USB generally needs a crystal or at least a ceramic resonator to meet its timing precision specs, though apparently Dmitry is getting by without one here. This

USB generally needs a crystal or at least a ceramic resonator to meet its timing precision specs, though apparently Dmitry is getting by without one here. This commonly adds extra cost to USB implementations, because dedicated silicon isn't sufficient; you also need some precisely tuned silicon dioxide or similar. In V-USB, usbdrv/.[ch] contains 1440 unique lines. The bitbanging stuff is mostly in .S *.inc, so correct me if I'm wrong, but I think this is roughly the non-timing-related complexity imposed by USB protocol stack. (This division is not perfect, because there are things in e.g. usbdrvasm.S which have nothing to do with bitbang timing, but I feel like it's a reasonable approximation.) The remaining complexity in, say, examples/hid-mouse/firmware/main.c is only a few dozen lines of code. And that's a USB device. Implementing a USB host is at least another order of magnitude more complexity. You definitely don't need 1000+ lines of code to implement, say, the PS/2 mouse protocol. From either side. So, while I agree that a lot of the difficulty of bitbanging USB results from its tight timing constraints, I don't agree that what's left over is "trivial".