Why does the setup-check equation subtract the capture clock path but the hold-check equation also subtract it - aren't they opposite checks?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Both equations subtract clockpath2 because the capture-clock delay always shifts the capture edge later. For setup that later edge helps, for hold it hurts - same algebraic sign, opposite consequence for margin.
Complete Technical Explanation
The sign in the equation and the effect on margin are two different things, and conflating them is what makes this question feel like a contradiction. Both checks subtract clockpath2 for the same physical reason: capture-clock delay shifts the capture edge later in time, which widens the launch-to-capture window. That is a single fact about the circuit, and it does not change depending on which check you are running. What changes is whether widening that window is good news. For setup, a later capture edge means the data has more time to arrive, so a larger clockpath2 relaxes the period requirement - the check gets easier. For hold, that same later capture edge is harmful, because it gives the new data more opportunity to arrive and be captured too soon, so a larger clockpath2 eats into hold margin - the check gets harder. Same algebraic sign in both equations, opposite consequence for the margin. This is exactly why OCV derates clockpath2 in opposite directions for the two checks. Since a larger clockpath2 helps setup, conservative setup analysis wants it scaled down; since a larger clockpath2 hurts hold, conservative hold analysis wants it scaled up. The derating direction is chosen by which way makes each check pessimistic, not by the sign the term carries in the equation. Read the equation for structure and the derate for intent.
In Pd / Signoff
This is what lets you read a report_timing path and predict which way a clock-tree change will move slack. Adding capture-side clock latency helps the setup check on that endpoint and hurts its hold check, so a useful-skew fix for a setup violation shows up as new hold violations at the same register. It also explains why the same path report shows clock-path derate factors above one for one check and below one for the other.
Common Trap
Reasoning from the minus sign to the conclusion that clockpath2 must hurt setup and help hold, or expecting the two equations to carry opposite signs because they are opposite checks. The sign is identical; only the consequence flips, and the OCV derate direction follows the consequence rather than the sign.
Expect Next
Given that, which direction does OCV derate clockpath2 for a hold check, and why is that the conservative choice?
Topic And Primetime Commands
Operating Conditions / On-Chip Variation. report_timingKeep building interview depth
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