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STM32F072 - Proper USB interface circuit

Question asked by west.charles on Sep 1, 2015

I am developing using the STM32F072C8T6.  I was hoping to use USB communication and the hardwired boot loader for initial programming of the chips.  I am running into some issues, however, and was wondering if anyone had used it before.

In my (nonfunctional) application, I connected the USB_DM, USB_DP and voltage/ground and left the ID pin floating.  I didn't have any resistors, as the documentation seemed to indicate that they weren't needed.  My device is getting power correctly, but is completely ignored by the computer even when it is restarted with the BOOT pin held high.

Looking at the schematic for the STM32F072-Discover board, they seem to have connected a 100K resistor to the ID pin.  All of the documents I've read seem to indicate that this should only be done if the device is a USB host (which it is not).  However, the bootloader shows up using the demo board.

If I may ask, does anyone have a success story/schematic that they could share?  Alternatively, does anyone know why there is a 100K resistor connecting the ID pin to ground in the eval board (instead of leaving it floating)?

Charlie West