The Best Flip-Flops to Scan

The full-scan and the nonscan versions of a circuit This paper presents a new algorithm for selecting the best flip-flops to scan for achieving maximum fault coverage in a partial-scan circuit. The algorithm, called PASCAL (PArtial Scan AnaLysis), ranks the flip-flops based on their contribution to the fault coverage. The results of PASCAL provide a global view of the entire partial-scan design spectrum (from no scan to full scan), and allow the designer to estimate the fault coverage achievable with any number of scanned flip-flops and to select the minimal subset of flip-flops to scan for obtaining a desired fault coverage. The number of scanned flip-flops can be reduced by taking into account faults detected by functional tests.