[Arm-netbook] [flying squirrel] need quick review of power input for RTC battery on ATSAM4S

[luke.leighton]

this is mainly a question for joe (and anyone else with schematics
review experience!)

the goal is to provide dual-power for the ATSAM4S.  atmel recommend
using 2 diodes however the voltage for the IO needs to be above 3.0v
in order to run USB, the AXP209 can only output 3.3v max and a diode
would suck 0.7v off of that.

so i've been advised to use a MOSFET instead, and need to check it's
going to work.  so here's the schematic that needs checking:

* http://hands.com/~lkcl/eoma/kde_tablet/rev3_review/atsam_vddio_power.png

* note the diode which provides backup power (from the AXP209 RTC supply)

* also i'd like to use the AXP209's "PWROK" signal to control when the
ATSAM4S powers up.  the datasheet says it's "HIGH" when power is "OK"
and it starts off "LOW" when the AXP209 starts up.

as the AXP209 PWROK signal is "HIGH" when "OK", that's why i *believe*
that the transistor (Q6) is needed, to invert that so that when PWROK
is "HIGH", the MOSFET delivers power.

greatly appreciated a quick review of that logic.

l.