At 02:47 AM 07/08/2012, you wrote:
>Hi,
>
>I'm connecting a 3.3V-powered PIC to a 5V-powered PIC via the hardware
>USARTs.  Currently I have these prototyped and working well with
>bi-directional mosfet level converters as shown here...
>http://www.nxp.com/documents/application_note/AN10441.pdf
>
>However, I need to fit this in a small enclosure and PCB real-estate
>is a problem once again.  I'm wondering if I can eliminate or at least
>reduce the components for the logic-level converter.
>
>In the 5V-->3.3V direction, the 3.3V RX input (PIC24) is 5V fault
>tolerant, so I should be able to direct-connect that.
>
>In the 3.3V-->5V direction, it seems the 5V RX1 input (18F24K22) is a
>Schmitt trigger, and the electrical specs for that in the datasheet
>are minimum input-High voltage of 0.8Vdd, so 4.0V, and thus the 3.3V
>signal won't work.
>
>So that brings the 6 components down to 3 (mosfet and 2 resistors),
>but is there a better (smaller footprint) way, or something I'm
>missing here that would let me direct-connect the other direction?
>
>FWIW, I did find monolithic converters such as this, but it doesn't
>save much space... http://www.nxp.com/documents/data_sheet/PCA9306.pdf
>
>Cheers,
>-Neil.

Something like a 74LVC1T45 in SOT363 will give you one channel
of translation in a single leaded package about 2mm x 2mm (and about
30 cents, if that matters). I don't think you'll do much
better than that unless you like packages without leads like
XSON6 (1mm x 1mm)-- the same part is available in that package.
You can direct connect the other way.

--sp


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