Connecting USB devices
USB attachment points areprovided by a special class of USB device known as a hub. The
additionalattachment points providedby a hub arec alledports.
A host includes anembeddedhub called the root hub that can provideone or more attachment
points. Hosts may also have other non-rootinternalhubs to provide furtherexpansion capability.
Theseinternal non-roothubs aretier 2 devices, andl imit the numberof external hubs that you can use
ina path.
Powering USB devices
A USB host can supplypower for USB devices that aredirectly connected. USB devices that relyon
powerfrom the cable arec alledbus-powereddevices.
USB devices that have their own externallyc onnectedpower supplies arecalled self-powered
devices.
Whena bus-powereddevice connects to t hehost’s USB interface,the host allocates powerin
incrementscalled unit loads. A unit load is defined to be 100mA. A device may be either a low-power
deviceusing one unit load ora high power device, consumingup to f ive unit loads. Whenc onnecting,
all devices defaultto low-power. If required by the device, the host’s software controls the transition
to high-powerby ensuringthat adequatepower is available. Hosts that are externallypowered must
beable to supply up to five unit loads ( 500 mA) of power but may supply significantly more. Battery
poweredhosts may supply from one to five unit loads but may limit the power to conserve battery
power.
AP P E N DI X A
USB2.0 specification primer
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