Chih-Tsun Huang

Built-in redundancy analysis for memory yield improvement

With the advance of VLSI technology, the capacity and density of memories is rapidly growing. The yield improvement and testing issues have become the most critical challenges for memory manufacturing. Conventionally, redundancies are applied so that the faulty cells can be repairable. Redundancy analysis using external memory testers is becoming inefficient as the chip density continues to grow, especially for the system chip with large embedded memories. This paper presents three redundancy analysis algorithms which can be implemented on-chip. Among them, two are based on the local-bitmap idea: the local repair-most approach is efficient for a general spare architecture, and the local optimization approach has the best repair rate. The essential spare pivoting technique is proposed to reduce the control complexity. Furthermore, a simulator has been developed for evaluating the repair efficiency of different algorithms. It is also used for determining certain important parameters in redundancy design. The redundancy analysis circuit can easily be integrated with the built-in self-test circuit.

A programmable BIST core for embedded DRAM

The programmable BIST design presented here supports various test modes using a simple controller. With the March C algorithm, the BIST circuits overhead is under 1.3% for a 1-Mbit DRAM and under 0.3% for a 16-Mbit DRAM. The BIST design presented for embedded DRAM supports built-in self-diagnosis by feeding error information to the external tester. Moreover, using a specific test sequence, it can test for critical timing faults, reducing tester time for ac parametric test. The design supports wafer test, pre-burn-in test, burn-in, and final test. It is field-programmable; the user can program test algorithms using predetermined test elements (such as march elements, surround test elements, and refresh modes). The user can optimize the hardware for a specific embedded DRAM with a set of predetermined test elements. Our design is different from the microprogram-controlled BIST described by J. Dreibelbis et al. (1998) which has greater flexibility but higher overhead. Because our design begins at the register-transfer language level, test element insertion (for higher test coverage) and deletion (for lower hardware overhead are relatively easy.

RAMSES: a fast memory fault simulator

In this paper, we present a memory fault simulator called the Random Access Memory Simulator for Error Screening (RAMSES). Although it was designed based on some well-known memory fault models, the algorithm that we developed ensures that new fault models can be included easily by adding new fault descriptors instead of modifying the algorithm or program. With RAMSES, the time complexity of memory fault simulation is improved from O(N/sup 3/) to O(N/sup 2/), where N is the memory capacity in terns of bits. Our approach requires only a small amount of extra memory space. Simulation results by RAMSES show that running the proposed cocktail-March tests can significantly reduce the test time. With the help of RAMSES, an efficient test algorithm called March-CW was developed for word-oriented memories.

Optimization of Pattern Matching Circuits for Regular Expression on FPGA

Regular expressions are widely used in the network intrusion detection system (NIDS) to represent attack patterns. Previously, many hardware architectures have been proposed to accelerate regular expression matching using field-programmable gate array (FPGA) because FPGAs allow updating of new attack patterns. Because of the increasing number of attacks, we need to accommodate a large number of regular expressions on FPGAs. Although the minimization of logic equations has been studied intensively in the area of computer-aided design (CAD), the minimization of multiple regular expressions has been largely neglected. This paper presents a novel sharing architecture allowing our algorithm to extract and share common subregular expressions. Experimental results show that our sharing scheme significantly reduces the area of pattern matching circuits for regular expression.

March-based RAM diagnosis algorithms for stuck-at and coupling faults

Diagnosis technique plays a key role during the rapid development of the semiconductor memories, for catching the design and manufacturing failures and improving the overall yield and quality. Investigation on efficient diagnosis algorithms is very important due to the expensive and complex fault/failure analysis process. We propose March-based RAM diagnosis algorithms which not only locate faulty cells but also identify their types. The diagnosis complexity is O(17N) and O((17+10B)N) for bit-oriented and word-oriented diagnosis algorithms, respectively, where N represents the address number and B is the data width. Using the proposed algorithms, stuck at faults, state coupling faults, idempotent coupling faults and inversion coupling faults can be distinguished. Furthermore, the coupled and coupling cells can be located in the memory array. Our word-oriented diagnosis algorithm can distinguish all of the inter-word and intra-word coupling faults, and locate the coupling cells of the intra-word inversion and idempotent coupling faults. With additional 2B-1 operations, the algorithm can further locate the intra-word state coupling faults. With improved diagnostic resolution and test time, the proposed algorithms facilitate the development and manufacturing of semiconductor memories.

Flash memory built-in self-test using March-like algorithms

Flash memories are a type of non-volatile memory based on floating-gate transistors. The use of commodity and embedded flash memories are growing rapidly as we enter the system-on-chip (SOC) era. Conventional tests for flash memories are usually ad hoc-the test procedure is developed for a specific design. We propose improved March-like algorithms (i.e., March FT) for both bit-oriented and word-oriented flash memory; to cover the disturbance faults derived from the IEEE 1005 Standard, as well as conventional faults. A novel flash memory fault simulator is used to analyze and generate the test algorithms. In addition, we present BIST designs for two industrial flash memories. The area overhead is only about 3% for a medium-sized flash memory.

Optimization of Regular Expression Pattern Matching Circuits on FPGA

Regular expressions are widely used in network intrusion detection system (NIDS) to represent patterns of network attacks. Since traditional software-only NIDS cannot catch up to the speed advance of networks, many previous works propose hardware architectures on FPGA to accelerate attack detection. The challenge of hardware implementation is to accommodate the regular expressions to FPGAs of the large number of attacks. Although the minimization of logic equations has been studied intensively in the CAD area, the minimization of multiple regular expressions has been largely neglected. This paper presents an architecture allowing our algorithm to extract and share common sub-regular expressions. Experimental results show that our sharing scheme significantly reduces the area of regular expression circuits

Single- and Multi-core Configurable AES Architectures for Flexible Security

As networking technology advances, the gap between network bandwidth and network processing power widens. Information security issues add to the need for developing high-performance network processing hardware, particularly that for real-time processing of cryptographic algorithms. This paper presents a configurable architecture for Advanced Encryption Standard (AES) encryption, whose major building blocks are a group of AES processors. Each AES processor provides 219 block cipher schemes with a novel on-the-fly key expansion design for the original AES algorithm and an extended AES algorithm. In this multicore architecture, the memory controller of each AES processor is designed for the maximum overlapping between data transfer and encryption, reducing interrupt handling load of the host processor. This design can be applied to high-speed systems since its independent data paths greatly reduces the input/output bandwidth problem. A test chip has been fabricated for the AES architecture, using a standard 0.25-¿m CMOS process. It has a silicon area of 6.29 mm2, containing about 200,500 logic gates, and runs at a 66-MHz clock. In electronic codebook (ECB) and cipher-block chaining (CBC) cipher modes, the throughput rates are 844.9, 704, and 603.4 Mb/s for 128-, 192-, and 256-b keys, respectively. In order to achieve 1-Gb/s throughput (including overhead) at the worst case, we design a multicore architecture containing three AES processors with 0.18-¿m CMOS process. The throughput rate of the architecture is between 1.29 and 3.75 Gb/s at 102 MHz. The architecture performs encryption and decryption of large data with 128-b key in CBC mode using on-the-fly key generation and composite field S-box, making it more cost effective (with better thousand-gate/gigabit-per-second ratio) than conventional methods.

An HMAC processor with integrated SHA-1 and MD5 algorithms

Cryptographic algorithms are prevalent and important in digital communications and storage, e.g., both SHA-1 and MD5 algorithms are widely used hash functions in IPSec and SSL for checking the data integrity. In this paper, we propose a hardware architecture for the standard HMAC function that supports both. Our HMAC design automatically generates the padding words and reuses the key for consecutive HMAC jobs that use the same key. We have also implemented the HMAC design in silicon. Compared with existing designs, our HMAC processor has lower hardware cost---12.5% by sharing of the SHA-1 and MD5 circuitry and a little performance penalty.

Error catch and analysis for semiconductor memories using march tests

We present an error catch and analysis (ECA) system for semiconductor memories. The system consists of a test algorithm generator called TAGS, a fault simulator called RAMSES, and an error analyzer (ERA). We use TAGS to generate a set of test algorithms of different lengths and diagnostic resolutions for the memory under test, and use RAMSES to generate the March dictionary for each test algorithm. With the March dictionaries, ERA is able to support March algorithms for easy diagnosis of faulty RAMs. Legacy test algorithms also can be reused. When integrated with a RAM tester, our ECA system can generate RAM bitmaps that are similar to the RAM layout. The bitmaps provide detail information about the error locations and faults causing the errors. Based on the information, diagnosis of the RAM chips for yield and reliability improvement can be done more easily.