That particular Rabbit core has several power supplies for the radio,
and while the radio may be off the power supplies do start up on power
up, IIRC.

This means that the core has a significant inrush current due to all
the bypass capacitores used for filtering - 0.9A may not be enough,
the Ron of the FET may not be low enough, etc, etc.

Measure the voltage across the FET with a high speed scope to see what
the inrush current looks like in combination with the turn on time of
the FET.

Also, are you able to reset the Rabbit when it doesn't come up using
the FET to power it?  If it resets ok, then you may need to control
the reset along with the power suplly.  If it doesn't reset then
you'll need to beef up the power supply and switching mechanism (the
FET and FET driver).  You may need to drive the FET very hard to get
it to turn on quickly enough especially since you don't have much
overhead.

You might consider trying the same thing with another Rabbit 4000 core
to determine if it's the added radio circuitry, or if there's a
problem with the reset and cpu itself.  It should just plug right in.

Good luck!

-Adam

On 7/24/08, Mauricio Jancic <listas@janso.com.ar> wrote:
> Hi,
>        I have a circuit with a MOSFET driving a RCM4400W (A Rabbit module
> with wifi, that need 80mA on startup and 450mA when the wifi module is on),
> and it is not working.
>
>        The power supply is fine, I think, because if I connect the Rabbit
> directly to the PSU it works.
>
>        The mosfet seems to be working, because I have 3.3V on the Rabbit
> terminals when it is on.
>
>        The FET is driven with a PIC pin with a 5K6 resistor to gate.
>
>        The source of the FET is to GROUND and the drain is connected to the
> NEGATIVE terminal of the Rabbit. The POSITIVE terminal of the rabbit is
> directly connected to 3.3V.
>
>        There is nothing else in the circuit, shouldn't it be working?
>
>        The FET is an FDV305N, 0.9A and the Vgsth is 1.5V.
>
> Regards,
>
> Mauricio Jancic
> Microchip Design Partner
> Janso Desarrollos
>
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