On Thu, 1 Jun 2006, linux-os (Dick Johnson) wrote:
> > If they do, they are violating the spec. A device in the unconfigured
> > (state 0)
> > state must not draw more than 100mA.
...
> Hmmm, the USB-IF recommends 100 mA per port, not requires.
See section 7.2.1 of the USB 2.0 specification (p. 177):
Devices must also ensure that the maximum operating current drawn
by a device is one unit load, until configured.
Note that a unit load is defined as to be 100 mA. This is a requirement,
not a recommendation.
On Thu, 1 Jun 2006, Mark Lord wrote:
> I think a far more sensible approach would be to just ensure that the
> total current draw for the (unpowered) hub and all connected devices,
> stays below the 500mA allowed. So a 200mA device could coexist with
> a 100mA device on a hub which itself steals 100mA.
On that same page the specification says:
Bus-powered hubs: Draw all of their power for any internal
functions and downstream facing ports from VBUS on the hub s
upstream facing port. Bus-powered hubs may only draw up to one
unit load upon power-up and five unit loads after configuration.
The configuration power is split between allocations to the hub,
any non-removable functions and the external ports. External ports
in a bus-powered hub can supply only one unit load per port
regardless of the current draw on the other ports of that hub.
This clearly states that a bus-powered hub cannot supply 200 mA on one
port, even if another port is unoccupied.
Alan Stern
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