[stdcall83]

I implemeted xmodem over uart as a recovery boot for several SOCs in my career. It's a simple protocol you can actually fit into a small boot ROM and test using RTL. We use it only to load the rest of the bootloader which is few hundreds of kb. Then we switch to networking using rj45 for the rest of the bootstrapping. When using xmodem 1k with higher bps you can get to a decent speed.