Robin2:
I don't understand why you are asking that question.Don't rely on my comments - study the Atmega 328 datasheet.
...R
what??
Please read my question closely.
You just wrote about 1000000 and that there is no error,
you didn't write about the absolute maximum clock speed (e.g., 1.5MHz or 2MHz or anything in between or even higher).
But instead I was asking not only about the 328p but about the absolute maximum clock speed of all those boards, which also comply with the raspi-supported speed set:
Nano, Uno or Mega2560,
Zero (M0, 48 MHz cpu) <=> Raspi,
Due (M3, 84 MHz cpu) <=> Raspi,
and esp8266 (80 MHz cpu) <=> Raspi
now what?