i'm not sure i understand why you want to do the switching: save power?
then the 470R resitance is rather small: 11mA when you drive the pin low.

you could use another circuit like this one for the 3.3V, but with a
p-channel fet, i guess -- if you find one with a really low g-s threshold
voltage. thinking of it, how's it with the gate-source threshold in your
circuit? it seems that the "vcc to circuit" is at least 2V below the 5.5V.
here you go... that's your 3.3V circuit... :)

use a p-channel fet for the 5.5V (source to 5.5V) and pull the gate to
ground -- this gives you -5.5V gate-source and lets the vcc output be lower
than the 5.5V only by the on-state resistance of the fet.
and use an n-channel fet for the 3.3V (drain to 3.3V) and pull the gate to
5.5V somehow (independently of the pic's supply voltage -- a transistor?)
-- this gives you >2.2V gate-source, which should be enough for a low
threshold fet. (if it have to be fets.)

ge

At 14:48 05/12/99 -0400, Andy Kunz wrote:
>I want to have a PIC switch it's power supply between 5.5V and 3.5V.  Any
>ideas?  It needs to default to 5.5V on powerup.  And it can only use 1 pin.
>
>I was thinking:
>
>                 5.5V
>                  |
>                  |
>     -------------o
>     |            |
>     \            |
>     /470         |
>     \            |
>     |    10      |
>PIC--o---\/\/-- FET
>                  |
>                  |
>                  |
>                 Vcc to circuit
>
>This would work for the 5.5V supply.  How do I do a 3.3V supply now?
>
>I'm using a FET because of the load I'm drawing.