The USB submodule of libmaple is a separate piece of the codebase for
reasons that are largely historical.
Current Status:
There's only support for the USB device peripheral found on
STM32F103s.
We rely on the low level core library provided by ST to implement
the USB transfer protocol for control endpoint transfers.
The virtual com port (which is exposed via
<libmaple/usb_cdcacm.h>) serves two important purposes.
1) It allows serial data transfers between user sketches an a
host computer.
2) It allows the host PC to issue a system reset into the DFU
bootloader with the DTR + RTS + "1EAF" sequence (see
leaflabs.com/docs/bootloader.html for more information on
this).
After reset, Maple will run the DFU bootloader for a few seconds,
during which the user can begin a DFU upload operation (uploads
application binary into RAM/FLASH). Thus, 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 (e.g. to use faster isochronous enpoints for streaming
audio, or implement the USB HID or Mass Storage specs), then
ensure that you leave some hook for resetting Maple remotely in
order to spin up the DFU bootloader. Please make sure to get
yourself a unique vendor/product ID pair for 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 storage 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 enabling the USB peripheral isn't "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, a USB_DISC pin is
asserted (e.g. on Maple, this is PC12). Alternatively, the NVIC
can be directly configured to disable the USB LP/HP IRQ's.
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.
TODO:
- Generic USB driver core with series-provided backends, like
libopencm3 has.
- Strip out ST code.
- Integration with a high level USB library (like LUFA/MyUSB) to
allow users to write custom USB applications.
