Fully X-tolerant combinational scan compression

Abstract

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.

A Case Study on Implementing Compressed DFT Architecture

“…The output side has a layer of masking logic dedicated to handling don't care bits (Xs) in the response, an XOR based compactor and a shift register that has additional XORs to accumulate the response as it streams out. The technology can scale from as low as one scan input/output to as many scan ports as available on the design It must be noted that a low pin count solution can be developed by designing serial-to-parallel structures around [15] by adding registers at the scan interface. However, this leads to a degradation in compression QoR as additional shift cycles are added per scan shift operation.…”

“…For designs with ample number of scan ports, hierarchical approaches have been presented in [15] using adaptive scan compression. Here the design is partitioned in to modules based on either blocks where DFT is to be inserted or blocks which are DFT ready and have abstracted test models that can be used for DFT closure rather than their full netlist representation.…”

On Determining the Real Output Xs by SAT-Based Reasoning

“…Others achieve X-tolerance by decoupling certain scan channels from each other in the response compactor [10]. In some cases a combination of X-tolerance with X-masking schemes can be found [11,12].…”

Constructing Augmented Multimode Compactors

“…The analysis was focused on the observability since this property had the highest impact on the number of test patterns and the test quality in the presence of a large number of Xs. More formally, the AMC(n,k,t) constructed based on Corollary 2 (using a simple output mapping) was compared with the fully X-tolerant compactor [19]. Accordingly, both compactors had the same number of inputs and outputs, and used control per shift.…”

“…Significant improvement in the X-tolerant capabilities of the compactors was achieved based on masking and selection using control per test, control per shift or combined control [13][14][15][16][17][18][19]. Most of the presented schemes are compatible with certain types of decompressors and compressors.…”