A substantial advantage of the Arduino Due MCU is the native USB High Speed interface allowing up to 480 Mbps transfers. Another option are the costlier Teensy boards that are really mature and provide a solid framework for USB bulk endpoints data interfaces not limited to ttyACM Serial over USB interfaces. However, it is absolutely possible to integrate a vendor (custom) bulk pair of endpoints (IN and OUT) on Arduino Due, it’s just that there is not much documentation besides examples such as the MIDI USB library and articles about it.<\/p>\n\n\n\n
In this article, we leveraged the power of LLMs advices and validated them through testing to add support for an additional pair of endpoints, such that the native SerialUSB object could still be used (for telemetry, communication control or debugging)<\/p>\n\n\n\n
Using a custom pair of endpoints has several advantages over SerialUSB.<\/p>\n\n\n\n
On the downside, detection of host ready or not conditions can be different or tricker than ttyACM, as the linestate logic (DTR emulation) is not present on a custom bulk interface. The idea is thus to keep the Serial USB link for telemetry, control, and debugging, and initiate data transfer once the proper Serial USB transmission. Note that on linux detecting a DTR down condition on SerialUSB does not seem to work as well as on MS Windows from a MCU use case perspective, This all situation needs unfortunately to resort to watchdogs to re-establish a broken connection when the host is electrically connected. but not listening.<\/p>\n\n\n\n
Also ttyACM or the tty stack seem to behave on linux so as to limit to 64 bytes max the data fetched fro; each read() call on the port. Since USB 2.0 max packet size is 512 byte, libusb allows more efficient call economy, although the mechanism is largely different, since IO read calls are not used, but library specific “transfer submits”.<\/p>\n\n\n\n
In our case, the packet size is 96 byte. All writes to USB from the MCU sides will be 96 byte. Note that packet sizes and URB sizes, ZLP (zero length packets) and libusb “transfer” sizes concepts are important as understanding and managing them properly can save you a lot of headaches.<\/p>\n\n\n\n
libusb also has a quite steep learning curve. However, through LLM assistance, we successfully made such an interface.<\/p>\n\n\n\n
Note that for “real time” transfers or transfers with timing constraints (continuous flow) a separate FIFO (sched_fifo) thread is preferable on the host side. This will be shown in that example.<\/p>\n\n\n\n
Optimized for 1 to 4 MB\/s transfers.<\/p>\n\n\n\n
The latest Arduino Due (SAM) framework available in Arduino IDE and Platform IO provide helper objects (Pluggable USB), defines \/ macros to help in creating additional endpoints.<\/p>\n\n\n\n
First we will need some defines that will setup USB endpoint types configuration bitmasks. Here comes the first versatile configuration as these elements are not exposed for the SerialUSB interface.<\/p>\n\n\n\n