Scan Chain Masking for Fault Coverage in IC Test Circuits
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Solution Overview
Problem
In integrated circuit testing, unknown values ('X') in scan chain outputs complicate fault detection and reduction, as they can interfere with known values and worsen test coverage, especially when scan data is compressed, making it difficult to localize faults accurately.
Innovation Solution
The implementation of a test circuit with a decompressor and compressor system that includes a mask block to selectively mask certain scan chains, allowing for configurable fanout modes to prevent the propagation of unknown values, thereby enhancing fault coverage and observability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If scan data is compressed to reduce data bandwidth and pins, then device complexity is reduced, but unknown values interfere with known values and reduce fault coverage
Solution Approach 1:
The patent segments scan chains into multiple groups, where each group is compressed independently into a separate bit stream. This segmentation prevents unknown values in one group from interfering with known values in other groups, thereby maintaining fault coverage while still achieving data compression. The test circuit includes multiple compressors that process different scan chain groups separately.
Solution Approach 2:
The patent introduces a mask block as an intermediary component between scan chains and the compressor. The mask block selectively masks (blocks) certain scan chains based on whether they contain unknown values, preventing those unknown values from propagating to the compressed output. This intermediary mechanism protects known values from contamination while allowing compression to proceed.
2Reliability
If scan chains are masked to prevent propagation of unknown values, then fault coverage is improved, but device complexity increases
Solution Approach 1:
The patent implements dynamic masking where the mask block can be configured to mask different scan chains based on the presence of unknown values in each chain. The masking is not fixed but adapts to the actual test conditions, allowing the system to maintain high fault coverage only when necessary while avoiding unnecessary complexity when all scan chains are clean.
Data Source
AI summary
Operating a scan chain of a test circuit of an integrated circuit to have either a single fanout or multiple fanout to a compressor. The test circuit receives a fanout control signal for configuring the fanout of the scan chain. If the fanout control signal indicates configuring of the scan chain with a single fanout, the output of the scan chain is sent to one input of a compressor. If the fanout control signal indicates configuring of the scan chain with multiple fanout, the output of the scan chain is sent to multiple inputs of the compressor.