Timing-Driven Synthesis · synthesis

Part 3 · Constraints and Timing · Intermediate

Timing-driven synthesis spends area where slack is worst. It restructures the critical path, upsizes its gates, and duplicates logic to shorten it.

How slack steers synthesis effort.

Slack picks the target, three techniques spend effort, and the two slack totals judge the result.

Technique	What it changes	What it costs
Restructuring	Logic depth of the critical path	Area varies; some paths grow while others shrink.
Sizing	Drive strength of gates on the worst path	Area grows with each upsize.
Duplication	Shared logic that slows the critical fanout	A second copy of the shared cone.
Groups and weights	Which paths the effort favours	Effort moved away from other paths.

Lessons

Lesson	What it covers
Critical-Path Focus	Slack ranks paths so effort lands on the worst one first.
Restructuring and Duplication	Shorter logic depth and a private copy for the critical fanout.
Sizing and Path Groups	Upsizing the worst gates, plus groups and weights that split effort.
Reading WNS and TNS	Worst and total negative slack: what each measures and how to read them.
Margin and When to Stop	Over-constraining margin, its harm, and rules for stopping.

Check yourself Worst slack improves but total negative slack barely moves. What does that say?

One path was fixed while many still fail. The design needs broader effort, not more work on the same path.

Technique

What it changes

What it costs

Restructuring

Logic depth of the critical path

Area varies; some paths grow while others shrink.

Sizing

Drive strength of gates on the worst path

Area grows with each upsize.

Duplication

Shared logic that slows the critical fanout

A second copy of the shared cone.

Groups and weights

Which paths the effort favours

Effort moved away from other paths.

Lessons

Lesson	What it covers
Critical-Path Focus	Slack ranks paths so effort lands on the worst one first.
Restructuring and Duplication	Shorter logic depth and a private copy for the critical fanout.
Sizing and Path Groups	Upsizing the worst gates, plus groups and weights that split effort.
Reading WNS and TNS	Worst and total negative slack: what each measures and how to read them.
Margin and When to Stop	Over-constraining margin, its harm, and rules for stopping.

Check yourself Worst slack improves but total negative slack barely moves. What does that say?

One path was fixed while many still fail. The design needs broader effort, not more work on the same path.