Scan Compression Output Encoding for On-Chip Test Comparison
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Solution Overview
Problem
As chips become increasingly complex, existing testing methods are inefficient, requiring longer times and higher costs due to the need for external comparison of test results, which can be bottlenecked by the slower scan-out process compared to scan-in.
Innovation Solution
Implementing an on-chip comparator that receives and compares test results directly, using a merged expected test-result and masking-instruction signal on fewer pins to reduce the number of output pins needed, thereby leveraging the speed advantage of scan-in and reducing test time and cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external comparison of test results is used, then test result accuracy is ensured, but test time increases and productivity decreases
Solution Approach 1:
The comparison function is extracted from the external testing equipment and implemented as an on-chip comparator unit. This allows the comparison operation to be performed internally on the chip rather than requiring external processing, thereby reducing test time while maintaining accuracy.
Solution Approach 2:
A merged signal is introduced as an intermediary that combines expected test results and masking instructions into a single encoded signal. This mediator enables the on-chip comparator to receive both comparison reference data and masking control simultaneously, facilitating efficient on-chip comparison operations.
2Reliability
If separate pins are used for expected test results and masking instructions, then signal integrity is maintained, but pin count increases and device complexity increases
Solution Approach 1:
Two separate signals (expected test results and masking instructions) are merged into a single encoded signal that is transmitted through fewer pins. The on-chip decoding logic reconstructs the original signals from this merged encoding, reducing pin count while preserving signal integrity through the encoding/decoding process.
Solution Approach 2:
The signal representation is transformed by encoding multiple pieces of information (expected results and masking instructions) into a compressed format with fewer bits. This parameter change in signal encoding reduces the number of pins required while maintaining all necessary information for accurate comparison and masking operations.
Data Source
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AI summary
A method for testing a chip comprising: receiving N scan-in chains of test data; using the N scan-in chains of test data to perform tests on the chip; receiving a merged expected test-result and masking-instruction signal on X pins of the chip from the off-chip test equipment, X being less than 2∗N; decoding the merged expected test-result and masking-instruction signal to extract N decoded output signals, each of the N decoded output signals corresponding to a respective chain of test results.