Bad baudrate below 600 Bps on Serial1


I'm trying to set 300 Bps on Serial1 but it seams to be impossible to set.
600 Bps and over seam to be OK.
I'm wrong?

Sounds right. With the relatively simple scheme the code uses to generate baudrates, you can go from 48e6/16 (3Mbps) to almost 48e6/(16*8192)=366bps

There are a couple ways that you could achieve lower baud rates, if it’s important.