Kate Temkin's Luna project is super cool, although I think it's still a WIP: https://github.com/greatscottgadgets/luna https://github.com/greatscottgadgets/luna

Kate Temkin's Luna project is super cool, although I think it's still a WIP: https://github.com/greatscottgadgets/luna https://github.com/greatscottgadgets/luna It's a USB stack implemented as pure gateware - no hard/soft CPU needed, although I think you can hook one up if you want to.