Ah, my mistake. I am indeed using a Serial with the USB port. I have a serial wrapper and then I’ve got it implemented as a bridge pattern with either no-serial, the debugger-serial, or usb-serial.

I used the usbd_ble_uart example as a starting point then ripped out all of the bluetooth code just leaving the usb serial I/o. That leaves the usb port event handler and the usb I/o event handlers — so all interrupt driven and neat and clean. My implementation is too rough to put onto Github atm.

Mark