重新设计了USB硬件，兼容所有F4系列64Pin封装芯片
但下载USB程序，PC检测不到USB设备
看了下ST提供的代码，主要是以下几个问题
- 晶振问题
库使用的是25MHz晶振，我焊接的是8MHz晶振
代码修改如下- #define PLL_M 8
- #define HSE_VALUE ((uint32_t)8000000) /!< Value of the External oscillator in Hz /
- VBUS检测问题
默认情况下，STM32用PA9来检测VBUS电压，并设置对应的上拉模式
所以可以在OTG_FS_GCCFG寄存器中，置位NOVBUSSENS，关闭VBUS检测功能，强制使用上拉模式
这样，外部DP，DM就不需要使用1.5K上拉电阻
手册摘录如下：
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.
参考资料