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Why configurable IP needs a coverage strategy across configurations
Parameters can change the RTL that is generated, so one configuration may contain logic that another does not. A coverage result from a single build therefore describes that build; it cannot automatically stand in for every other parameter choice.
For example, a Synopsys-authored 2010 article describes a DesignWare USB 2.0 HS OTG IP with 39 configuration parameters. Two of them are DMA mode—slave, external DMA or internal DMA—and PHY interface—UTMI+, ULPI or both. Those two parameter sets alone produce nine combinations. The article’s regression example used 60 configurations overall; that is an example set, not a claim that all combinations of the 39 parameters were tested.
Code coverage commonly tracks line, toggle, condition and finite-state-machine (FSM) behavior. The challenge is deciding which configuration’s evidence applies to which generated logic, rather than treating every coverage database as interchangeable.
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Choose how to measure coverage across configurations
There are three practical approaches. The right choice depends on whether you need exact results per build, a consolidated view of shared RTL, or a quicker representative estimate.
| Approach | What it does | Trade-off |
|---|---|---|
| Golden or maximum-overlap configuration | Run coverage on the configuration whose RTL overlaps most with the others, and use it as a representative. | Efficient, but its numbers accurately describe only that selected configuration; they do not precisely describe the others. |
| Independent coverage for every configuration | Run coverage-enabled regressions separately for each configuration. | Produces configuration-accurate results, but may require more simulation cycles as the number of configurations grows. |
| Base/sub-design merge | Choose one configuration as the base, merge coverage from the others as sub-designs, and consolidate common RTL while retaining configuration-specific RTL. | Improves the consolidated view without additional simulation cycles solely to generate the merged report. The report still needs engineering review. |
When a representative configuration is enough
A maximum-overlap build can help with an early coverage check or a quick view of common behavior. Treat it as a sample, not as coverage closure for unmeasured configurations. If a delivery depends on a specific customer configuration, that build needs results that actually represent its RTL.
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Independent regressions are the clearest option when sign-off requires a precise result for each configuration. They also make it easier to see which build has an uncovered branch or state, but regression time can rise with configuration count.
When shared RTL makes merging useful
A base/sub-design merge is useful when configurations share significant RTL but also have distinct blocks or branches. Synopsys describes using VCS Unified Report Generator (URG) for this flow. The published example uses the following command shape:
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urg -dir Config1/simv.cm ... Config60/simv.cm
Here, Config1 is the base configuration and the listed coverage databases provide the other designs. The merged report can improve consolidated coverage compared with looking at individual reports alone. Select a customer-specific build as the base when the report should emphasize that delivery while incorporating reusable coverage from the other configurations.
How to close coverage without mistaking a merge for sign-off
- Define the configuration set. Record the supported parameter combinations, identify configurations that materially change generated RTL, and decide whether sign-off requires an individual result for every build.
- Run the required regressions with coverage enabled. Use independent runs when exact per-configuration results are required. Keep each configuration’s database identifiable so results can be traced back to its build.
- Choose the base deliberately if merging. Select the configuration that best represents the delivery or shared RTL, then include the intended sub-design coverage databases in URG.
- Inspect the merged report and the per-configuration evidence. A consolidated increase can reveal coverage gathered across configurations; it does not establish that every distinct block or customer build is covered.
- Classify every uncovered item. Decide whether it is a real test gap, unreachable behavior supported by proof, dead code, or a specification issue. Document justified waivers rather than silently excluding holes.
- Close gaps and measure again. Add tests for genuine holes, rerun the relevant regressions, and repeat analysis until coverage stabilizes against the verification goals.
Does 100% code coverage prove configurable IP is verified?
No. The Synopsys authors of the 2010 explanation put it directly: “It is required to meet 100 percent coverage goals but it does not mean that verification is complete.” Code coverage indicates which implementation structures were exercised; it does not by itself show that the tests checked the intended behavior or that the specification is satisfied.
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Review code coverage alongside functional coverage, assertion coverage, formal checking or equivalence where applicable, passing tests, and specification intent. These forms of evidence answer different questions: whether implementation structures ran, whether required behaviors were exercised, whether properties held, and whether the design matches its requirements.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical coverage-closure checklist
- Are all supported configurations represented in the regression or explicitly justified as outside the sign-off scope?
- Does each coverage database map unambiguously to its configuration and generated RTL?
- Are merged results interpreted only for common RTL, with configuration-specific logic reviewed separately?
- Have uncovered lines, toggles, conditions and FSM behavior been classified and dispositioned?
- Do functional coverage, assertions, formal checks and specification review provide complementary evidence?
- Are waivers documented with rationale, and are coverage goals rechecked after fixes?
The 60-configuration Synopsys example reported significant improvement in merged coverage over individual reports without extra verification cycles for the reporting improvement. Engineers still analyzed the results and added tests where real holes remained. That distinction is central: merging can make existing simulation evidence more useful, but it does not create new stimulus or eliminate the work of closing actual gaps.
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