WebUSB Serial Support¶
To date, this has only been tested on one port (esp32s2), on one board (espressif_kaluga_1).
What it does¶
If you have ever used CircuitPython on a platform with a graphical LCD display, you have probably already seen multiple “consoles” in use (although the LCD console is “output only”).
New compile-time option CIRCUITPY_USB_VENDOR enables an additional “console” that can be used in parallel with the original (CDC) serial console.
Web pages that support the WebUSB standard can connect to the “vendor” interface and activate this WebUSB serial console at any time.
You can type into either console, and CircuitPython output is sent to all active consoles.
One example of a web page you can use to test drive this feature can be found at:
How to enable¶
Update your platform’s mpconfigboard.mk file to enable and disable specific types of USB interfaces.
CIRCUITPY_USB_HID = xxx CIRCUITPY_USB_MIDI = xxx CIRCUITPY_USB_VENDOR = xxx
On at least some of the hardware platforms, the maximum number of USB endpoints is fixed. For example, on the ESP32S2, you must pick only one of the above 3 interfaces to be enabled.
Original espressif_kaluga_1 mpconfigboard.mk settings:
CIRCUITPY_USB_HID = 1 CIRCUITPY_USB_MIDI = 0 CIRCUITPY_USB_VENDOR = 0
Settings to enable WebUSB instead:
CIRCUITPY_USB_HID = 0 CIRCUITPY_USB_MIDI = 0 CIRCUITPY_USB_VENDOR = 1
Notice that to enable VENDOR on ESP32-S2, we had to give up HID. There may be platforms that can have both, or even all three.
CircuitPython uses the tinyusb library.
The tinyusb library already has support for WebUSB serial. The tinyusb examples already include a “WebUSB serial” example.
Sidenote - The use of the term "vendor" instead of "WebUSB" was done to match tinyusb.
Basically, this feature was ported into CircuitPython by pulling code snippets out of the tinyusb example, and putting them where they best belonged in the CircuitPython codebase.
There was one complication:
tinyusb uses C preprocessor macros to define things like USB descriptors.
CircuitPython uses a Python program (tools/gen_usb_descriptor.py) to create USB descriptors (etc.) using “helper objects” from another repo (adafruit_usb_descriptor). This means some of the example code had to be adapted to the new programing model, and gen_usb_descriptor gained new command-line options to control the generated code.
The generated files go into the “build” directory, look for autogen_usb_descriptor.c and genhdr/autogen_usb_descriptor.h.
Also worth pointing out - the re-use of the CDC connect/disconnect mechanism is not actually part of the WebUSB standard, it’s more of “common idiom”. We make use of it here because we need to know when we should be paying attention to the WebUSB serial interface, and when we should ignore it..
Possible future work areas¶
The current code uses the existing Python infrastructure to create the Interface descriptor, but simply outputs the code snippets from the original tinyusb demo code to create the WEBUSB_URL, BOS, and MS_OS_20 descriptors. I suppose additional work could be done to add these to the adafruit_usb_descriptor project, and then gen_usb_descriptor.py could be modified to make use of them.
Program gen_usb_descriptor.py creates objects for most interface types, regardless of whether or not they are actually enabled. This increases the size of a generated string table. I made the new vendor-interface-related code not do this (because some of the ARM platforms would no longer build), but I did not go back and do this for the other interface types (CDC, MIDI, HID, etc.) Some FLASH savings are probably possible if this is done.