USB

USB 元件簡介
分為Host Controller, Hub and Device

Host controller
Also known as the root, the root tier, or the root hub,
the host controller can be built into the motherboard of the computer

or installed as an add-in CardBus or PCI card in the computer

to gain additional ports and bandwidth.
The host controller controls all traffic on the bus and also functions as a hub.

Hub
Provides multiple ports, for attaching devices to the USB bus.
Hubs are also responsible for detecting devices that are plugged in or unplugged,
and for providing power for attached devices. Hubs are either bus-powered,
drawing power directly from the USB bus, or self-powered, drawing power from an external AC adapter.Bus-powered hubs are capable of providing 100 milliamperes (mA) of power per port for attached devices, and they can provide a maximum of four ports for devices to be plugged into. Self-powered hubs, on the other hand, typically provide 500 mA of power per port,
and they can provide more than four ports. Hubs can be stand-alone devices, or they can be integrated into other devices such as keyboards and monitors.

Device
A USB device, which is attached to the bus through a port. A USB device can be any kind of peripheral device,
such as a keyboard, mouse, game controller, printer, and so forth.
Certain USB input devices such as keyboards and mice require only 100 mA of power to function. Thus, they can be plugged into both bus-powered and self-powered hubs, in addition to being plugged directly into a root port. Other devices such as printers, scanners, storage devices, and video-conferencing cameras might require 500 mA of power to function. These kinds of devices can only be plugged into root ports or self-powered hubs. If the device requires more than 500 mA of power, it includes a wall plug provided by the vendor for power.

[Ref] http://technet.microsoft.com/en-us/library/bb457107.aspx


USB的三種速度
1. low speed: 1.5Mbps
2. full speed:12Mbps
3. high speed: 480Mbps


         

You will notice we have not included speed identification for High Speed mode. High speed devices will start by connecting as a full speed device (1.5k to 3.3V). Once it has been attached, it will do a high speed chirp during reset and establish a high speed connection if the hub supports it. If the device operates in high speed mode, then the pull up resistor is removed to balance the line.