[GreatFET] hw design: gpio-controlled power

[Michael Ossmann]

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.

Thanks,

Mike