I'm sorry if this is a trivial (or already answered) question, but I really couldn't find an answer on the web. I'm trying to connect an AVR board, which operates at 5V, to a Raspberry Pi 3 Model B's embedded UART converter, via the GPIO interface. So, I noticed that the most common practice is building a voltage divider such that the GPIO won't operate at 5V - a 2.2kohm resistor and a 3.3kohm one, the latter's low voltage terminal being grounded. Assuming zero current on the output wire that connects the GPIO's RXD to the AVR's TX, this would lead to about 3V for the GPIO, which is fine - but I wonder, is RXD pin's input impedance so high that it would make almost no difference when it's connected to the voltage divider? Do you know a typical impedance value for such pin? I suppose it is very high because inputs are usually like that, but I also wondered if that's the case.