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STM32F207 USB device with no VBUS monitoring

Question asked by Bettega Stefano on Jun 10, 2014
Latest reply on Feb 25, 2015 by Tim B.
Hi all,
I'm modifying the VCP demo for STM32F207 to work without VBUS sensing, just using D+/D- for USB communications. As stated in RM0033 (page 935/1334 Doc ID 15403 Rev 5, chapter 29.5 USB peripheral): “The VBUS pin can be freed by disabling the VBUS sensing option. This is done by setting the NOVBUSSENS bit in the OTG_FS_GCCFG register. In this case the VBUS is considered internally to be always at VBUS valid level (5 V).

So I modified my usb_conf.h and commented the line

#define VBUS_SENSING_ENABLED

Code is perfeclty working on STM3220G evaluation board, but the same code loaded on our board does not work: device is not detected and inspecting USB_OTG_dev variable revelas that device does not change its status from the initial one.

I'm using USB FS, and the only difference between STM3220G board and our board is that PA9 pin is tied to VBUS only on STM3220G, while our board leaves it unconnected.

My question is: as VBUS is not sensed, how the USB peripheral detects host enumeration and exits the initialization state to become fully operative?

Thanks in advance,
Stefano

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