IntermediateQuestion 69 of 112Source: Synopsys PrimeTime User Guide: Case and Mode Analysis

What does the static value in set_case_analysis do that a plain 0 or 1 does not?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

set_case_analysis static (SDC) fixes a pin at a constant value without committing to whether that value is logic 0 or logic 1, which matters specifically for signal integrity analysis. A net driven by a static pin is treated as never switching, so it cannot act as a crosstalk aggressor or be pushed around as a crosstalk victim, the same protection a plain 0 or 1 gives for timing but stated in a way that also covers noise analysis correctly.

A configuration strap net shown with two possible resistor-option values, marked static in the SDC, running alongside a high-activity bus that crosstalk analysis excludes from any delta-delay penalty because the strap can never switch. — Technical Reference DiagramWhat does the static value in set_case_analysis do that a plain 0 or 1 does not?

Technical Explanation

The three constant options in set_case_analysis look similar but carry slightly different meaning once crosstalk analysis is involved.

Plain 0 and 1 are specific constant values. set_case_analysis 0 [get_ports SEL] and set_case_analysis 1 [get_ports SEL] (SDC) each fix the pin at one exact logic level.
static also fixes the pin as non-switching, without naming the value. set_case_analysis static [get_ports SEL] tells the tool the net never transitions, enough for crosstalk analysis even when the exact value need not be pinned down.
Why crosstalk cares about "never switches" specifically: a net that cannot switch cannot act as a crosstalk aggressor pushing a neighboring net around, and cannot be pushed around as a victim, since both effects need an actual transition.
A rising or falling value is the opposite case. set_case_analysis rising [get_ports RESET] fixes the pin to one controlled transition, which the tool still needs to time correctly, unlike a static net.
Some flows apply static automatically to disabled tie-off cells, but a designer can also apply it directly wherever a net is known to be quiet for the whole run.
Choosing static over a plain 0 or 1 rarely changes timing on its own. It mainly matters once crosstalk analysis needs to know which nets can never be an aggressor or a victim.

Common Mistake

The Trap: treating static, 0, and 1 as interchangeable spelling choices for "this pin is constant", without considering that crosstalk analysis reads them slightly differently.

A designer picks 0 or 1 out of habit for a pin whose exact value genuinely does not matter to the surrounding logic, just because it happens to be one value in the current netlist.
If crosstalk analysis is enabled and something downstream depends on the net being flagged as fully static rather than a fixed 0 or 1, the wrong choice can produce a subtly different noise-analysis result than intended, even though timing alone looks unaffected.

Follow-up Question & Model Response

A tie-off pin is wired to a fixed value in the netlist, and you also want crosstalk analysis to treat its net as guaranteed quiet. Would set_case_analysis 0 be enough, or would you use static?

Candidate Model Response: If the pin's wired value is known and fixed, set_case_analysis 0 (or 1, matching the real value) already tells the tool the net cannot switch, which crosstalk analysis reads the same way it reads static for practical purposes. I would reach for static when the exact value is not the point — a pin whose value could plausibly be either 0 or 1 depending on an unfinalized strap option, but which is guaranteed not to switch during analysis either way. There, static states the fact that matters, that the net is quiet, without asserting a value the design has not committed to.

Practical Example

A configuration strap net is tied through a resistor option that could be populated pull-up or pull-down depending on the board variant, so its value is undecided at the SDC-writing stage. The team applies set_case_analysis static [get_ports CFG_STRAP] (SDC) rather than guessing 0 or 1, letting crosstalk analysis treat CFG_STRAP as non-switching regardless of the board variant. A neighboring high-activity bus routed alongside it is then excluded from any crosstalk delta-delay penalty from that strap, since the tool knows it never switches.

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