Physical Interface and Power Management

Physical Interface

USB transfers data and provides power using four-wire cables. The four wires are: Vbus, D+, D- and Ground. Signaling occurs on the D+ and D- wires.

Speed

The USB 2.0 specification defines three different speeds.

• Low Speed: 1.5 Mb/s
• Full Speed: 12 Mb/s
• High Speed: 480 Mb/s

Power Distribution

The host can supply power to USB devices that are directly connected to the host. USB devices may also have their own power supplies. USB devices that use power from the cable are called bus-powered devices. Bus-powered devices can draw a maximum of 500 mA from the host. USB devices that have an alternative source of power are called self-powered devices.