Abhijit Jas

Test vector decompression via cyclical scan chains and its application to testing core-based designs

A novel test vector compression/decompression technique is proposed for reducing the amount of test data that must be stored on a tester and transferred to each core when testing a core-based design. A small amount of on-chip circuitry is used to reduce both the test storage and test time required for testing a core-based design. The fully specified test vectors provided by the core vendor are stored in compressed form in the tester memory and transferred to the chip where they are decompressed and applied to the core (the compression is lossless). Instead of having to transfer each entire test vector from the tester to the core, a smaller amount of compressed data is transferred instead. This reduces the amount of test data that must be stored on the tester and hence reduces the total amount of test time required for transferring the data with a given test data bandwidth.

Scan vector compression/decompression using statistical coding

A compression/decompression scheme based on statistical coding is presented for reducing the amount of test data that must be stored on a tester and transferred to each core in a core-based design. The test vectors provided by the core vendor are stored in compressed form in the tester memory and transferred to the chip where they are decompressed and applied to the core. Given the set of test vectors for a core, a statistical code is carefully selected so that it satisfies certain properties. These properties guarantee that it can be decoded by a simple pipelined decoder (placed at the serial input of the cores scan chain) which requires very small area. Results indicate that the proposed scheme can use a simple decoder to provide test data compression near that of an optimal Huffman code. The compression results in a two-fold advantage since both test storage and test time are reduced.

An efficient test vector compression scheme using selective Huffman coding

This paper presents a compression/decompression scheme based on selective Huffman coding for reducing the amount of test data that must be stored on a tester and transferred to each core in a system-on-a-chip (SOC) during manufacturing test. The test data bandwidth between the tester and the SOC is a bottleneck that can result in long test times when testing complex SOCs that contain many cores. In the proposed scheme, the test vectors for the SOC are stored in compressed form in the tester memory and transferred to the chip where they are decompressed and applied to the cores. A small amount of on-chip circuitry is used to decompress the test vectors. Given the set of test vectors for a core, a modified Huffman code is carefully selected so that it satisfies certain properties. These properties guarantee that the codewords can be decoded by a simple pipelined decoder (placed at the serial input of the cores scan chain) that requires very small area. Results indicate that the proposed scheme can provide test data compression nearly equal to that of an optimum Huffman code with much less area overhead for the decoder.

Test vector encoding using partial LFSR reseeding

A new form of LFSR reseeding that provides higher encoding efficiency and hence greater reduction in test data storage requirements is described. Previous forms of LFSR reseeding have been static (i.e. test generation is stopped and the seed is loaded at one time) and have required full reseeding (i.e. n=r bits are used for an r-bit LFSR). The new form of LFSR reseeding proposed here is dynamic (i.e. the seed is incrementally modified while test generation proceeds) and allows partial reseeding (i.e. n<r bits can be used). Full static forms of LFSR reseeding are shown to be a special case of the new partial dynamic form of LFSR reseeding. In addition to providing better encoding efficiency, partial dynamic LFSR reseeding has a simpler hardware implementation than previous schemes based on multiple-polynomial LFSRs, and can generate each test vector in fewer clock cycles. Experimental results demonstrate the advantages of the new partial dynamic LFSR reseeding approach.

Using an embedded processor for efficient deterministic testing of systems-on-a-chip

If a system-on-a-chip (SOC) contains an embedded processor, the paper presents a novel approach for using the processor to aid in testing the other components of the SOC. The basic idea is that the tester loads a program along with compressed test data into the processors on-chip memory. The processor executes the program which decompresses the test data and applies it to scan chains in the other components of the SOC to test them. This approach both reduces the amount of data that must be stored on the tester and reduces the test time. Moreover, it enables at-speed scan shifting even with a slow tester (i.e. a tester whose maximum clock rate is slower than the SOCs normal operating clock rate). A procedure is described for converting a set of test cubes (i.e., test vectors where unspecified inputs are left as Xs) into a compressed form. A program that can be run on an embedded processor is given for decompressing the test cubes and applying them to scan chains on the chip. Experimental results indicate that a significant amount of compression can be achieved.

Hybrid BIST based on weighted pseudo-random testing: a new test resource partitioning scheme

This paper presents a new test resource partitioning scheme that is a hybrid approach between external testing and BIST. It reduces tester storage requirements and tester bandwidth requirements by orders of magnitude compared to conventional external testing, but requires much less area overhead than a full BIST implementation providing the same fault coverage. The proposed approach is based on weighted pseudo-random testing and uses a novel approach for compressing and storing the weight sets. Three levels of compression are used to greatly reduce test costs. No test points or any modifications are made to the function logic. The proposed scheme requires adding only a small amount of additional hardware to the STUMPS architecture. Experimental results comparing the proposed approach with other approaches are presented.

Instruction-Level Impact Analysis of Low-Level Faults in a Modern Microprocessor Controller

We investigate the correlation between low-level faults in the control logic of a modern microprocessor and their instruction-level impact on the execution of typical workload. Such information can prove immensely useful in accurately assessing and prioritizing faults with regards to their criticality, as well as commensurately allocating resources to enhance online testability and error/fault resilience through concurrent error detection/correction methods. To this end, we developed an extensive fault simulation infrastructure which allows injection of stuck-at faults and transient errors of arbitrary starting time and duration, as well as cost-effective simulation and classification of their repercussions into various instruction-level error types. As a test vehicle for our study, we employ a superscalar, dynamically-scheduled, out-of-order, Alpha-like microprocessor, on which we execute SPEC2000 integer benchmarks. Extensive fault injection campaigns in control modules of this microprocessor facilitate valuable observations regarding the distribution of low-level faults into the instruction-level error types that they cause. Experimentation with both Register Transfer (RT-) and Gate-Level faults, as well as with both stuck-at faults and transient errors, confirms the validity and corroborates the utility of these observations.

Weighted pseudorandom hybrid BIST

This paper presents a new test data-compression scheme that is a hybrid approach between external testing and built-in self-test (BIST). The proposed approach is based on weighted pseudorandom testing and uses a novel approach for compressing and storing the weight sets. Three levels of compression are used to greatly reduce test costs. Experimental results show that the proposed scheme reduces tester storage requirements and tester bandwidth requirements by orders of magnitude compared to conventional external testing, but requires much less area overhead than a full BIST implementation providing the same fault coverage. No test points or any modifications are made to the function logic. The paper describes the proposed hybrid BIST architecture as well as two different ways of storing the weight sets, which are an integral part of this scheme.

Achieving high encoding efficiency with partial dynamic LFSR reseeding

Previous forms of LFSR reseeding have been static (i.e., test application is stopped while each seed is loaded) and have required full reseeding (i.e., the length of the seed is equal to the length of the LFSR). A new form of LFSR reseeding is described here that is dynamic (i.e., the seed is incrementally modified while test application proceeds) and allows partial reseeding (i.e. length of the seed is less than that of the LFSR). In addition to providing better encoding efficiency, partial dynamic LFSR reseeding has a simpler hardware implementation than previous schemes based on multiple-polynomial LFSRs.

RT-Level Deviation-Based Grading of Functional Test Sequences

Functional test sequences are often used in manufacturing testing to target defects that are not detected by structural test. Therefore, it is necessary to evaluate the quality of functional test sequences. However, it is very time-consuming to evaluate the quality of functional test sequences by gate-level fault simulation. Therefore, we propose output deviations as a metric to grade functional test sequences at the register transfer (RT)-level without explicit fault simulation. Experimental results for the open-source Parwan processor and the Scheduler module of the Illinois Verilog Model (IVM) show that the deviations metric is computationally ef¿cient and it correlates well with gate-level coverage for stuck-at, transition-delay, and bridging faults. Results also show that functional test sequences that are reordered based on output deviations provide steeper gate-level fault coverage ramp-up compared to other ordering methods.