simpkins / ausb

Adam's USB Stack

Adam's USB Implementation

This is a USB stack for embedded devices.

I wrote it as an alternative USB stack for ESP32 devices, rather than using tinyusb. This is mainly just for some hobby projects of mine.

This only supports a few device classes and hardware platforms. I am primarily using it to implement HID class devices, and non-HID functionality hasn't been very well tested. If you are looking for a more full-featured, robust USB stack you probably should use tinyusb instead. tinyusb is much more mature, and is officially supported by several vendors.

Motivation

My primary motivation for writing this library was since I don't trust the synchronization model of tinyusb on FreeRTOS, and particularly the way ESP-IDF recommends using it. To be fair, it appears like tinyusb was designed for simple single-core MCUs, and FreeRTOS support was added later. However, the model they use in their example code, with USB state be manipulated from many tasks possibly simultaneously, doesn't seem safe, particularly on multi-core MCUs. When I've submitted some bug reports Espressif has indicated that they are working on a re-write of the tinyusb DCD implementation for ESP32. In the meantime, I figured I might as well tackle just building my own stack.

C++

AUSB is implemented in C++. It doesn't use RTII or exceptions, since it is intended to also play well when used with pure-C code.

Compared to pure C, C++ provides a number of additional convenience functionality. C++ also allows writing constexpr functions that can perform compile-time construction of USB descriptors, which can then be embedded in the firmware's read-only data section. This seems nicer than using C macros or a completely separate build-time tool for generating descriptors.

I did consider using Rust, since Rust has growing support for embedded devices. However, one factor that caused me to not use Rust is that I do have some older projects using 8-bit AVR MCUs (atmega32U4), and from a cursory investigation it seems unlikely that Rust will be usable on these 8-bit microcontrollers. At the moment AUSB is using some standard library functionality from ESP-IDF, but I have generally designed it so that it should be able to work on other MCUs without any standard library support or any memory allocation capabilities. (At the moment some transaction buffers are allocated dynamically, but this should be easy to replace with a static buffer since there is only ever a single transaction in flight at a time.)