Delay Models and Slack · synthesis

Part 3 · Constraints and Timing · Intermediate

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5lessons

Synthesis scores every timing path with two numbers: when the signal arrives and when it is required. Their difference is the slack, and the path with the worst slack is the critical path.

Term	Meaning
Arrival time	When the signal actually reaches a point, adding clock-to-Q, cell and net delays along the path.
Required time	When the signal must reach a point, set by the clock period minus setup (or by the hold bound).
Slack	Required minus arrival. Positive means margin; negative means the path fails.
Critical path	The path with the worst (smallest) slack. It sets the maximum speed of the block.
Cell delay	Delay through a gate, from a table or linear model in the library.
Net delay	Delay along a wire, estimated at synthesis time and known only after layout.

How arrival, required time, slack and the critical path relate.

Arrival and required time give slack; the worst slack names the critical path.

Lesson	What it covers
Arrival, Required Time and Slack	Arrival, required, slack and the critical path with a worked EXAMPLE.
Setup, Hold and Clock-to-Q	Launch and capture edges, setup and hold checks, and a worked pair.
Cell Delay Models	Table versus linear cell models with a worked interpolation.
Wire Delay at Synthesis Time	Ideal-wire and wire-load estimates with a worked slack comparison.
A Static Timing Calculator	A runnable Python calculator over a small cell DAG.

Check yourself Arrival is 3.2 ns and required time is 2.9 ns. What is the slack, and does the path meet timing?

Slack is 2.9 minus 3.2, which is minus 0.3 ns. Negative slack means the path fails.

Up nextArrival, Required Time and Slack

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Term

Meaning

Arrival time

When the signal actually reaches a point, adding clock-to-Q, cell and net delays along the path.

Required time

When the signal must reach a point, set by the clock period minus setup (or by the hold bound).

Slack

Required minus arrival. Positive means margin; negative means the path fails.

Critical path

The path with the worst (smallest) slack. It sets the maximum speed of the block.

Cell delay

Delay through a gate, from a table or linear model in the library.

Net delay

Delay along a wire, estimated at synthesis time and known only after layout.

Lesson

What it covers

Arrival, Required Time and Slack

Arrival, required, slack and the critical path with a worked EXAMPLE.

Setup, Hold and Clock-to-Q

Launch and capture edges, setup and hold checks, and a worked pair.

Cell Delay Models

Table versus linear cell models with a worked interpolation.

Wire Delay at Synthesis Time

Ideal-wire and wire-load estimates with a worked slack comparison.

A Static Timing Calculator

A runnable Python calculator over a small cell DAG.