A little margin guards the handoff

Synthesis models differ slightly from later stages, so closing exactly at zero can reopen downstream. A small margin holds slack back for that gap.

Margin means constraining synthesis to a target tighter than the real one. The calc block works a 1.0-unit margin on a 10.0-unit period.

EXAMPLE parameters: clock period 10.0 time units, margin 1.0 time units
synthesis target = 10.0 - 1.0 = 9.0 time units
tool closes with arrival 8.8: tool slack = 9.0 - 8.8 = +0.2
handoff slack at the real period = 10.0 - 8.8 = +1.2

Too much margin distorts the design

A large margin spends area and power on a fiction. The tool reshapes logic for a target the design never needs to meet.

Those shapes can hurt the true paths: deeper area, larger loads, and structures that later stages must undo. Margin beyond the real uncertainty buys risk, not safety.

Rules for stopping

Effort stops when the numbers and the budget agree. Each rule below is a separate reason to quit a failing path or a finished job.

• WNS reaches zero at the synthesis target including margin: the job is done.

• TNS stays flat across two runs while area still grows: restructure instead of sizing further.

• Each run moves the worst path somewhere new with no global gain: the structure is spent, so change it or accept the result.

• Area or power hits its budget: further timing effort spends what the design cannot afford.

Common mistakes

Related topics

• Up: Timing-Driven Synthesis

• Critical-Path Focus

• Area, Power and Timing Tradeoffs