@lefty, yes thanks I did find that thread and review it. My first 1284P boards arrived earlier today! The first one has just been running for maybe an hour now, but I'm happy to report that I don't see any sign of this issue where serial input causes a reset, even at 3.3V. It doesn't even seem to mind having the supply voltage switched on the fly, which is consistent with my experience trying similar shenanigans on an ATmega328P.
