Why must you query .mean/.std_dev on POCV attributes instead of the bare attribute?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Because under POCV a slack or arrival attribute is a distribution rather than a scalar, so there is no single value to return - the bare attribute yields an empty list {}. .mean returns the average and .std_dev returns the sigma.
Complete Technical Explanation
In a deterministic analysis an arrival or slack attribute is one number, so querying it directly makes sense. Under POCV it is not a number at all - it is a statistical distribution, and the tool has nothing sensible to hand back as a scalar. Querying the bare attribute therefore returns an empty list, shown as {}, which people frequently misread as "the attribute does not exist" or "POCV is not enabled". It does exist; you simply have to ask for a specific statistic of it. The .mean sub-attribute returns the average of the distribution and .std_dev returns the sigma. This convention applies across the POCV attributes: the {min,max}_{rise,fall} _variation_{arrival,slack} family on pins, and variation_arrival and variation_slack on timing paths and timing points. So the pattern in a script is always to append the statistic you want rather than to read the attribute raw. Practically this also tells you something about how to write POCV post-processing: because you get mean and sigma separately, any sorting, thresholding or margin arithmetic you do in Tcl has to be explicit about which statistic it is using, and a script written for deterministic slack will silently produce empty results rather than wrong ones when pointed at a POCV run.
In Pd / Signoff
You hit this the first time you write a Tcl script to collect POCV slacks for a block summary. The loop returns nothing until you query variation_slack.mean and variation_slack.std_dev, and then you can rank endpoints by mean slack or by sigma to see which failures are variation-driven rather than nominal.
Common Trap
Reading the empty list {} from the bare attribute as evidence that POCV is off or the attribute is unsupported, and abandoning the query. The distribution is there - only .mean and .std_dev can be returned as scalars, and a script that forgets the suffix fails silently with empty output rather than an error.
Expect Next
Which POCV attributes does this mean and std_dev convention apply to on pins as opposed to timing paths?
Topic And Primetime Commands
Variation / POCV Distributions. variation_arrival, variation_slack
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