Acing the Conformal LEC Interview: A Comprehensive Guide

In the intricate world of VLSI design, ensuring the logical integrity of your design is paramount. One crucial technique that plays a pivotal role in this endeavor is Logical Equivalence Checking (LEC). As you navigate the interview process for roles involving ASIC or FPGA design, you can expect questions related to Conformal LEC, a widely used tool for LEC. This article aims to equip you with the knowledge and insights necessary to tackle Conformal LEC interview questions confidently.

Understanding Logical Equivalence Checking (LEC)

Before we delve into the specifics of Conformal LEC, let’s establish a solid foundation by understanding the concept of Logical Equivalence Checking (LEC). LEC is a crucial verification step in the ASIC design cycle, ensuring that the logical functionality of a design remains intact throughout various stages, such as synthesis, place and route, sign-offs, engineering change orders (ECOs), and optimizations.

The VLSI design cycle is divided into two phases: the front-end, which covers architectural specifications, coding, and verification, and the back-end, which involves the physical implementation of the design on the targeted technology node. LEC plays a vital role in bridging these phases, ensuring that the design’s functionality remains uncompromised during the transition from one phase to another.

The Importance of LEC

With shrinking technology nodes and increasing design complexity, LEC has become an indispensable part of the chip design process. Any unintended change in the logical functionality can render an entire chip useless, making LEC one of the most critical checks in the entire design flow.

LEC not only provides a nearly exhaustive proof of equivalence without the need for writing test patterns but also offers several benefits, including:

• Reduced reliance on gate-level simulation
• Boosted confidence in new tool revisions for synthesis and place & route
• Identification of poor RTL coding areas in the design
• Decreased risk of missing bugs introduced by the back-end process

Conformal LEC: The Industry-Standard Tool

Conformal, developed by Cadence Design Systems, is an industry-standard tool for performing Logical Equivalence Checking. It is widely used across the semiconductor industry and is likely to be a topic of discussion during interviews for roles involving ASIC or FPGA design.

The Conformal LEC Flow

The Conformal LEC flow comprises three main steps: Setup, Mapping, and Compare.

1. Setup

In the setup mode, the Conformal tool reads two designs: the Golden design (typically the synthesized netlist) and the Revised design (the modified or post-processed design that will be compared against the Golden design).

For the execution of LEC, the Conformal tool requires three types of files:

• <design_name>.lec: This file guides the Conformal tool to execute different commands in a systematic way.
• <design_name>.scan_const: This file provides scan-related constraints, such as ignoring specific scan connections, SERDES input/output pins, or other pins that need to be excluded from the LEC process.
• <design_name>.stdlib: This file contains pointers to the standard cell library.

During the transition from the setup mode to the LEC mode, the Conformal tool flattens and models the Golden and Revised designs, and automatically maps the key points. Key points are defined as:

• Primary Inputs
• Primary Outputs
• D Flip-Flops
• D Latches
• TIE-E Gates (error gates, created when x-assignment exists in the Revised design)
• TIE-Z Gates (high impedance or floating signals)
• Black boxes

2. Mapping

In the mapping phase, the Conformal tool automatically maps the key points and compares them. It employs two name-based methods (useful for gate-to-gate comparisons with minor logic changes) and one no-name method (useful when the designs have completely different names).

The key points that the Conformal tool cannot map are classified as unmapped points, which fall into three categories:

• Extra unmapped points: Key points present in only one of the designs (Golden or Revised).
• Unreachable unmapped points: Key points that do not have an observable point, such as a primary output.
• Not-mapped unmapped points: Key points that are reachable but do not have a corresponding point in the logic fan-in cone of the corresponding design.

3. Compare

After mapping the key points, the Conformal tool examines them to determine if they are equivalent or non-equivalent. The comparison classifies the compared points as:

• Equivalent
• Non-equivalent
• Inverted-equivalent
• Aborted (in case of aborted compare points, changing the compare effort to a higher setting can allow the Conformal tool to continue the comparison)

Upon completion of the LEC run, the Conformal tool generates multiple reports, including:

• Non-equivalence report
• Unmapped report
• Blockbox report
• Abort report
• Unreachable report
• Floating report
• Mapped report

Debugging LEC Failures

LEC failures are not uncommon, and it’s crucial to understand how to debug and resolve them effectively. Here’s a step-by-step approach to tackle LEC failures:

Step 1: Non-equivalent Report

The first step is to check the “non-equivalent.rpt” file, which lists the compare points that are failing the LEC. It’s important to note that due to a single broken connection, a higher number of cell names may be reported as non-equivalent. This is because many paths going through the failed connection will have their endpoints (compare points) reported as non-equivalent.

Step 2: Unmapped Report

Next, examine the “unmapped” file, which shows the unmapped nets where the logical connectivity is broken. Tracing these nets can help pinpoint the missing connections.

Step 3: Fix the LEC Issue

After identifying the root cause of the LEC failure, the necessary fix typically involves inserting the missing cell or restoring the broken connection in the LEC failed database. Once the fix is implemented, rerunning the LEC should result in a passing report with zero non-equivalent points.

Common Areas Where LEC Fails

While debugging LEC failures, it’s helpful to be aware of some common areas where LEC issues might occur:

• Multibit flops: Mapping issues can arise when multibit flops are used in the design, as flop names may change in the Revised netlist.
• Clock gating cells: Clock gating cells may not get mapped correctly after cloning in the Revised netlist.
• Logical connectivity breaks: Logical connectivity can break during timing fixing or while performing manual ECOs.
• Functional ECO implementation: Implementing functional ECOs can introduce LEC issues.
• Missing DFT constraints: Failing to include necessary DFT constraints can lead to LEC failures.

Conclusion

Mastering Conformal LEC is crucial for anyone aspiring to work in the field of ASIC or FPGA design. By understanding the LEC flow, its importance, and the debugging process, you’ll be well-equipped to tackle Conformal LEC interview questions with confidence. Remember, practice and hands-on experience are key to solidifying your knowledge and impressing potential employers in this domain.