mbed-os/TESTS/usb_device/serial/README.md

udev rules for Mbed USB CDC device

Before running tests-usb_device-serial test suite on Debian-based Linux distros, make sure to update the udev rules as follows:

sudo tee /etc/udev/rules.d/99-ttyacms.rules >/dev/null <<EOF
ATTRS{idVendor}=="1f00" ATTRS{idProduct}=="2013", ENV{ID_MM_DEVICE_IGNORE}="1"
ATTRS{idVendor}=="1f00" ATTRS{idProduct}=="2012", ENV{ID_MM_DEVICE_IGNORE}="1"
EOF
sudo udevadm control --reload-rules

This will prevent the ModemManager daemon from automatically opening the port and sending the AT commands, which it does for every new /dev/ttyACM device registered in system.