Explain, at the edge level, why a simultaneous source edge is not counted as the launch edge in the different-clock setup analysis, and why that's conservative-correct.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
For a capture edge at time t, PrimeTime pairs the nearest source edge strictly before t, because data launched at t cannot also be captured at t. Reaching back to the previous edge yields the larger, more restrictive requirement.
Complete Technical Explanation
PrimeTime builds the different-clock setup relationship by taking each capture edge at some time t and pairing it with the nearest source edge occurring strictly before t. A source edge at exactly t is excluded, and the reason is physical: in real hardware, data launched by an edge cannot also be validly captured by that same-instant edge, because there is no time for it to propagate through the launch flop and the logic between them. Treating the simultaneous edge as the launch would allot the path zero time and would describe nothing the hardware ever does. Excluding it forces PrimeTime to reach back to the previous source edge, and in the cases where this matters that yields a larger, more restrictive launch-to-capture requirement - the conservative and therefore safe choice for a setup check. The standard mixed-period case shows it directly: with CLK1 at period 10 and CLK2 at period 25, the capture at 30 pairs with the launch at 20, not with the simultaneous source edge at 30. The pairing that survives is the one that demands the most of the logic, and the simultaneous edge is skipped rather than treated as an easy pass. Understanding this is what lets you read a cross-domain report without suspecting the tool of arithmetic errors.
In Pd / Signoff
This changes what you believe when you read the launch and capture times in a cross-domain report_timing . When the launch edge is not the one you expected on a mixed-period crossing, check whether a simultaneous source edge was skipped before you start doubting the clock definitions - the pairing PrimeTime chose is the conservative one and is usually right.
Common Trap
Deriving the setup relationship by hand and pairing the simultaneous edges on a mixed-period crossing, which gives a zero-length launch-to-capture window and a slack that disagrees with the tool. The follow-on error is assuming PrimeTime picked the wrong edge and adding a multicycle exception to 'correct' it.
Expect Next
How does the same nearest-edge-before rule determine the hold relationship on that crossing?
Topic And Primetime Commands
Timing Exceptions / General. report_timing
Keep building interview depth
Continue practising STA
Continue learning free
Get a practical low-power chapter
Receive the existing “Low Power and Multivoltage Fundamentals” PDF chapter and its download link by email.
Continue practising