oric_dan:
A dozen - that's pretty serious. Can you explain the bit about "fast signals nearby" a little more.
What exactly are you talking about here? I'm not even sure what fast signals on an Arduino even
means, as I/O is pretty slow in general. The crystal oscillator is by far the fastest. Thanks.
actually avr has faster io than most other chips of its type. not high frequency, high drive capability. it contributes to fast rise and fall times and avr are very high current. the xtl signal is quite slow by comparison and may actually be a sine wave if ckopt aint enabled.
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.