Scan Compression Selector for X-Tolerant Test Observation
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Solution Overview
Problem
Existing scan compression techniques face challenges in handling unknown values (Xs) effectively, leading to reduced test quality and increased pattern count due to coarse selection of non-X data and limited observability, especially in designs with high X densities and aggressive timing.
Innovation Solution
A scan-based circuit with a selector operable on a per-shift basis, coupled between scan chains and a compressor, allowing for multi-fanout overlap and direct observation modes to selectively handle Xs, ensuring data of interest is extracted and observed without masking non-X values, thereby maintaining high observability and compression efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If scan compression is applied to reduce test data volume, then test application time is reduced, but test quality deteriorates due to masking of non-X values by unknown values (Xs)
Solution Approach 1:
The patent segments the scan chain data into multiple groups, with each group processed by a separate compactor. This segmentation allows selective observation of specific scan cells while compressing others, preventing X-masking from propagating across the entire test set and thereby maintaining test quality while achieving compression.
Solution Approach 2:
The patent implements per-group observation control where each compactor can independently select which scan cell groups to observe. This local quality approach ensures that only necessary data is observed with high fidelity while other data is compressed, resolving the conflict between compression efficiency and test quality.
2Device complexity
If coarse selection of non-X data is used to handle unknown values, then circuit complexity is reduced, but observability deteriorates leading to increased pattern count
Solution Approach 1:
The patent divides scan cells into multiple groups and assigns each group to a separate compactor with independent observation control. This segmentation enables fine-grained observability control at the group level rather than coarse全院 level, maintaining high observability while keeping individual compactor circuits simple.
Solution Approach 2:
The patent introduces dynamic observation control where the observation status of each scan cell group can be independently adjusted based on the presence of X values. This dynamic approach allows the system to adapt observability levels to actual test needs, preventing information loss without requiring complex static circuitry.
3Reliability
If per-shift basis selection is implemented to handle Xs, then test quality is improved, but device complexity increases
Solution Approach 1:
The patent segments scan cells into groups processed by separate compactors, each with independent observation control. This segmentation distributes the complexity across multiple simple units rather than requiring one complex unit, achieving per-shift selection capability while keeping individual device complexity manageable.
Solution Approach 2:
The patent combines multiple simple compactors with independent observation control into a unified scan compression system. This merging approach achieves the functionality of complex per-shift selection by coordinating simpler units, improving test quality without proportionally increasing overall device complexity.
Data Source
AI summary
A scan-based circuit includes a selector that is implemented by multiple observation logics. Each observation logic is coupled to a scan chain to receive data to be supplied to a combinational compressor. Each observation logic is also coupled to a single input line in a corresponding group of input lines of the combinational compressor, to selectively supply data from the coupled scan chain. Each observation logic may be coupled to additional input lines (if present) in the corresponding group. The selector is operable on a per-shift basis in (a) transparent mode wherein data is supplied to all input lines and (b) several direct modes wherein data from only one scan chain is supplied at each compressor output without overlap.