USB power direction in host mode

[Andy Green]

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Somebody in the thread at some point said:

| Is it possible to charge the Neo Freerunner when using the USB port in
| host mode, by ascerting LOGICAL hostmode and by NOT asserting EN_USBHOST?

I verified this a couple of months ago.  EN_USBHOST asserted is
deliberately fatal for any charging activity (we do not want to charge
ourselves off of the 5V we just generated!), but you can have logical
host mode up with EN_USBHOST down if you externally add 15K each to D+
and D-.

This is how we formulated the "Y cable" recipe, which features the 15Ks
and the charger, together with a USB A socket for your USB device.  With
this you can have charging, wallsocket power, USB host mode and a socket
for your USB device.

- -Andy
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