@Robin2, @Grumpy-Mike:
You sound like blatherers and smart-alecks, not willing to provide a reasonable answer but just trolling my thread. Please shut up and move along.
@all, who are willing to give a reasonable answer:
which subset of the following UART clock freqencies
230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000 and 4000000
will work by Serial.begin(x) with either one of the following cpu boards:
16MHz Nano
16MHz Uno
16MHz Mega2560
Zero (M0, 48 MHz cpu)
Due (M3, 84 MHz cpu)
and esp8266 (80 MHz cpu)