Unstable reading from Raspberry Pi I'm trying to send a signal from Raspberry Pi GPIO to Arduino. I made a sketch that basically reads the state of the digital read, but the reading are very unstable. I understand that there is a floated voltage and the obvious solution is to configure the pullup resistor and connect the input to ground, but my goal is to read the incoming signal.
I'm using Arduino Mega 2560 and Raspberry Pi 2B.

I hope this info is enough!
As Majenko commented, you need to not only connect the data wires together but also the ground wire (from the Arduino to the Pi).


  ... the reading are very unstable ...


Without a consistent "ground reference" the readings would indeed be unstable.