Вот прямые требования
9.1.2 Bus Enumeration
4. The hub maintains the reset signal to that port for 10 ms (See Section 11.5.1.5). When the reset signal
is released, the port has been enabled. The USB device is now in the Default state and can draw no
more than 100mA from VBUS. All of its registers and state have been reset and it answers to the
default address.
8. Based on the configuration information and how the USB device will be used, the host assigns a
configuration value to the device. The device is now in the Configured state and all of the endpoints in
this configuration have taken on their described characteristics. The USB device may now draw the
amount of VBUS power described in its descriptor for the selected configuration. From the device’s
point of view it is now ready for use.
А вот как себя вести если питание двойное:
7.2.1.4
9.1.1.2
Table 9-8. Standard Configuration Descriptor
A device may not increase its power draw
from the bus, when it loses its external power
source, beyond the amount reported by its
configuration
