PICdude wrote:

>Hi,
>
>I'm connecting a 3.3V-powered PIC to a 5V-powered PIC via the hardware =20
>USARTs.  Currently I have these prototyped and working well with =20
>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 =20
>is a problem once again.  I'm wondering if I can eliminate or at least =20
>reduce the components for the logic-level converter.
>
>In the 5V-->3.3V direction, the 3.3V RX input (PIC24) is 5V fault =20
>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 =20
>Schmitt trigger, and the electrical specs for that in the datasheet =20
>are minimum input-High voltage of 0.8Vdd, so 4.0V, and thus the 3.3V =20
>signal won't work.
>
>So that brings the 6 components down to 3 (mosfet and 2 resistors), =20
>but is there a better (smaller footprint) way, or something I'm =20
>missing here that would let me direct-connect the other direction?
>
>FWIW, I did find monolithic converters such as this, but it doesn't =20
>save much space... http://www.nxp.com/documents/data_sheet/PCA9306.pdf
>
>Cheers,
>-Neil.
>
>
>
> =20
>
Neil,

Have you considered trying a SN74LVC2G07DBVR and a couple of pull up=20
resistors?  Pull up to appropriate level on each output.  Run the G07=20
off either the 3.3V or 5V.  That gets you down to 3 parts total.  I've=20
been using this method to interface my PICs to Digi's Rabbit modules for=20
years and it works well.

Bill
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.