The USB submodule of libmaple is responsible for:

    Initializing the USB peripheral, scaling the peripheral clocks
    appropriately, enabling the interrupt channels to USB, defining
    the USB isr, resetting the USB disc pin (used to tell the host
    were alive). Additionally, the USB submodule defines the virtual
    com port USB applications that is available to all user sketches
    via Usb.print() and others.

To use it:
   Call Usb.init() to enable the IRQ channel, configure the clocks,
     pull down usb_disc, and setup the vcom buffers
   Usb.print/ln, available(), read(), write() implement the same
     interface as Serial1/2/3
   
   
Current Status:
    Currently, the USB submodule relies on the low level core library
    provided by ST to access the USB peripheral registers and
    implement the USB transfer protocol for control endpoint
    transfers.  The high level virtual com port application is
    unfortunately hard to untangle from this low level dependence, and
    when a new USB core library is written (to nix ST dependence)
    changes will likely have to be made to virtual com application
    code. Ideally, the new core library should mimick the form of
    MyUSB (LUFA), since this library (USB for AVR) is growing in
    popularity and in example applications. Additionally, the USB lib
    here relies on low level hardware functions that were just ripped
    out of the bootloader code (for simplicity) but clearly this
    should be replaced with direct accesses to functions provided
    elsewhere in libmaple.

    The virtual com port serves two important purposes. 1) is allows
    serial data transfers between user sketches an a host computer. 2)
    is allows the host machine to issue a system reset by asserting
    the DTR signal. After reset, Maple will run the DFU bootloader for
    a few seconds, during which the user can begin a DFU download
    operation ('downloads' application binary into RAM/FLASH). This
    without this virtual com port, it would be necessary to find an
    alternative means to reset the chip in order to enable the
    bootloader.

    If you would like to develop your own USB application for whatever
    reason (uses faster isochronous enpoints for streaming audio, or
    implements the USB HID or Mass Storage specs for examples) then
    ensure that you leave some hook for resetting Maple remotely in
    order to spin up the DFU bootloader.  Please make sure to give
    yourself a unique vendor/product ID pair in your application, as
    some operating systems will assign a host-side driver based on
    these tags.

    It would be possible to build a compound USB device, that
    implements endpoints for both the virtual COM port as well as some
    other components (mass sotrage etc.) however this turns out to be
    a burden from the host driver side, as windows and *nix handle
    compound USB devices quite differently.

    Be mindful that running the USB application isnt "free." The
    device must respond to periodic bus activity (every few
    milliseconds) by servicing an ISR. Therefore the USB application
    should be disabled inside of timing critical applications. In
    order to disconnect the device from the host, the USB_DISC pin can
    be asserted (on Maple v1,2,3 this is GPIOC,12). Alternatively, the
    NVIC can be directly configured to disable the USB LP/HP IRQ's

    This library should exposed through usb.h, do not include any
    other files direcly in your application.

    The files inside of usb_lib were provided by ST and are subject to
    their own license, all other files were written by the LeafLabs
    team and fall under the MIT license.

Integration with libmaple:

    The current USB lib is ported directly from the maple bootloader
    code, adapted to be a virtual com rather than a DFU device. That
    means several functions are redefined locally that could have been
    pulled from elsewhere in libmaple. Thus, ths USB module depends
    absolutely zero on libmaple, it even ensures that clocks are
    configured correctly for its operation.

Todo:

    - write custom low level USB stack to strip out any remaining
      dependence on ST code
    - remove dependence on hardware.c, since any functions here really
      should have their own analogues elsewhere inside libmaple
    - add a high level USB application library that would allow users
      to make their own HID/Mass Storage/Audio/Video devices.
    - implement a Usb.link(SerialX) that forces a passthrough
      the host computer virtual com to SerialX, and utilizes the
      line_config commands correctly (sets baud etc)