I have two F103ZGT boards from a Chinese OEM. I can config the boards to start the ROM boot loader, via the COM port. This works fine on one board, the other always took several hard resets before the ZGT identified itself to the loader utility. I was looking at Option Bytes, but not as far as I know saving anything, and now the boot loader won’t start at all. I was able to load my DFU code before the problem arose, and the board app can be updated via USB, so it is functional. Is it possible I wrote some option bytes and this has disabled the boot loader, or is there a chip/board problem? Maybe the LF oscillator? it seems to be functioning on my ‘scope.