In this paper, we propose a high-level variable selection for partial-scan approach to improve the testability of digital systems. The testability of a design is evaluated at the high level based on previously proposed controllability and observability measures. A testability grading technique is utilized to measure the relative testability improvement in a design as the result of making a subset of the variables fully controllable and observable. The variables that cause the greatest

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... y improvement are selected, and the selection process is performed incrementally until no further testability improvement can be achieved. Then the registers that correspond to the selected variables are placed in the scan-chain for partial-scan implementation. The experimental results shows that the variable selection approach produces partial-scan implementations that can achieve high fault coverage, while the logic overheads are fairly low.