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F107 USB VBUS clash with uart1 TX

Question asked by goosen.kobus.001 on Mar 13, 2015
Latest reply on Mar 16, 2015 by goosen.kobus.001
I have a board based on the STM32F107 where the VBUS pin is unused, and USART1 TX is used on PA9. This has worked fine for me up until now, as I used a pin previously to define whether the CI should respond to USB or UART, and in general the device would be in either state. Because this worked so well, I idly put the issue of USB detection out of my mind and a few months later updated the PCB, now without the selection pin with the idea that the micro responds to whatever port the message came from. 
Now the problem is that I can't actually use both ports simultaneously because every time I send something over the uart, the USB disconnects! What's more, I no longer have a concrete way to determine if the USB is connected...
As mentioned in I have tried to disable VBUS sensing in the USB stack, but this simply disables the callback functions, the device still disconnects. 
I can confirm that this happens both on the PC (I can hear the disconnect sound) and the micro(the dive enters the disconnect callback if enabled).
What's More I'm relatively certain this is not a firmware issue because I created a cubeMX project with the same uart/usb configuration and the code it generates does the same. 

What I'd like to know is if there is any way to tell the USB hardware to disregard what's happening on the uart pin. Barring that, is there any way to know if the device is connected without the VBUS?