On the thread https://www.avrfreaks.net/forum/quiz-fun-io-pin-and-4-levels we found out that, by adding two external resistors (pull-up and pull-down), an I/O pin (as of ATmega8) can output 4 voltages (by setting its DDR/PORT bits as 1/1,
1/0 0/1, 0/0 and 1/0).
Therefore, in theory, by combining the outputs of two such I/O pins, it is possible to get 16 different voltages (4*4).
Ideally, the 15 steps, from 0V to Vcc, are equal.
Any idea?... before I present mine