Re: USB Mass Storage devices continually reenumerate

On Wed, 17 Oct 2007, Mike Nuss wrote:

> I'm not very familiar with how USB power works but it sounds as if
> there are two possible issues; the device not drawing enough current
> to operate properly, and the device drawing so much current that it
> disconnects (either on the host side or the device side). Is that
> right so far?

Your second case should be: "the device drawing so much current that it
overwhelms the host's ability to provide electricity and causes a fault
of some kind".

In fact, the two scenarios aren't really distinguishable without
detailed knowledge of how the device operates and careful measurements
of its internal voltages and currents.  Besides, for your purposes it
doesn't much matter whether the disconnect is caused by the host's
reaction to an overcurrent or the device's reaction to insufficient
current, right?

> It seems like only one of those scenarios is likely to be visible on
> the host (if the host sees an overcurrent, and even that isn't
> guaranteed). How would I debug the device side?

About all you can do is what I suggested earlier: Plug the device into
a powered hub, which most likely will provide enough current for the
device's needs.  If the device then works, you will know that the host
doesn't provide enough current.  If it doesn't, that's a reasonably 
good indication that the current draw isn't the source of the problem.

Alan Stern


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