There are 2 versions of the firmware.
Both versions have one USB serial port dedicated to AT Command interface.
The 1st version has one USB serial port dedicated to debug output (for MTK monitor) and this port cannot be used in application. If you want the serial communication with application you have to use UART. It doesn’t make much sense to use it under Linux as we don’t have MTK monitor for Linux anyway.
The 2nd version has debug output disabled, and the USB serial port can be used for communicating with the application (initiated in retarget.c)
uart_handle = vm_dcl_open(VM_DCL_SIO_USB_PORT1, g_owner_id);
....
vm_dcl_control(uart_handle, VM_DCL_SIO_COMMAND_SET_DCB_CONFIG, (void *)&settings);
I’m using the second version for couple of weeks, and it works as expected.
I’m not sure which version I’ve uploaded to GitHub, maybe it is the 1st one, I’ll check it tomorrow.