First off, my overall project is to bring life to several dozen 3pi robots (formerly used in a course) by connecting them to Raspberry Pi Zero’s using USB AVR Programmers (the original ones: https://www.pololu.com/product/1300).
I’ve been able to connect an RPi to a 3pi robot through the programmer just by connecting the RX/TX lines on the programmer to the 3pi’s PD0/PD1 pins. This connection works, however I’m worried that I won’t get a robust connection with just 2 wires. Should the programmer’s 5V and GND lines be tied to the corresponding pins on the 3pi? Or maybe just tie the grounds? What’s the best strategy for the 3.3V RPi to talk to the 5V 3pi using these programmers?
Note, I see others have used logic level shifters and that makes great sense. I’m just wondering if I can use my existing programmers because I have so many of them already. So it would just be much more efficient and less costly to re-use these versus purchasing (and worse, soldering) a bunch of logic level shifters.
Thanks for any help!