sterretje:
Explain 5V vs 3.3V and why a 5V output can't drive a 3.3V input and why a 3.3V output can't drive a 5V input (might depend on micro controller; in practice it might work but according to the 328 datasheet it will not (0.7 * Vcc = 3.5V for Vil(min)).
Where in the datasheet did you see that?
I see
32.2. Common DC Characteristics
Minimum Input High Voltage, except XTAL1 and RESET pins =
0.7 VCC when VCC = 1.8 V to 2.4 V
0.6 VCC when VCC = 2.4 V to 5.5 V
(06/2016 revision of the datasheet; section 30.2 in 05/2015 revision)
That means anything from 3 to 5 volts is high on a 5 V input.
PS. The two-wire section does say minimum VIH = 0.7 VCC regardless of VCC.