The Pi Zeros only have two serial ports, only one can be mapped to the GPIO pin header at a time. So, how to free up the serial port for a user/app to use?
This is for a buildroot generated file system.
The edit cmdline.txt:
root=/dev/mmcblk0p2 rootwait console=tty1 console=ttyAMA0,115200
remove console=ttyAMA,115200
so it looks like this:
root=/dev/mmcblk0p2 rootwait console=tty1
next edit /etc/inittab to start a tty shell
add this line at the bottom of the file:
ttyGS0::respawn:/sbin/getty -L ttyGS0 115200 vt100
edit config.txt
add dtoverlay=dwc2
This will start the OTG USB driver.
I did something simple, edit S40network add:
modprobe dwc2
modprobe g_serial
This will start the serial interface. A new serial interface will be created /dev/ttyGS0
I'm using a Linux PC, so, when the USB is plugged in Zero will create a serial connection to Linux PC over USB OTG. Then use minicom --dev /dev/ttyACM0
