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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.
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.
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
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