Re: USB devices fail unnecessarily on unpowered hubs

[Alan Stern <stern@rowland.harvard.edu>]

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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