but this is not what im talking about. it was external signals coming from outside the chip and not so much self-interference. specially if there were long wires from external serial. even 50ohm or 100ohm suppressed so im guessing its transmission line ringing. probably really skinny hv pulses causing clamp zeners to forward bias. that can show up in completely unrelated areas of the chip and totally unpredictable. even 100k+ worked but because im fond of avr pull-ups 1-2k would be a better choice for the series resistor there. but like i said we decided on pcb shielding instead in the case of the m1284p. btw as others noted m1284 in the same socket did not show the problem.
Ok, thanks, John, external signals. "even 50ohm or 100ohm suppressed" - is this on the pcb or at
the source? There is such a thing as "source series termination", which is using a 50R series-R at
the source end to quench ringing on a long line, and it works really really well.
There is also an issue with the RX0 pin floating. On my proto-test pcb, I use an FTDI cable, and
when it was disconnected, I found the chip would reset and reboot by touching the RX0 pin [no
other pins did this, except sometimes the xtal pins]. A pullup fixed the problem.