Does PrimeTime (per the supplied PTUG) provide a command to create or schedule 'useful skew,' or is that a different tool's job — and what does PrimeTime actually give you regarding skew?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Technical Explanation
Based on the sampled PTUG material, PrimeTime does not include a command to create or schedule useful skew — that is implemented upstream in synthesis/CTS tools. PrimeTime's role is analysis and reporting of skew, not skew synthesis.
Timing-path Reasoning
The handbook notes only a single passing mention that useful skew can sometimes satisfy setup and hold together, without developing it as a PrimeTime feature. In the PTUG material provided, PrimeTime's skew-related capability consists of clock latency and uncertainty modeling (set_clock_latency with -rise/-fall/-early/-late/-dynamic, set_clock_uncertainty) and skew reporting (report_clock_timing -type skew), which computes and reports the actual skew between clock points rather than scheduling or deliberately introducing skew as an optimization technique.
Equation
Skew = (latency at B) - (latency at A) [ - (clock reconvergence
pessimism) ]Command
report_clock_timing -type skewSwitch-by-switch
-type skew selects the skew reporting mode of report_clock_timing (other -type options
include latency, transition, interclock_skew). -clock, -from/-to, -hold/-setup, -capture/-rise,
-nworst N, -show_clocks, -include_uncertainty_in_skew, -verbose further scope and
detail the skew report, but none of these create or schedule skew — they only analyze
and present it.Expected Result
PrimeTime reports the actual computed skew between two clock points (with optional CRPR credit factored in), giving visibility into existing skew, but it does not generate a skew schedule or CTS target — that remains the domain of the synthesis/clock-treesynthesis tool.
Failure Symptoms
An engineer looking for a 'create useful skew' command in PrimeTime will not find one in this material; attempting to use PrimeTime alone to schedule skew for setup/hold cooptimization would be a misapplication of the tool's intended role.
Root Cause
Useful skew scheduling is a clock-tree construction/optimization decision made during synthesis or CTS, whereas PrimeTime is a static timing analysis and signoff tool that verifies and reports the resulting skew rather than creating it.
Mentor Note — Debugging Procedure
If useful skew is suspected or desired on a path, use report_clock_timing -type skew (and set_clock_uncertainty/set_clock_latency to model expectations) to characterize the existing or intended skew, then coordinate any actual skew scheduling changes with the upstream CTS/synthesis flow. PrimeTime reports and verifies skew — including with CRPR-aware computation — but per the supplied PTUG, it does not offer a mechanism to create or schedule useful skew; that responsibility sits with the CTS/synthesis tool.
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