Weird SignalTap behaviour

Metastability was mentioned as one possible cause.

It is related to the present discussion because it precisely matches OPs behavior.

I think you are on to something on the data transmission from JTAG as I have observed that the glitches occur in different signals at the same time - so JTAG signal integrity had occurred to me as well. I will try "read data" at my next opportunity.

Thanks, SysTom

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The previous discussion was about unconstraint and possibly too fast Signaltap clock rather than metastability. I explicitly mentioned that metastability won't cause fake signal transitions for stable signals.

Not knowing any details about your design, it's impossible to relate it to the present discussion.

I rarely see unexpected behavior of Signaltap. Few cases I remember are failure of trigger or storage qualifier logic with signals asynchronous to the Signaltap clock. I don't remember a case where a known stable signal showed fake transitions, except for situations where the acquired signal couldn't be correctly transferred due to JTAG signal quality issues. In the latter case, I got different recording each time the signal was retransmitted by pressing "read data".

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