AVR-Chat

--- In AVR-Chat@yahoogroups.com, STEVEN HOLDER <s.holder123@...> wrote:
> The wave form at this pin is a little odd, it looks as though the waveform  is being integrated by something switching the reference, possibly through the on board reference select fet and some parasitic capcitance that is 'seen' at this pin, it seems to be very periodic (not noise) and consistant.

The AREF waveform is the sourcing transistor turning on when AREF gets below 2.56V, sourcing lots of current into the pin capacitance, and overshooting by ~2V. The pin then discharges relatively slowly at around 140uA, until the cycle repeats. The 2MHz observed matches a 140uA discharge rate of 35pF, which seems like a reasonable ballpark figure for AREF internal capacitance.

This is the same waveform you get if you pick a voltage regulator that requires capacitive load on the output, and neglect to give it this load.

When I put capacitance on AREF, the slew rate of the charge/discharge changes (and thus the amplitude and frequency drops), and around 300pf-1nF, depending on the part, it becomes stable. 

/Kasper Pedersen