I have a gizmo that is based around an ATMEGA16. And there's a particular pot that if I twist it, it can cause the AVR to reset... sometimes. The behavior is a bit weird. The pot has about 1.7V across it. I was assuming there was some sort of (intermittent) short between it and the reset line, but, I couldn't find anything obvious either through visual inspection or electrically. It also doesn't really make much sense... because it fails when I INCREASE the voltage on the wiper... So if I was somehow shorting the wiper to reset, why wouldn't it fail when the wiper was closer to ground.
But I decided to try probing other stuff. The RESET pin only goes to a JTAG connector which is right next to the AVR. I tried probing the RESET line with my multimeter (Fluke 88 -- specs say 10M input impedance and < 100pF cap). When I touched my probe to the reset line, the chip reset. And it was reading ~ 4.75V. Interestingly, VCC is about 5.04V. So if Fluke is to be trusted that means that the AVR is pulling the RESET line up with 650k, and the capacitance in the probe was enough to pull the RESET line down long enough to reset. This all strikes me as a bit... weird.
Is the RESET line really that weakly pulled up that a reset can be triggered so easily or is this a defective part?