I have built and installed HCI_UART to nRF52840 Nordic Development kit.  When I connect this device it appears with "lsusb", seemingly I can attach it using "btattach -B /dev/ttyACM0 -S 1000000", revealing hci0.  I have made the recommended adaptation to "/lib/systemd/system/bluetooth.service", appending -E to the "ExecStart=/usr/lib/bluetooth/bluetoothd -E" line.

When issuing commands with "btmgmt", "btmon" reports "Invalid Index (0x11)".  Normal HCI commands fail such as "hciconfig hci0 up".  

Using Bionic-Beaver Ubuntu, 4.15 kernel, Bluez 5.48, also tested with Xenial Ubuntu 4.15 kernel, Bluez 5.50.  Any suggestions would be greatly appreciated. 

Best regards,