Does anyone have details (hopefully a schematic) of the reset spike filter? I've been testing ADC0 on 8-pin tinies like the t13 and t85, without disabling reset. I'm getting noise, about 3LSB, that I don't see on other ADC inputs. For example just reading ADC0 with nothing connected gives a reading of 1023 only about 50% of the time, whereas ADC1, 2, & 3 all give a consistent, stable, and noise-free reading of 1023 with the pullup enabled.
The "noise" seems to have a pattern, so I suspect there's some sort of RC oscillation going on between capacitance in the reset spike filter and the ~10pf S/H ADC capacitor. Adding an external 10nf capacitor does not eliminate the noise.