Hi Josep,
sorry for the late reply, I just seen your questeion.
The hardware chip for UART3 (/dev/ttyS3) is a 16550A and supports a Base-baud of 150000.
So the highest possible baud rate you can get out of this chip is 150 000.
To set it to 150000 Baud you have to:
1) enable the UART3 via armbian.config,
2) install the tool setserial
3) enable for customer speed by:
setserial -a /dev/ttyS3 spd_cust
4) set the divisor to 1 by:
setserial -a /dev/ttyS3 divisor 1
5) activate the UART by:
stty -F /dev/ttyS3 38400 <-- will get an error, but this is o.k.
Here is my output:
pi@bananapim2zero:~$ setserial -a /dev/ttyS3
/dev/ttyS3, Line 3, UART: 16550A, Port: 0x0000, IRQ: 46
Baud_base: 1500000, close_delay: 50, divisor: 0
closing_wait: 3000
Flags: spd_normal
pi@bananapim2zero:~$ setserial -a /dev/ttyS3 spd_cust
pi@bananapim2zero:~$ setserial -a /dev/ttyS3 divisor 1
pi@bananapim2zero:~$ setserial -a /dev/ttyS3
/dev/ttyS3, Line 3, UART: 16550A, Port: 0x0000, IRQ: 46
Baud_base: 1500000, close_delay: 50, divisor: 1
closing_wait: 3000
Flags: spd_cust
pi@bananapim2zero:~$ stty -F /dev/ttyS3 38400
pi@bananapim2zero:~$
Regards
wollik