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Cost-free scan: a low-overhead scan path design

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4 Author(s)
Chih-Chang Lin ; California Univ., Santa Barbara, CA, USA ; M. Marek-Sadowska ; M. T. -C. Lee ; Kuang-Chien Chen

Conventional scan design imposes considerable area and delay overheads. To establish a scan chain in the test mode, multiplexers at the inputs of flip-flops and scan wires are added to the actual design. However, the functionality of the functional logic has not been utilized for the test purposes. We propose a low-overhead scan design methodology, called cost-free scan, which exploits the controllability of primary inputs to establish scan paths through the functional logic. We show how to analyze the circuit to determine all the free-scan flip-flops and select the best input vector to establish the maximum number of free-scan flip-flops for the scan chain design. Significant reduction in the scan overhead is achieved on ISCAS89 benchmarks. In full-scan designs, as many as 89% of the flip-flops are found free-scannable. In the partial-scan designs, we assume that selecting flip-flops for scan to break sequential cycles is used to increase circuit testability. Reduction can be as high as 97% in scan flip-flops needed to break sequential cycles

Published in:

IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems  (Volume:17 ,  Issue: 9 )