Problem with swinging Atmel AVR comparator

Hello Wolfgang,

Your name and the word "swinging" makes me believe you could be German. Anyway, you probably meant oscillation (swinging sounds nicer...). Can you post more information?

When comparators are run as zero crossers and there is no hysteresis their output will be pretty wild. It depends a bit on the offset of the particular chip or uC. Normally it should look like random noise on the scope. Do you see a real oscillation, like a certain more or less fixed frequency? If yes, then the input could somehow "see" the output or couple into it inductively. Traces too close etc. Remember that the gain of a comparator is huge. It's basically an opamp with no feedback so

Is anything connected to its output or is that used inside the uC only?

Gruesse, Joerg

No, there is no oszillation with a definite frequency. It seem, that the comparator could very easyly disturbed. Therefor I=B4m searching for someone with experience with the ATMEGA88 controller.

Best regards

Wolfgang

Hello Wolfgang,

Well, maybe there is an expert on that uC here who can shed some light. Anyway, comparators without a hysteresis (resistor for positive feedback) are bound to be "wild".

How wild? Depends on the offset. There will be a spec on the data sheet that says how much that can be and it varies between batches of chips. So one uC's comparator might be kind of quiet because it has a large offset. Another might produce lots of noise because it happens to be very low in offset. The noisy one is actually the better one, more sensitive.

Regards, Joerg