Writing a USB stack



The USB protocol stack can be laid out in 2 or three levels
Hardware management (which may include low-level protocol)
Optionally, common protocol procedures
The actual device code (usb-serial, network, storage)

Then, higher levels (UDP/IP, FATFS, whatever) will be generic
The USB device driver will offer a non-usb API

Most USB implementations are state machines with callbacks
This can happen based on interrupts
Or you can just poll the status bits