oric_dan:
In the end, after everything these mega1284 threads have been through, I'm still
not certain why many people are having the uploading problem. I haven't needed
the low-pass filter on RX0, and my tests earlier indicated I couldn't even reproduce
the upload problem by running the RX0 line directly through the crystal area, or
under and between the xtal pins. So ????
...........However, I did discover a sensitivity that I forgot to mention earlier. On my mega1284
proto-test board, I am using a 6-pin header to connect the FTDI cable and FTDI Friend.
Otherwise, the RX0 pin was floating.I found you "definitely" need to have a pullup on RX0, for when the FTDI interface is
not connected. This stands to reason, of course, as you will otherwise get noise
injected on that pin. However, unlike essentially every "other" pin on the chip, which
will be floating unless specifically terminated, noise coming in on RX0 will actually reset
the chip.I did this test by touching each of the 1284 pins with my finger [my usual try to kill
the chip test]. Doing this had no effect for other pins, but touching the floating RX0
pin "always" reset the processor immediately. So, this is interesting....
I believe this is the same failure mode/symptom that people having trouble uploading via RX0, noise being injected via RX0 corrupting internal data bus.
Lefty