Change UART serial port speed (baud rate) on OpenWrt

Related documentation:

• port.serial
• uboot.config
• https://dev.openwrt.org/ticket/1509

I saw a lot of forum questions asking “how to change serial port speed”, and I thought it was easy, but it is not that easy.

OpenWrt has uart serial port baudrade speed set via kernel command line option, so it is hardwired into kernel, and as far as I know this method is the only way to change baudrate speed of serial port.

If you are lucky maybe your kernel supports changing serial port speeds, then you can just try stty or mgetty commands:

mgetty -s 19200 /dev/ttyS0 

OR

stty -F /dev/ttyS0 9600 clocal cread cs8 -cstopb -parenb 

For reading serial port settings use:

stty -F /dev/ttyS0 -a 

In recent builds, you can easilly install stty from packages as follows:

opkg update
opkg install coreutils-stty

You will find stty in your menuconfig under

Base system -> busybox -> Coreutils -> stty

For example for TP-Link WR741ND V4.x it is set to: “console=ttyATH0,115200”

Now the question is how to change this kernel command line, only way to do this is to compile your own kernel with different “console=” line.

First you need to know which speed you need to set. In this example we use TP-LINK WR741ND which uses has it's uart serial port baudrate speed set to 115200 and we need to change it to 19200.

First check check what baudrate and which port your current router uses, telnet or ssh to your router and then find out your current settings. Usual uart serial ports are ttyS0 or ttyATH0.

Use these command to check your serial port settings

• fw_printenv
  • baudrate=115200
  • console=console=ttyS0,115200
• OR: logread | grep tty
  • ...

find where is your config file:

• grep ttyATH0 ./trunk/target/linux/ar71xx/ -R

edit file before building your own image:

• ./trunk/target/linux/ar71xx/image/Makefile

find line for your modem:

• tlwr740_cmdline=board=TL-WR741ND console=ttyS0,115200
• tlwr740v4_cmdline=board=TL-WR741ND-v4 console=ttyATH0,115200

Change “console=ttyATH0,115200” to speed you need:

• tlwr740_cmdline=board=TL-WR741ND console=ttyS0,19200
• tlwr740v4_cmdline=board=TL-WR741ND-v4 console=ttyATH0,19200

You can change the Kernel command line within kernel_menuconfig:

• kernel_menuconfig > Kernel hacking > Default kernel command string

Then you have to recompile the Kernel with 'make target/clean world' and reflash.

https://forum.openwrt.org/viewtopic.php?id=38419