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15996 Discussions

Can SignalTap ignore periods of inactivity?

New Contributor II

Hi. I have some UART code that sends text. My top level module sends a sequence of (English) words to the UART. The UART runs at 9600 Baud which is really slow compared to the FPGA's 50MHz clock.

What I want to do is monitor the state of the top level module but I am not interested in the hundreds (if not thousands) of clock cycles where there are no changes to my top level module's state. I also don't have enough memory on my FPGA to record enough samples between the transmission of individual words. I have the trigger set so that I don't start recording until I move out of my idle state but there are still too many clock cycles to capture. I can't use a sample depth of more than 16k.

Is there anyway in which SignalTap can be configured to ignore these clock cycles of inactivity?

The only other way I can think of diagnosing this problem is to create some sort of pseudo-clock signal that only oscillates a few 50MHz cycles either side of my top module's state transitions. Is that the only way to get around something like this?

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2 Replies
Honored Contributor III

Yes.  Use one of the Storage Qualifier options to choose what samples you want to store in the buffer.  You have it enabled in your screenshot using the Conditional option.  Set values in the Storage Qualifier column and when the conditions are true, a sample gets stored.  You could use the Transitional qualifier to store samples only when particular signal(s) change state.  This sounds like what you want.

You already have the "Record data discontinuities" option enabled.  This will show you in the output data where samples were skipped and not stored because of your qualifier.

This training goes into detail on storage qualification:

New Contributor II

Thanks for the info. Transitional definitely sounds the way to go. I'll be sure to give that a try next time. I did manage to use the Storage Qualifiers some weeks ago but I tend to find when I go back to SignalTap after a period of time, I can never get things working again. Blame that on my notes I guess.