Finally got remote PV battery charging data through a Raven wireless link, using USI-TWI interfaces in my Tiny85 chargers, AVR2002 in DC sleep mode with added TWI-TWI interfaces to the Raven 1284P, and hacking of the USB stick sniffer mode to get ASCII logging to a terminal program. I'll post the project when it's cleaned up. But I am worried about damaging the Raven 1284P TWI ports.
The Raven's J201 gives access to the 1284 PORTC TWI pins, and can also power the board with a charger 5 volt supply. But the Raven regulator drops that to 3V3. So the TWI involves 5 volt pins on the '85s and 3.3 volt pins on the 1284P. Internal pullups on the Raven don't work, not high enough voltage for the '85. Internal pullups on the '85 do work, but push the 1284P pins beyond spec. I'd like bidirectionality so I can also send commands to the chargers (log interval and the like). How can I get level shifting in both directions? Oh, and lightning protection would also be nice :)