Scan Chain Selector for X State Masking
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Solution Overview
Problem
Current built-in self-test techniques for integrated circuits face challenges in handling unknown states (X states) during testing, as even advanced time compactors do not effectively tolerate or manage these states, leading to inefficiencies and compromised test quality.
Innovation Solution
The implementation of a specialized scan chain selector coupled with a time compactor, which enables selective masking of X states through a scan chain selection scheme, allowing for efficient suppression of X states while preserving test quality and observability of scan cells, and handling various unknown state profiles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional time compactors are used for test response compaction, then compaction is performed, but unknown states (X states) cannot be effectively tolerated or managed, leading to compromised test quality
Solution Approach 1:
The test response is segmented into multiple scan chain groups, and a selector is used to selectively mask specific scan chains based on the presence of unknown states. This segmentation allows the system to handle X states in specific regions without affecting the entire test response, thereby maintaining test quality while tolerating unknown states.
Solution Approach 2:
A scan chain selector is introduced as an intermediary component between the scan chains and the time compactor. This selector acts as a mediator that can dynamically mask specific scan chains containing unknown states before the test response reaches the compactor, allowing the compactor to process clean data while the selector handles the X state management.
2Reliability
If scan chains are masked to suppress X states, then unknown states are eliminated, but observability of scan cells may be reduced
Solution Approach 1:
The masking configuration is made dynamic rather than static. The selector can be reconfigured between test cycles to adapt to different patterns of unknown states. This dynamic reconfiguration allows the system to mask only the necessary scan chains containing X states while preserving observability of other scan chains, thus balancing X state suppression with maintainability of scan cell observability.
Solution Approach 2:
Instead of uniformly masking all scan chains or using a fixed masking pattern, the system applies local quality by selectively masking only specific scan chains that contain unknown states. This localized masking approach ensures that scan cells with valid test data remain observable while only the problematic regions are masked, preserving overall test quality.
3Productivity
If selective masking is implemented per cycle, then X states are eliminated efficiently, but additional control information is required
Solution Approach 1:
The system performs preliminary analysis to identify which scan chains contain unknown states before the actual test cycle executes. By determining the masking configuration in advance, the selector can be pre-configured to mask only the necessary scan chains. This preliminary action approach improves efficiency by avoiding the need to mask all scan chains while keeping control information requirements manageable through targeted identification of problematic regions.
Data Source
AI summary
Built-in self-test techniques for integrated circuits that address the issue of unknown states. Some implementations use a specialized scan chain selector coupled to a time compactor. The presence of the specialized scan chain selector increases the efficiency in masking X states. Also disclosed are: (1) an architecture of a selector that works with multiple scan chains and time compactors, (2) a method for determining and encoding per cycle scan chain selection masks used subsequently to suppress X states, and (3) a method to handle an over-masking phenomenon.