SLAU533D September 2013 – April 2017
USB_setup() is called next. This initializes the USB API and enables all of the USB events. It then checks to see if a USB host is already attached, which it determines by the presence of 5 V on the VBUS pin. If attached, it pulls the D+ signal high, telling the host it is there. The host responds by enumerating the device.
Finally, global interrupts are enabled, and execution enters the main loop.