Kuang-Chien Chen

Logic Synthesis for Engineering Change

In the process of VLSI design, specifications are often changed. It is desirable that such changes will not lead to a very different design so that a large part of engineering effort can be preserved. We consider synthesis algorithms for handling such engineering changes. Given a synthesized network, our algorithm modifies it minimally to realize a new specification.

ErrorTracer: a fault simulation-based approach to design error diagnosis

This paper addresses the problem of locating error sources in an erroneous combinational circuit. We use a fault simulation-based technique to approximate each signals correcting power. The correcting power of a particular signal is measured in terms of the signals correctable set, namely, the maximum set of erroneous input vectors that can be corrected by re-synthesizing the signal. Only the signals that can correct every erroneous input vector are considered as a potential error source. Our algorithm offers three major advantages over existing methods. First, unlike symbolic approaches, it is applicable for large circuits. Secondly, it delivers more accurate results than other simulation-based approaches because it is based on a more stringent condition for identifying potential error sources. Thirdly, it can be easily generalized to identify multiple errors. Experimental results on diagnosing circuits with one and two random errors are presented to show the effectiveness and efficiency of this new approach.

An Efficient Sequential SAT Solver With Improved Search Strategies

A sequential SAT solver, Satori, was recently proposed (Iyer, M.K. et al., Proc. IEEE/ACM Int. Conf. on Computer-Aided Design, 2003) as an alternative to combinational SAT in verification applications. This paper describes the design of Seq-SAT, an efficient sequential SAT solver with improved search strategies over Satori. The major improvements include: (1) a new and better heuristic for minimizing the set of assignments to state variables; (2) a new priority-based search strategy and a flexible sequential search framework which integrates different search strategies; (3) a decision variable selection heuristic more suitable for solving the sequential problems. We present experimental results to demonstrate that our sequential SAT solver can achieve orders-of-magnitude speedup over Satori. We plan to release the source code of Seq-SAT.

Logic synthesis for engineering change

During the process of very large scale integration design, specifications are often changed. To preserve as large a portion of the engineering effort as possible, it is desirable that such changes will not lead to a very different design. In this work, we consider logic synthesis algorithms for handling engineering changes. To solve it, we propose a combination of multiple-error diagnosis and logic minimization techniques. Given a new specification and an existing synthesized network, our algorithms first identify the candidate signals in the network, and then synthesize the candidate functions. The synthesis step utilizes the existing network as much as possible so that the new specification can be realized with minimal changes.

AQUILA: An equivalence verifier for large sequential circuits

In this paper, we address the problem of verifying the equivalence of two sequential circuits. A hybrid approach that combines the advantages of BDD-based and ATPG-based approaches is introduced. Furthermore, we incorporate a technique called partial justification to explore the sequential similarity between the two circuits under verification to speed up the verification process. Compared with existing approaches, our method is much less vulnerable to the memory explosion problem, and therefore can handle larger designs. The experimental results show that in a few minutes of CPU time, our tool can verify the sequential equivalence of an intensively optimized benchmark circuit with hundreds of flip-flops against its original version.

Error correction based on verification techniques

In this paper we address the problem of correcting a combinational circuit that is an incorrect implementation of a given specification. Most existing error-correction approaches can only handle circuits with certain types of errors. Here, we propose a general approach that can correct a circuit with multiple errors without assuming any error model. We identify internal equivalent pairs to narrow down the possible error locations using local BDDs with dynamic support. We also employ a technique called back-substitution to correct the circuit incrementally. This approach can also be used to verify circuit equivalence. The experimental results of correcting fully SIS-optimized benchmark circuits with a number of injected errors are presented.

Fault-simulation based design error diagnosis for sequential circuits

This paper addresses the problem of locating design errors in a sequential circuit. For single-error circuits, we consider a signal ƒ as a potential error source only if the circuit can be completely rectified by re-synthesizing ƒ (i.e., changing the function of signal ƒ). In order to handle larger circuits, we do not rely on Binary Decision Diagram. Instead, we search for potential error sources by a modified sequential fault simulation process. The main contributions of this paper are two-fold: (1) we derive the necessary and sufficient condition of whether an erroneous input sequence (i.e., an input sequence producing erroneous responses) can be corrected by changing the function of a particular internal signal; and (2) we propose a modified fault simulation procedure to check this condition. Our approach does not rely on any error model, and thus, is suitable for general types of errors. Furthermore, it can be easily extended to identify multiple errors. Experimental results on ISCAS89 benchmark circuits are presented to demonstrate its capability.

On verifying the correctness of retimed circuits

We address the problem of verifying a retimed circuit. After retiming, some latches in a sequential circuit are repositioned to reduce the clock cycle time and thus the behavior of the combinational portion is changed. Here, we present a novel approach to check the correctness of a retimed circuit according to the definition of 3-valued equivalence. This approach is based on our verification framework using sequential ATPG techniques. We further incorporate an algorithm to pre-process the circuits and make the verification process even more efficient. We will present the experimental results of verifying the retimed circuits with hundreds of flip-flops on ISCAS89 benchmark circuits to show its capability.

Cost-free scan: a low-overhead scan path design methodology

Conventional scan design imposes considerable area and delay overhead by using larger scan flip-flops and additional scan wires without utilizing the functionality of the combinational logic. We propose a novel low-overhead scan design methodology, called cost-free scan, which exploits the controllability of primary inputs to establish scan paths through the combinational logic. The methodology aims at reducing scan overhead by (1) analyzing the circuit to determine all the cost-free scan flip-flops, and (2) selecting the best primary input vector to establish the maximum number of cost-free scan flip-flops on the scan chain. Significant reduction in the scan overhead is achieved on ISCAS89 benchmarks, where in full scan environment, as many as 89% of the total flip-flops are found cost-free scannable, while in partial scan environment, reduction can be as high as 97% in scan flip-flops needed to break sequential loops.

Verifying sequential equivalence using ATPG techniques

In this paper we address the problem of verifying the equivalence of two sequential circuits. State-of-the-art sequential optimization techniques such as retiming and sequential redundancy removal can handle designs with up to hundreds or even thousands of flip-flops. However, the BDD-based approaches for verifying sequential equivalence can easily run into memory explosion for such designs. In an attempt to handle larger circuits, we modify test pattern-generation techniques for verification. The suggested approach utilizes the popular efficient backward-justification technique used in most sequential ATPG programs. We present several techniques to enhance the efficiency of this approach by (1) identifying equivalent flip-flop pairs using an induction-based algorithm, and (2) generalizing the idea of exploring the structural similarity between circuits to perform verification in stages. This ATPG-based framework is suitable for verifying circuits either with or without a reset state. In order to extend this approach to verify retimed circuits, we introduce a delay-compensation-based algorithm for preprocessing the circuits. The experimental results of verifying the correctness of circuits after sequential redundancy removal and retiming with up to several hundred flip-flops are presented.