Synthesis Design Scenarios · synthesis

Part 9 · Interview and Judgment · Intermediate

5topics
5lessons

Real synthesis work arrives as symptoms: timing fails after an RTL change, area grows after a feature, a latch appears, hold breaks after scan, equivalence fails, power runs too high. Each scenario here is worked the same way: symptoms, a ranked investigation, the fix, and the trade-off.

Decision flow used by every scenario in this topic.

Every scenario moves from symptom to investigation to a priced fix.

Symptom	First suspicion
Setup fails after an RTL edit	Added logic depth on the failing path.
Area jumps after a feature	Replicated or widened datapath logic.
A latch appears; hold fails after scan	Incomplete assignment; scan skew and routing at fixed arrival.
The netlist fails equivalence	The two sides come from different revisions.
Power too high at the same timing	Activity higher than the estimate assumed.

Lesson	What it covers
Timing fails after an RTL change	Slack before and after, ranked causes, and the fix.
Area blows up after enabling a feature	Area split in library units, with the fix and its cost.
A latch and a hold surprise	An inferred latch and a scan hold violation, each priced.
A netlist fails equivalence	Localizing one wrong gate with a brute-force proof.
Power too high at the same timing	Hand power estimate, then gating at the same timing.

Check yourself Timing fails the day after an RTL change. What is the first step?

Diff the inputs: RTL, constraints, and library versions against the last green run. Blaming the tool before the diff wastes the investigation.

Up nextTiming Fails After an RTL Change

Practice this lesson Report an error

Symptom

First suspicion

Setup fails after an RTL edit

Added logic depth on the failing path.

Area jumps after a feature

Replicated or widened datapath logic.

A latch appears; hold fails after scan

Incomplete assignment; scan skew and routing at fixed arrival.

The netlist fails equivalence

The two sides come from different revisions.

Power too high at the same timing

Activity higher than the estimate assumed.

Lesson

What it covers

Timing fails after an RTL change

Slack before and after, ranked causes, and the fix.

Area blows up after enabling a feature

Area split in library units, with the fix and its cost.

A latch and a hold surprise

An inferred latch and a scan hold violation, each priced.

A netlist fails equivalence

Localizing one wrong gate with a brute-force proof.

Power too high at the same timing

Hand power estimate, then gating at the same timing.