To begin with, I realize that voltage is relative but I do not understand how to get a common ground so that everything is relatively correct to go in and out of the various chips.
The picture summarizes the setup:
Xbox plugged into the wall -> Wired USB Xbox controller which the input runs at 1.8v. Streaming the controller button input through a level shifter (SN74LVC245A) which shifts the logic to 3.3v -> SPI chip (MCP23S17) -> FPGA (Basys 2). The FPGA is connected to the computer via USB which is plugged in to the wall.
I would expect the multimeter would read 1.8v because of the assumed common ground. But I guess this is not the case because of what the value suggests. Right now both the laptop and Xbox are plugged into the same power strip but I could see how there might be further mismatch if they were on separate rails.
How do I work around this issue? Have I overlooked something simple or is it a flaw in design?
I realize that 4 buttons would be easier to go straight into the FPGA after coming out of the level shifter but I plan to add a lot more of the controllers input which I do not have enough pins for.