Mastering AI Chip Complexity
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In semiconductor design, “signoff” is often treated as a single milestone. In practice, however, it encompasses distinct verification phases with unique objectives.
Functional signoff and RTL signoff represent two such phases. Both are essential, and each one is focused on different facets of correctness. While functional signoff verifies whether a design behaves as intended, RTL signoff focuses on whether the Register Transfer Level implementation is structurally sound and ready for downstream synthesis and physical implementation.
As System-on-Chip (SoC) complexity increases — with more IP blocks, multiple asynchronous clock domains, and aggressive power management schemes — these signoff stages have become increasingly specialized. Verification flows must now capture both the intent and the integrity of a design before it can move forward. And the line between what is considered complete at the functional level versus the RTL level is not always clear — especially when there are overlapping concerns around coverage, power intent, and timing readiness.
Fortunately, Synopsys offers a cohesive portfolio of tools that bring structure and intelligence to both RTL and functional signoff.
Multi-Die Start Guide
This guide provides essential information for a successful multi-die design, including:
- Advantages and motivating factors
- Key considerations and guidelines for new projects
What is functional signoff?
Functional signoff determines whether a design correctly demonstrates the desired behavior. It is performed at the RTL abstraction level, at which point the design is still represented in source code (typically Verilog or VHDL), before any synthesis or layout transformation occurs. The goal is to validate whether the RTL meets the specification across all defined use cases, operating modes, and corner conditions, without necessarily accounting for physical implementation concerns.
This stage typically involves simulation and formal analysis. Engineers use testbenches — often based on the Universal Verification Methodology (UVM) — to apply stimuli and check for expected responses. They define functional coverage through covergroups and assertions, which measure whether important events and behaviors have occurred during simulation. Code coverage complements this by tracking whether all parts of the RTL have been exercised.
While code coverage is relatively easy to maximize, functional coverage remains more subjective. It depends on the quality of the specification and the verification team's ability to anticipate real-world scenarios. Even when coverage metrics show 100%, there’s often lingering doubt about whether the right things were tested. This uncertainty is why verification often continues indefinitely after tapeout.
To help close this gap, we provide AI-driven machine learning in our VSO.ai solution — which is integrated with our industry-leading VCS functional verification solution — to identify untested behaviors, generate targeted inputs, and streamline test selection. In addition to improving coverage results, VSO.ai increases confidence in the thoroughness and effectiveness of verification.
What is RTL signoff?
RTL signoff focuses on structural correctness rather than behavior. It confirms the RTL code is implementation-ready and free of issues that could compromise synthesis, timing, or physical integration.
Important checks performed during RTL signoff include clock domain crossing (CDC), reset domain crossing (RDC), linting, and low-power intent validation. These analyses look for hazards such as unsynchronized signals crossing between independent clock domains, incomplete reset logic, or violations of naming conventions and design rules. Tools like Synopsys VC SpyGlass and Synopsys VC Formal are used to identify these issues early in the flow, before they become bugs at the gate or layout level.
Power-aware verification is also a central tenet of RTL signoff. Engineers are expected to confirm whether isolation cells, level shifters, and retention strategies have been correctly defined and integrated through techniques like UPF (Unified Power Format). These structures are typically invisible at the pure RTL level and thus require synthesis or static elaboration to reveal their behavior in context.
How RTL and functional signoff overlap
Although functional and RTL signoff serve different purposes, they are deeply interconnected. One verifies design behavior while the other verifies structural viability.
- Both are needed for design closure, and each can expose issues the other cannot.
- They often overlap in time, but they answer fundamentally different questions.
- Issues in one domain can influence the other.
A missing synchronizer uncovered during RTL signoff, for example, might cause intermittent behavior that would never be caught through functional coverage metrics alone. Similarly, an untested state transition in functional signoff might pass structural checks but lead to failure in silicon.
Treating these stages as separate but aligned helps avoid false confidence. It also lets teams apply the right tools and techniques to the right problems. Simulation and testbenches are better suited to explore use cases and logic conditions, whereas static analysis is more effective for uncovering design rule violations or missing synchronizers.
Across both phases, Synopsys Verdi serves as the common debug platform for viewing, analyzing, and identifying the root causes of failures and coverage gaps.
AI in modern signoff
As verification complexity grows, so does the effort required to reach signoff. Historically, coverage closure can stall for weeks as engineers chase elusive corner cases or struggle to generate the right input sequences. Today, AI is beginning to reshape the process.
VSO.ai addresses one of the hardest problems in functional signoff — figuring out how to hit the coverage targets that matter. Instead of relying purely on random test generation or manual tuning, VSO.ai analyzes simulation data, identifies gaps, and autonomously generates input conditions that are more likely to trigger untested behaviors. These features reduce the need for human intervention and shorten the path to meaningful coverage.
Our latest generation Verdi platform also leverages AI to enhance triaging and root-cause analysis of failures, significantly boosting productivity.
While AI can help improve the quality of coverage itself, unreachability analyses (a key feature of VC Formal) help determine which parts of the RTL can never be exercised. This allows design teams to exclude dead code from coverage goals and ensures time isn’t wasted chasing targets that don’t contribute to functional confidence.
Looking forward, agentic AI offers even more potential. By translating natural-language specs into formal assertions, it will help teams define functional coverage faster and more accurately.
Closing the loop on verification confidence
Functional signoff and RTL signoff represent two sides of the same verification coin. One confirms a design behaves as expected while the other confirms it’s constructed in a way that supports reliable implementation.
Both are important, and neither can replace the other. In a world of increasingly heterogeneous and power-sensitive SoCs, overlooking either milestone introduces risk that no amount of post-silicon debug can easily fix.
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