Stimulus Optimization in Hardware Verification Using Machine-Learning

Simulation-based functional verification is a commonly used technique for hardware verification, with the goal of exercising critical scenarios in the design, detecting and fixing bugs, and achieving close to 100% of the coverage targets required for tape-out. As chip complexity continues to grow, functional verification is also becoming a bottleneck for the overall chip design cycle. The primary goal is to shorten the time taken for functional coverage convergence in the volume verification phase, which in return, accelerates the bug detection in the design. In this thesis, I have investigated the application of machine learning towards this objective. I accessed the machine learning-guided stimulus generation with two approaches: coarse-grained test-level optimization and fine-grained transaction-level optimization. The effectiveness of machine learning was first confirmed on test-level optimization, which rests on achieving full coverage for a certain group of functional coverage metrics in reduced time with a minimal number of simulated tests. It was observed that test-level optimization was limited to some common functional coverage metrics. This was the motivation to explore and implement transaction-level optimization in two novel ways: transaction pruning and directed sequence generation for accelerated functional coverage closure. These techniques were applied on FSM (Finite State Machine) and Non-FSM based coverage metrics and compared the gains using different ML classifiers. Experimental results showed that the fine-grained implementation can potentially reduce the overall CPU time for the verification coverage closure; thus, I propose that complementary application of both the levels of stimulus optimization is the recommended path for efficiency improvements in functional verification coverage convergence

Data Learning Methodologies for Improving the Efficiency of Constrained Random Verification

Functional verification continues to be one of the most time-consuming steps in the chip design cycle. Simulation-based verification is well practised in industry thanks to its flexibility and scalability. The completeness of the verification is measured by coverage metrics. Generating effective tests to achieve a satisfactory coverage level is a difficult task in verification. Constrained random verification is commonly used to alleviate the manual efforts for producing direct tests. However, there are yet many situations where unnecessary verification efforts in terms of simulation cycles and man hours are spent. Also, it is observed that lots of data generated in existing constrained random verification process are barely analysed, and then discarded after simplistic correctness checking. Based on our previous research on data mining and exposure to the industrial verification process, we identify that there are opportunities in extracting knowledge from the constrained random verification data and use it to improve the verification efficiency.In constrained random verification, when a simulation run of tests instantiated by a test template cannot reach the coverage goal, there are two possible reasons: insufficient simulation, and improper constraints and/or biases. There are three actions that a verification engineer can usually do to address the problem: to simulate more tests, to refine the test template, or to change to a new test template. Accordingly, we propose three data learning methodologies to help the engineers make more informed decisions in these three application scenarios and thus improve the verification efficiency.The first methodology identifies important ("novel") tests before simulation based on what have been already simulated. By only simulating those novel tests and filtering out redundant tests, tremendous resources such as simulation cycles and licenses can be saved. The second methodology extracts the unique properties from those novel tests identified in simulation and uses them to refine the test template. By leveraging the extracted knowledge, more tests similar to the novel ones are generated. And thus the new tests are more likely to activate coverage events that are otherwise difficult to hit by extensive simulation. The third methodology analyses a collection of existing test items (test templates) and identifies feasible augmentation to the test plan. By automatically adding new test items based on the data analysis, it alleviates the manual efforts for closing coverage holes.The proposed data learning methodologies were developed and applied in the setting of verifying commercial microprocessor and SoC platform designs. The experiments in this dissertation were conducted in the verification environment of a commercial microprocessor and a SoC platform in Freescale Semiconductor Inc. and were in parallel with the on-going verification efforts. The experiment results demonstrate the feasibility and effectiveness of building learning frameworks to improve verification efficiency

Learning Approaches to Analog and Mixed Signal Verification and Analysis

The increased integration and interaction of analog and digital components within a system has amplified the need for a fast, automated, combined analog, and digital verification methodology. There are many automated characterization, test, and verification methods used in practice for digital circuits, but analog and mixed signal circuits suffer from long simulation times brought on by transistor-level analysis. Due to the substantial amount of simulations required to properly characterize and verify an analog circuit, many undetected issues manifest themselves in the manufactured chips. Creating behavioral models, a circuit abstraction of analog components assists in reducing simulation time which allows for faster exploration of the design space. Traditionally, creating behavioral models for non-linear circuits is a manual process which relies heavily on design knowledge for proper parameter extraction and circuit abstraction. Manual modeling requires a high level of circuit knowledge and often fails to capture critical effects stemming from block interactions and second order device effects. For this reason, it is of interest to extract the models directly from the SPICE level descriptions so that these effects and interactions can be properly captured. As the devices are scaled, process variations have a more profound effect on the circuit behaviors and performances. Creating behavior models from the SPICE level descriptions, which include input parameters and a large process variation space, is a non-trivial task. In this dissertation, we focus on addressing various problems related to the design automation of analog and mixed signal circuits. Analog circuits are typically highly specialized and fined tuned to fit the desired specifications for any given system reducing the reusability of circuits from design to design. This hinders the advancement of automating various aspects of analog design, test, and layout. At the core of many automation techniques, simulations, or data collection are required. Unfortunately, for some complex analog circuits, a single simulation may take many days. This prohibits performing any type of behavior characterization or verification of the circuit. This leads us to the first fundamental problem with the automation of analog devices. How can we reduce the simulation cost while maintaining the robustness of transistor level simulations? As analog circuits can vary vastly from one design to the next and are hardly ever comprised of standard library based building blocks, the second fundamental question is how to create automated processes that are general enough to be applied to all or most circuit types? Finally, what circuit characteristics can we utilize to enhance the automation procedures? The objective of this dissertation is to explore these questions and provide suitable evidence that they can be answered. We begin by exploring machine learning techniques to model the design space using minimal simulation effort. Circuit partitioning is employed to reduce the complexity of the machine learning algorithms. Using the same partitioning algorithm we further explore the behavior characterization of analog circuits undergoing process variation. The circuit partitioning is general enough to be used by any CMOS based analog circuit. The ideas and learning gained from behavioral modeling during behavior characterization are used to improve the simulation through event propagation, input space search, complexity and information measurements. The reduction of the input space and behavioral modeling of low complexity, low information primitive elements reduces the simulation time of large analog and mixed signal circuits by 50-75%. The method is extended and applied to assist in analyzing analog circuit layout. All of the proposed methods are implemented on analog circuits ranging from small benchmark circuits to large, highly complex and specialized circuits. The proposed dependency based partitioning of large analog circuits in the time domain allows for fast identification of highly sensitive transistors as well as provides a natural division of circuit components. Modeling analog circuits in the time domain with this partitioning technique and SVM learning algorithms allows for very fast transient behavior predictions, three orders of magnitude faster than traditional simulators, while maintaining 95% accuracy. Analog verification can be explored through a reduction of simulation time by utilizing the partitions, information and complexity measures, and input space reduction. Behavioral models are created using supervised learning techniques for detected primitive elements. We will show the effectiveness of the method on four analog circuits where the simulation time is decreased by 55-75%. Utilizing the reduced simulation method, critical nodes can be found quickly and efficiently. The nodes found using this method match those found by an experienced layout engineer, but are detected automatically given the design and input specifications. The technique is further extended to find the tolerance of transistors to both process variation and power supply fluctuation. This information allows for corrections in layout overdesign or guidance in placing noise reducing components such as guard rings or decoupling capacitors. The proposed approaches significantly reduce the simulation time required to perform the tasks traditionally, maintain high accuracy, and can be automated