Re: USB Mass Storage devices continually reenumerate

On 10/17/07, David Brownell <[email protected]> wrote:
> 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)..

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?

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?

Mike

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