IntermediateQuestion 3 of 20Source PDF page 37

What is clock propagation, and what does set_propagated_clock actually change?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Explanation

Before layout, STA typically assumes an idealized clock (arriving everywhere at the same time, zero skew). set_propagated_clock switches the tool to compute the clock's actual insertion delay through the real, built clock tree once clock tree synthesis (CTS) has produced that physical network.

Timing-path Walkthrough

Pre-CTS, every flop's clock pin might be treated as arriving at t=0. After CTS builds an actual tree with buffers and routing, applying set_propagated_clock lets the tool compute the true arrival time at each flop's clock pin, which now carries real skew between launch and capture flops — directly changing both the S term in the setup inequality and the S term in the hold inequality.

Command

set_propagated_clock

Switch-by-switch

No sub-switches are given in the notes; the command itself is the toggle applied once the real clock network exists so the tool computes actual insertion delay from the physical network.

Expected Report Behavior

Clock path sections of report_timing move from flat, idealized zero-latency entries to perstage insertion delay reflecting the real buffer/wire tree, and skew values become nonzero and path-specific.

Mentor Note — Common Mistake

Leaving the clock idealized (never applying set_propagated_clock) after CTS, so every flop still looks like it receives the clock at t=0.

How To Debug

Compare clock path entries in report_timing before and after applying set_propagated_clock; real per-flop insertion delay and non-zero skew confirm propagation is active.

Visual explanationSTA context: What is clock propagation, and what does set_propagated_clock actually change?
STA context: What is clock propagation, and what does set_propagated_clock actually change?A three-step concept map summarizes the focus, core answer, and practical verification for What is clock propagation, and what does set_propagated_clock actually change?Question focusWhat is clockpropagation, and whatdoesset_propagated_clock…Core answerBefore layout, STAtypically assumes anidealized clock(arriving everywhere at…Verify in practiceset_propagated_clockUnderstand → explain the mechanism → verify the assumptions

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