Improved:


                   5.5V
                    |
                    |
       -------------o
       |            |
       \            |
       /470         |
       \            |
       |    10      |
  PIC--o---\/\/-- FET
                    |
                    o------|<|------- 3.3V
                    |
                   Vcc to circuit


When the FET is on, supplying the 5.5V source, the diode
is reverse biased and off.  When the pic pin is activated,
turning off the FET, the voltage at the diode's cathode is
close to zero, thus forward biasing the diode and allowing
supply of your 3.3V source. Chose a low dropout diode as
suggested recently in the list.

What kind of FET are you using that you need such a heavy
pullup?

Craig

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Andy Kunz
> Sent: May 12, 1999 12:48 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: PIC select Vcc
>
>
> 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.
>
> Thanks.
>
> Andy
>
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