Support serial over USB (CEC ADC)

[negz]

I'd like to have a log.zig logFn that I could use to emit debug logs over USB, rather than UART.

Currently Microzig has support for acting as a USB HID device. I believe emitting debug logs over USB would require support for acting as a "serial device", which uses USB CDC/ADC.

[negz]

https://github.com/tinygo-org/tinygo/blob/731532cd2/src/machine/usb.go#L261

I'm attempting to take a shot at this, using TinyGo's implementation as a reference. In part because it seems more minimal than alternatives like TinyUSB. I believe TinyGo have implemented just enough of CEC to be a serial device, not support for modems etc. I'm also trying to use TinyGo as a reference because I'm fluent in Go, but not Zig, or microcontrollers, or USB. 😄

[rosshadden]

Any luck with this? I read that picotool needs the firmware running on a microcontroller to implement USB-CDC in order for its load --force argument to work (letting you reboot picos outside of BOOTSEL mode). When using C you can set pico_enable_stdio_usb(<project-name> 1) in CMakeLists.txt to accomplish this.

Apparently this is because picotool uses PICO_STDIO_USB_ENABLE_RESET_VIA_VENDOR_INTERFACE though I don't know enough of how this stuff works to provide any of my own insight.

[negz]

Unfortunately I didn't get very far before my vacation ended.

I was finding it pretty hard to figure out how to cleanly extend the existing USB implementation. A lot of this is probably just because I don't know Zig, but I think it was exacerbated by the fact that the implementation isn't designed for extensibility.

My understanding based on ZigEmbeddedGroup/raspberrypi-rp2040#40 is that the USB implementation is a port of https://github.com/cbiffle/rp2040-usb-device-in-one-file/blob/4878f4/src/main.rs, which per its header comments is intended "for reference and study".

Here's what I ended up with. It's mostly just all the structs that I think are needed to make CDC work: 846e994e