Peter Wohl

Efficient compression and application of deterministic patterns in a logic BIST architecture

We present a novel method to efficiently generate, compress and apply test patterns in a logic BIST architecture. Patterns are generated by a modified automatic test pattern generator (ATPG) and are encoded as linear feedback shift register (LFSR) initial values (seeds); one or more patterns can be encoded into a single LFSR seed. During test application, seeds are loaded into the LFSR with no cycle overhead. The method presented achieves reductions of at least 100x in test data and 10x in tester cycles compared to deterministic ATPG while maintaining complete fault coverage, as confirmed by experimental results on industrial designs.

X-tolerant compression and application of scan-atpg patterns in a bist architecture

We present X-tolerant deterministic BIST (XDBIST), a novel method to efficiently compress and apply scan pattems generated by automatic test pattern generation (ATPG) in a logic built-in self-test architecture. Our method allows test patterns to have any number of unknown values with no degradation in compression and application efficiency. XDBIST does not require changing the core logic of the device under test (DUT); no test points or X-blockage logic need be inserted. The proposed solution guarantees the same high test coverage and diagnosis ability as deterministic scan-ATPG and uses the same tester flow, while reducing test data volume and tester cycles by more than 10 times.

Design of compactors for signature-analyzers in built-in self-test

Originally developed decades ago, logic built-in self-test (BIST) evolved and is now increasingly being adopted to cope with rapid growth in design size and complexity. Compared to deterministic pattern test, logic BIST requires many more test patterns, and therefore, increased test time unless many more internal scan chains can be shifted in parallel. To match this large number of scan chains, the width of the signature analyzer would have to be enlarged, which would result in large area overhead and signature storage space. Instead, a combinational space-compactor is inserted between the scan chain outputs and the signature analyzer inputs. However, the compactor may deteriorate the ability to test and diagnose the design. This paper analyzes how compactors affect test and diagnosis and shows that compactors can be designed to actually improve the testability of certain faults, while providing full diagnosis capability. Algorithms that allow automated design of optimal compactors are presented and results are discussed.

Effective diagnostics through interval unloads in a BIST environment

Logic built-in self test (BIST) is increasingly being adopted to improve test quality and reduce test costs for rapidly growing designs. Compared to deterministic automated test pattern gener¿ation (ATPG), BIST presents inherent fault diagnostic challenges. Previous diagnostic techniques have been limited in their diagnosis resolution and/or require significant hardware overhead. This paper proposes an interval-based scan-unload method that ensures diagnosis resolution down to gate-level faults with minimal hard¿ware overhead. Tester fail-data collection is based on a novel con¿struct incorporated into the design-extensions of the standard test-interface language (STIL). The implementation of the proposed method is presented and analyzed.

Fully X-tolerant combinational scan compression

Traditional scan and, more recently, scan compression are increasingly accepted for reducing test cost and improving quality in ever more complex designs. Combinational scan compression techniques are attractive for their low impact on area, timing and design flow, but are best suited for designs with a limited number of unknowns (Xs). However, recent design performance and cost tradeoffs create a much higher density of Xs than previously expected. We present a combinational scan compression method that preserves the low-impact advantages, while also allowing any number and distribution of Xs with virtually no loss of test quality. Results on industrial designs with a varied density of Xs demonstrate consistent data and test time compressions with negligible impact on all design parameters.

Minimizing the Impact of Scan Compression

Scan is widely accepted as the basis for reducing test cost and improving quality, however its effectiveness is compromised by increasingly complex designs and fault models that can result in high scan data volume and application time. The authors present a scan compression method designed for minimal impact in all aspects: area overhead, timing, and design flow. Easily adopted on top of existing scan designs, the method is fully integrated in the scan synthesis and test generation flows. Data and test time compressions of over 10times were obtained on industrial designs with negligible overhead and no impact on schedule.

Efficient compression of deterministic patterns into multiple PRPG seeds

Recent test-cost reduction methods are based on controlling the initial state (seed) of a pseudo-random pattern generator (PRPG) so that deterministic values are loaded in selected scan cells. Combined with an unload-data compression technique, PRPG seeding reduces test data volume and application time. This paper presents a method of mapping each scan load to multiple PRPG seeds, computed so that test pattern count, data volume, and, therefore, test cost are minimized. This method also allows smaller and fewer PRPGs, reducing the area overhead of test-compression circuitry. The results on deep-submicron industrial designs, show significant test cost reduction when this method is applied with either X-tolerant or X-free unload-data compression

Scalable selector architecture for X-tolerant deterministic BIST

X-tolerant deterministic BIST (XDBIST) was recently presented as a method to efficiently compress and apply scan patterns generated by automatic test pattern generation (ATPG) in a logic built-in self-test architecture. In this paper we introduce a novel selector architecture that allows arbitrary compression ratios, scales to any number of scan chains and minimizes area overhead. XDBIST test-coverage, full X-tolerance and scan-based diagnosis ability are preserved and are the same as deterministic scan-ATPG.

Fully X-tolerant, very high scan compression

This paper presents a new X-blocking system which allows very high compression and full coverage even if the density of unknown values is very high and varies every shift. Despite the presence of Xs in scan cells, compression can be maximized by using PRPG and MISR structures. Results on industrial designs with various X densities demonstrate consistently high compression and full test coverage.

Analysis and design of optimal combinational compactors [logic test]

Scan and logic built-in self-test (BIST) are increasingly used to reduce test cost. In these test architectures, many internal signals are observed through a small number of output pins or into a small signature analyzer, requiring a combinational space compactor. This paper analyzes the basic requirements of compactors to support efficient test and diagnosis, focusing on practical compactors where all inputs have a fanout of two. We show how graph theory can be used to model compactors and design compactors with robust non-aliasing properties that have minimal area and delay overhead and are independent of the test set, the fault model, and the circuit tested.