I use a third-party modified Arduino that has the correct voltages and current limitations to control my robot. I additionally want to connect a Raspberry Pi Zero W to the Arduino that is evaluating a USB webcam. The Zero should send some picture evaluation data to the Arduino. I've gone through several possibilities to connect the two controllers, but each of them is not very easy:
I²C: My custom Arduino has only one I²C port that is used to communicate with external sensors. As far as I know, the Raspberry is not able to act as an I²C slave, so I'd have to connect all sensors to the Raspberry Pi which is very bad.
USB: Since the Pi is using a USB webcam, I would have to use a hub to connect it to the Arduino. I don't want to have a big USB hub in my robot.
UART: If I understood the information on the internet correctly, it is possible to use the GPIO UART interface parallel to the USB interface of my Raspberry Pi Zero. But because of the fact that my Arduino is custom, there is no free accessible RX/TX or other digital pin. I would have to solder a cable on the PCB. Or can I use a UART to USB adaptor to connect to the Arduino?
Do you have any other (better) ideas? What is the best way for connection? Help is highly appreciated :)