How do you read the arc-by-arc breakdown in a report_timing output to find the dominant delay contributor?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Explanation
report_timing prints a Point/Incr/Path table walking down the launch path arc by arc, showing each cell or net's incremental delay and the running cumulative total. To find the dominant contributor, scan the Incr column for the single largest jump rather than just looking at the final cumulative arrival time.
Timing-path Walkthrough
In the worked example: reg_a/Q (clk-to-Q) contributes 0.09, u1/Z (buffer) 0.12, u2/Z (logic) 0.31 — flagged as the big contributor — u3/Z (logic) 0.28, and reg_b/D (net) 0.07, giving a cumulative data arrival time of 0.87. Against a data required time of 0.93 (after clock skew +0.03, library setup −0.06, and uncertainty −0.04 are applied to the 1.00 capture edge), the slack comes out to +0.06, MET. The notes advise reading slack first: if it had instead FAILED, u2 at 0.31 would be the fix target.
Command
report_timing -delay_type max -transition_time -capacitance -input_pins netsSwitch-by-switch
-delay_type max: reports max-delay (setup-relevant) arcs. -transition_time: shows the
slew at each point. -capacitance: shows the load at each point. -input_pins: shows input
pin timing detail. -nets: includes net delays explicitly in the breakdown.Expected Report Behavior
The Point/Incr/Path table shows per-arc cell/net delay, slew, and load down the path, ending in data arrival time, data required time, and a final slack line (MET or VIOLATED).
Mentor Note — Common Mistake
Fixating on the final cumulative arrival number instead of scanning each Incr value individually, missing that one specific arc (like u2/Z at 0.31) is the actual dominant contributor worth targeting.
How To Debug
Read the slack line first to know pass/fail, then scan the Incr column top to bottom for the largest single jump as the primary fix target, following up with -transition_time and capacitance to understand why that arc is slow.
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