[GreatFET] hw design: gpio-controlled power

[David Hubbard]

On Friday, January 27, 2017, Michael Ossmann <mike at ossmann.com> wrote:
> On Wed, Jan 18, 2017 at 02:04:38PM -0600, David Hubbard wrote:
>>
>> I see that the GreatFET One schematic includes a way to disconnect the
USB
>> VBUS power from the AP7333 regulator and VCC.
>
> There isn't a way to disconnect it, but the USB host is protected from
reverse
> current that might occur if a neighbor is supplying power.
>
>> I was wondering if a hardware change could be made which allows the
LPC4330
>> to switch off power to the header pins, neighbors, etc. (I began thinking
>> about this as I worked on USB suspend on the HackRF One.)
>
> That could be done, but I would prefer not to do it.  Neighbors that draw
> significant current should have a mechanism allowing the microcontroller
to put
> them in a low power state, but this should be separate from cutting the
power
> supply to the neighbor.  It should be possible to shut down a single
neighbor
> without shutting down all attached neighbors.
>
> This is also complicated by the fact that a neighbor may be used to supply
> power to GreatFET One.  If power is supplied by a neighbor, then a "turn
off
> neighbor power" function would become a "turn off power to self"
function, and
> it would leave any attached neighbors powered on without microcontroller
> supervision.  That might be an acceptable risk if it were the only way to
power
> down neighbors during normal (USB-attached) use, but I think it is better
for
> neighbors to provide a separate power-down function.

Ok, that's a good way to do it.

David
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