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AM335x里没有指定的寄存器查询设备连接状态，是通过检测DP/DM管脚状态来判断。
Assuming that the voltage level of the USBX_VBUSIN is found to within the Vbus Valid range, the USB 2.0 host controller will wait for a device to connect; that is for it to see one of its data lines USB0/1_DP/DM be pulled high.
