On the upcoming RevC's with a working EHCI port, will there be a tap point to
disconnect the Vbus on the USB A jack from the main power supply? I am
thinking of maybe a place on either the top or bottom where the trace can be
cut; a pair of test points with a bridge link or a jumper header or even a 0
ohm resistor in a 0805 (to simplify reworking) or bigger package would be
nice but that is a luxury. Unless I am wrong, right now the only reason for
needing 5V to power the beagle is due to the Vbus needs of the EHCI port. It
would be nice to be able to run the rest of the board from as low of a
voltage as possible. I can think of several reasons why such a tap point
would be handy:
1. Current measurement of the USB port. The TPS2061 power switch seems to only
current limit.
2. Ability use an off board charge/pump switcher to provide Vbus so the rest
of the board can run off 3.3V + LDO overhead. This should be more efficient
then boosting with a switcher and then having the LDO drop it back for other
things.
3. A test point for diagnosing poorly designed USB devices that backdrive the
Vbus line. This violates the USB spec but I have a hub that appears to be
backdriving the bus in self powered mode.
-- Hunyue