Below please find links to information on using I2C and SPI to communicate between Arduinos and RPis. There are others and I’ll post as able.
I2C
- i2c.info
- https://create.arduino.cc/projecthub/…
- http://www.instructables.com/id/I2C-between-Arduinos/
- http://www.robot-electronics.co.uk/i2c-tutorial
- https://learn.sparkfun.com/tutorials/i2c
- https://www.youtube.com/…
- https://blog.adafruit.com/…
- https://gist.github.com/carlosefr/5849220
SPI
Newbury Pi 