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Bit 21 NOVBUSSENS: VBUS sensing disable option
When this bit is set, VBUS is considered internally to be always at VBUS valid level (5 V).
This option removes the need for a dedicated VBUS pad, and leave this pad free to be used for other purposes such as a shared functionality.
VBUS connection can be remapped on another general purpose input pad and monitored by software.
This option is only suitable for host-only or device-only applications.
0: VBUS sensing available by hardware
1: VBUS sensing not available by hardware.
如果需要使用 VBUS 引脚作为其他功能，NOVBUSSENS = 1
According to the reference manual for the STM32F205, page 945, you want to do the following:
First, you want to disable the VBUS sensing option.
This is done by setting the NOVBUSSENS bit in the OTG_FS_GCCFG register.
In this case VBUS is considered internally to always be at VBUS valid level (5 V),
so you don‘t need to run a trace from the VBUS pin on the USB connector over to the STM32F205 at all.
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原文地址：http://www.cnblogs.com/shangdawei/p/4334250.html
