digitalRead flip flops between HIGH and LOW

[yourtechjohn]

Thanks @Ric and @peekay123. Found another post about the INPUT:

Difference between INPUT, INPUT_PULLUP & INPUT_PULLDOWN

When in any `INPUT´ mode the pin goes in a high impedance state which means that it should (almost) not influence the external circuitry and for this the signal to read. If there is no ext circuitry attached INPUT will not give you any predictable reading (floating signal). To prevent this floating you can instruct the µP to internally attach a somewhat lower (but still high) resistance either towards GND (INPUT_PULLDOWN) or towards Vcc (INPUT_PULLUP). This will give you predictable readings …

I did check the document and it looks like all pins except A3 and DAC are 5V tolerant (when not in analog mode).

I then tried switching from INPUT to INPUT_PULLDOWN (and INPUT_PULLUP for good measure). I still get the floating between HIGH/LOW with nothing connected.

Even when I connected a cable from A5 to D4 it still continues to float. D1 however, does seem to stay in a state with INPUT_PULLDOWN. Unattached or attached to GND, it stays LOW. Connected to A5, it goes HIGH. It takes a while to test each pin and let it cycle through several times, but D1 seems reliable while D2-D5 do not (they float between states).