Here's the scenario: I have a radio that displays information on a 1602A LCD. The 1602A is plugged into a header so I can easily pull it out and instead plug in logic analyzer leads. The logic analyzer decodes the HD44780 protocol and shows me the data that would have gone to the 1602A. The logic analyzer, not surprisingly, doesn't have any noticeable effect on the radio.
Now I want to replace the logic analyzer with a Nano Every. I wired it up so that the signals to be observed are connected to digital input pins on the Every. The Every is powered through USB and doesn't intentionally draw any power from the radio. Everything runs as expected and the Every shuffles readings to the PC via serial. HOWEVER, in this combination, the radio draws at least twice as much current from the power supply and the radio's voltage regulators begin to heat up substantially!
I would have thought that the digital input pins of the Every are high enough impedance that it would have no significant effect on the radio, just like the logic analyzer didn't. I can only assume that "significant" current is flowing from the radio's LCD header into the Nano Self's input pins, since they are the only connection between the radio and the Nano.
So, I guess my question is this: How can I make the Nano Self work more like a logic analyzer? Do I need additional circuitry between the signals being tested and the Self's input pins, perhaps logic buffers? Or might something else be going on here?