USB Function and Specification
USB connector provides a single 5 volt wire from which connected
USB devices may power themselves. A given segment of the
bus is specified to deliver up to 500 mA. This is often
enough to power several devices, although this budget must
be shared among all devices downstream of an unpowered hub.
A bus-powered device may use as much of that power as allowed
by the port it is plugged into. Bus-powered hubs can continue
to distribute the bus provided power to connected devices
but the USB specification only allows for a single level
of bus-powered devices from a bus-powered hub. This disallows
connection of a bus-powered hub to another bus-powered hub.
Many hubs include external power supplies which will power
devices connected through them without taking power from
the bus. Devices that need more than 500 mA or higher than
5 volts must provide their own power. When USB devices (including
hubs) are first connected they are interrogated by the host
controller, which enquires of each their maximum power requirements.
However, seems that any load connected to USB port may be
treated by operating system as device. The host operating
system typically keeps track of the power requirements of
the USB network and may warn the computer's operator when
a given segment requires more power than is available and
may shut down devices in order to keep power consumption
within the available resource.
hubs: Draw Max 100 mA at power up and 500 mA normally.
hubs: Draw Max 100 mA, must supply 500 mA to each port.
power, bus-powered functions: Draw Max 100 mA.
power, bus-powered functions: Self-powered hubs: Draw
Max 100 mA, must supply 500 mA to each port.
functions: Draw Max 100 mA.
device: Max 0.5 mA
simple USB charger should short the 2 data lines together.
The device will then not attempt to transmit or receive
data, but can draw up to 1.8A, if the supply can provide
voltage by a host or a powered hub ports is between 4.75
V and 5.25 V. Maximum voltage drop for bus-powered hubs
is 0.35 V from its host or hub to the hubs output port.
All hubs and functions must be able to send configuration
data at 4.4 V, but only low-power functions need to be working
at this voltage. Normal operational voltage for functions
is minimum 4.75 V.