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

[luke.leighton]

On Tue, Nov 5, 2013 at 3:46 PM, joem <joem at martindale-electric.co.uk> wrote:
> On Tue, 2013-11-05 at 13:48 +0000, luke.leighton wrote:
>> perfect.  thanks.
>>
>> On Tue, Nov 5, 2013 at 1:18 PM, joem <joem at martindale-electric.co.uk> wrote:
>> > On Tue, 2013-11-05 at 12:04 +0000, luke.leighton wrote:
>> >> 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
>
>
> hmmmp....
>
> I just scanned it again - notice a problem - the WNM2306 is an n-FET.
> You need a p-FET with similar ratings to work it correctly.

 you missed the message saying "please disregard".  i've already
replaced this with an SS12.