32khz Crystal and internal RC (u-blox BMD-380)

I've been running the core on both a Nordic DK and recently a u-blox BMD-380 evaluation board as well as Nano 33 BLEs. All run without issues. I am now looking at running the BMD-380 part without the external 38khz crystal and instead using the internal oscillator for the low-frequency clock.

I have modified the mbed_app.json files to correctly set the clock source & associated defines, rebuilt the core and my application. If I run the application on the Nordic board (with the external crystal), the application runs fine and I can confirm that the LFCLK is the internal RC one.

If I run the application on the u-blox board (crystal now disconnected), it hangs in the PluggableUSB.begin() call in main.cpp. If I remove that call, my application boots and runs without further issue, but without any USB/Serial connection either.

It feels like there is still something in mbed or the ArduinoCore that is trying to use the external crystal. If I re-enable the PluggableUSB.begin() call, I also notice the the host machine can see the USB device, but it's not identified as a Arduino board. The bootloader has no problem running on the u-blox board, or identifying itself correctly as a Arduino to the host.

Any suggestions as to what I might have missed? Has anybody else tried running the core on SoC without an external crystal?

Start with the modified mbed_app.json files and main.cpp, focusing on the PluggableUSB.begin() call and the low-frequency clock configuration. Compare the BMD-380 and Nordic board configurations, then trace USB initialization with the external crystal disconnected. Done means the BMD-380 application boots and the host identifies its USB/Serial connection correctly while using the internal oscillator.

Written by the indexing model from the issue text.