At 04:55 PM 2/26/2004 +0000, you wrote:
> >Don't forget that there is a simple way to drive from a
> >PIC at 5V to a 3.3V peripheral. With PORT bit always low,
> >just use the TRIS bit to drive the output low, and use
> >a pullup to 3.3V to set the level.
>
>Trouble is that the lines the OP wants to interface to 3.3V are the UART
>lines, so he cannot use the TRIS trick.
>
>As an afterthought, the OP should look at the Philips I2C interface doc
>which has a section on doing exactly this on I2C lines.
>
>See
>http://www.semiconductors.philips.com/acrobat/literature/9398/39340011.pdf

That's a nice circuit. Just 2 pullup resistors and 2 2N700x transistors.
total for both directions.

Come to think of it, you could use BJTs too, with base resistors as thus:


                   3.3V  5V
                    o    o
                    |    |
                   .-.  .-.
               10K | |  | | 10K
                   | |  | |
                   '-'  '-'
                    |    |
                    |    |
                   ---   |
      0/3.3 in o---v \---+--o 0/5V out


             3.3v   5V
              o     o
              |     |
             .-.   .-.
         10K | |   | | 10K
             | |   | |
             '-'   '-'
              |     |
              |     |
              |    ---
            o-+----/ v-------o 0/5v in

     0/3.3v out


Only disadvantage I see is that it requires access to (and draws
current from) the 3.3V supply (or you could create a local 3.3V supply
from the 5V supply, but that would add at least one part.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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