/*
Hardware Prerequisites:
=======================
USB lines D+ and D- MUST be wired to the same I/O port. We recommend that D+
triggers the interrupt (best achieved by using INT0 for D+), but it is also
possible to trigger the interrupt from D-. If D- is used, interrupts are also
triggered by SOF packets. D- requires a pull-up of 1.5k to +3.5V (and the
device must be powered at 3.5V) to identify as low-speed USB device. A
pull-down or pull-up of 1M SHOULD be connected from D+ to +3.5V to prevent
interference when no USB master is connected. If you use Zener diodes to limit
the voltage on D+ and D-, you MUST use a pull-down resistor, not a pull-up.
We use D+ as interrupt source and not D- because it does not trigger on
keep-alive and RESET states. If you want to count keep-alive events with
USB_COUNT_SOF, you MUST use D- as an interrupt source.

As a compile time option, the 1.5k pull-up resistor on D- can be made
switchable to allow the device to disconnect at will. See the definition of
usbDeviceConnect() and usbDeviceDisconnect() further down in this file.