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64 lines
2.7 KiB
Plaintext
64 lines
2.7 KiB
Plaintext
The USB submodule of libmaple is a separate piece of the codebase for
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reasons that are largely historical.
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Current Status:
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There's only support for the USB device peripheral found on
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STM32F103s.
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We rely on the low level core library provided by ST to implement
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the USB transfer protocol for control endpoint transfers.
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The virtual com port (which is exposed via
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<libmaple/usb_cdcacm.h>) serves two important purposes.
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1) It allows serial data transfers between user sketches an a
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host computer.
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2) It allows the host PC to issue a system reset into the DFU
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bootloader with the DTR + RTS + "1EAF" sequence (see
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leaflabs.com/docs/bootloader.html for more information on
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this).
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After reset, Maple will run the DFU bootloader for a few seconds,
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during which the user can begin a DFU upload operation (uploads
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application binary into RAM/FLASH). Thus, without this virtual com
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port, it would be necessary to find an alternative means to reset
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the chip in order to enable the bootloader.
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If you would like to develop your own USB application for whatever
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reason (e.g. to use faster isochronous enpoints for streaming
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audio, or implement the USB HID or Mass Storage specs), then
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ensure that you leave some hook for resetting Maple remotely in
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order to spin up the DFU bootloader. Please make sure to get
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yourself a unique vendor/product ID pair for your application, as
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some operating systems will assign a host-side driver based on
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these tags.
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It would be possible to build a compound USB device, that
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implements endpoints for both the virtual COM port as well as some
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other components (mass storage etc.). However, this turns out to
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be a burden from the host driver side, as Windows and *nix handle
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compound USB devices quite differently.
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Be mindful that enabling the USB peripheral isn't "free." The
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device must respond to periodic bus activity (every few
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milliseconds) by servicing an ISR. Therefore, the USB application
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should be disabled inside of timing critical applications.
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In order to disconnect the device from the host, a USB_DISC pin is
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asserted (e.g. on Maple, this is PC12). Alternatively, the NVIC
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can be directly configured to disable the USB LP/HP IRQ's.
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The files inside of usb_lib were provided by ST and are subject to
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their own license, all other files were written by the LeafLabs
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team and fall under the MIT license.
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TODO:
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- Generic USB driver core with series-provided backends, like
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libopencm3 has.
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- Strip out ST code.
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- Integration with a high level USB library (like LUFA/MyUSB) to
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allow users to write custom USB applications.
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