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+The USB submodule of libmaple is responsible for:
+ initilizing 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) \ No newline at end of file