Raspberry Pi Pull Down GPIO Pin using Arduino
Use Open-Drain Configuration. As far as I know, there is no method to set a digital pin of UNO in open-drain mode. You can use an external MOSFET in open drain configuration like this:







simulate this circuit &ndash; Schematic created using CircuitLab


Dout = Digital output pin of UNO

Din = Digital input of Pi, in internal pull-up mode.
When Dout of UNO = '1', N-MOSFET will turn on and pull down the Din of Pi to '0'.
When Dout of UNO = '0', N-MOSFET will turn off and the Din of Pi will get pulled up by the internal pull-up of Pi.