Explained: Verification in the RTL to GDS flow
In the intricate realm of Very Large-Scale Integration (VLSI), where the tiniest errors can have monumental consequences, the importance of robust verification processes cannot be overstated.
As semiconductor technologies advance, the complexity of designs increases exponentially, making it imperative for engineers to employ a comprehensive verification strategy throughout the entire design flow.
In this blog, we will unravel the intricacies of verification in VLSI, exploring the various types and their roles in ensuring a flawless semiconductor design.
RTL Simulation: Laying the Foundation
The journey begins at the Register Transfer Level, where the high-level functionality of the design is captured using a Hardware Description Language (HDL) like Verilog or VHDL. The first type of verification in the VLSI design flow is RTL simulation, a critical step in ensuring that the design behaves as intended at the algorithmic level.
RTL simulation involves testing the design against a set of testbenches to validate its functionality. Engineers create a suite of test cases that cover various scenarios to verify the correctness of the design’s logic. Any discrepancies or functional errors are addressed and refined before moving on to the next stage.
Read More: My Experience of Doing a NPTEL Course on VLSI: RTL to GDS
Functional Verification: Ensuring Correct Operation
Once the RTL simulation provides confidence in the design’s high-level functionality, the focus shifts to functional verification. It acts as a safety net to catch any issues that might have slipped through the initial RTL simulation.
The types of functional verification are:
Simulation-based verification checks whether the RTL design is producing the same results as given in the specification. It employs test vectors. Test vectors are sequence of zeros/ones and the associated timing information.
Demerits of this type of verification is that it is incomplete and we may miss out on some of the essential test vectors. Hence, we go for formal verification.
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Formal Verification: This method applies mathematical reasoning to establish proof of correctness i.e. the system behaves correctly irrespective of input vectors.
1. Model Checking:
- Exhaustive Exploration: Model checking involves systematically exploring all possible states of a design.
- Corner-Case Detection: It excels in detecting subtle errors and corner cases that might be missed by other verification methods.
- Automation: Model checking is highly automated, making it efficient for large and complex designs.
2. Theorem Proving:
- Mathematical Proofs: Theorem proving relies on mathematical proofs to establish the correctness of a design.
- Rigorous Demonstration: It involves formally demonstrating that logical assertions hold true under all conditions.
- Manual Approach: Unlike model checking, theorem proving is a more manual, meticulous approach to verification.
Thus, model checking automates the exploration of design states and theorem proving adds a human-guided, rigorous layer of assurance through mathematical validation. This dual approach ensures a more comprehensive and robust verification process, crucial in the complex landscape of VLSI design.
Read More: Explained: What the hell is RTL?
Gate-Level Simulation: Transitioning to Implementation
With functional verification providing a solid foundation, the next stage in the VLSI design flow is gate-level simulation. This verification step takes the design from its abstract RTL representation to the gate-level netlist that will be implemented in silicon.
Gate-level simulation involves validating the timing, power, and connectivity aspects of the design. Engineers use accurate delay models to simulate the propagation of signals through the gates and verify that the design meets its timing requirements. This step is crucial for identifying issues related to clock domain crossing, signal integrity, and other timing-sensitive aspects of the design.
- Combinational Equivalence Checking
Will the RTL and the netlist generated by a logic synthesis tool always produce the same (equivalent) output?
Combinational equivalence checking (CEC) establishes the functional equivalence of two models using formal methods.
- CEC required whenever non-trivial design changes occur
- Carried out multiple times in a design flow
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1. Timing Verification: Meeting Critical Deadlines
Timing closure is a significant challenge in modern VLSI design due to the increasing complexity and shrinking process nodes. At this stage, engineers use Static timing analysis (STA) tools to analyze the timing characteristics of the design and identify any violations.
This includes
- setup and hold time violations,
- clock-to-q delays, and
- Clock skew and jitter
By addressing these issues, engineers can achieve timing closure and guarantee that the design meets its performance specifications.
2. Power Verification:
- Objective: Evaluate and verify the power consumption of the gate-level netlist.
- Techniques: Utilize power analysis tools to simulate and analyse power characteristics.
- Significance: Ensures the design aligns with power constraints and optimization goals, optimizing for power efficiency.
Physical Verification: Guarding Against Manufacturing Challenges
Physical verification includes checks for design rule violations, such as minimum spacing, width of metal traces, and other geometric constraints.
The three aspects of physical verification are:
1. DRC (Design Rule Checking):
– Identifies violations of manufacturing rules in the physical layout.
-Ensures adherence to minimum spacing, width, and geometric constraints.
3. LVS (Layout vs. Schematic):
– Compares physical layout with the original schematic to ensure consistency.
– Verifies that the physical implementation matches the intended logical representation.
3. ERC (Electrical Rule Checking):
– Checks for electrical rule violations, focusing on signal integrity.
– Identifies issues like short circuits, open circuits, and electrical connectivity problems.
Design for Test (DFT): Facilitating Testability
DFT involves incorporating features into the design that facilitate efficient testing of the fabricated semiconductor device.
One key aspect of DFT is the insertion of scan chains, which allow for the efficient application of test patterns and the observation of output responses.
Two aspects of Design for Test (DFT),
Automatic Test Pattern Generation (ATPG) plays a pivotal role by autonomously crafting test patterns using sophisticated algorithms. This process optimizes fault coverage, bolstering the circuit’s resilience during manufacturing tests.
Built-In Self-Test (BIST) seamlessly integrates self-testing structures within the design. BIST empowers the circuit to autonomously generate test patterns and evaluate its own functionality. This self-sufficiency enhances overall testability, reduces external testing dependencies.
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Conclusion: A Holistic Approach to VLSI Verification
In the design flow often, verification emerges as the unsung hero. It is a multifaceted journey that spans the entire design flow from RTL to GDS. Each type of verification plays a crucial role in ensuring the correctness, performance, and manufacturability of the semiconductor design. This holistic approach safeguards against functional errors, timing issues, and manufacturing challenges. As technology advances, the role of verification remains paramount and demands constant innovation.
