full speed mode, the wire segment between a hub and a device is terminated via a pull-up resistor 480 on the D+ data line on the downstream end of the segment. In the high-speed mode, both signal wires are terminated by the LS/FS driver 420 generating a single-ended zero via resistors 490. The device emulates a disconnect, while it operates in the high-speed mode, by activating the pull-up resistor 480. Including the D+ pull-up resistor is practically equivalent to an open end, enabling a reliable disconnect detection in the hub using the disconnection envelop detector 460. Detection of an (emulated) disconnect, triggers the reset en enumeration process. This allows the device to report a change in functionality without the user having to physically remove the device from the bus.
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