Re: USB Mass Storage devices continually reenumerate

> > > One way to check for too much power draw is to insert a powered hub
> > > between the device and the computer.
> > 
> > Yeah, this should probably make things works... but also enabling USB
> > DEBUGGING you should see some sort of VBUS error thing... or at least
> > you'd be able to track vbus interrupts and check where they're failing
> > ;-)

VBUS errors are uncommon except on OTG hosts ... they'd be
more likely to appear as "overcurrent" interrupts on non-OTG
systems.  But few PCs seem to bother wiring up overcurrent
detection logic.


> Not necessarily.  What the host considers to be an overcurrent might
> differ from what the device needs.  That is, if the device is drawing
> too little current to operate, then it probably isn't drawing so much 
> current that it generates an error on the host.

Plus, remember there's capacitance at several points in the system,
including the cable connecting the PC to thd drive.

So if the drive draws so much power that the VBUS level on its side
falls below the limit, causing a disconnect ... there's no guarantee
that the VBUS level at the host end of the cable would fall below its
(different, ISTR) limit.

It may only take a brief current spike at the slave end of the cable
to cause a power-failure induced disconnect.  Such a spike may not
even be visible at the host side ... and as I commented above, even
if it *is* visible, many hosts ignore such errors (since overcurrent
interrupts aren't wired up)..

- Dave


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