Autoscan-Invert: An Improved Scan Design without External Scan Inputs or Outputs

Autoscan is a design-for-testability approach proposed earlier that uses scan chains without external scan inputs or outputs in order to reduce the test application time and test data volume of scan. We describe three improvements to the basic autoscan design-for-testability approach based on the following observation. Under autoscan, due to the elimination of external scan inputs, the first flip-flop of a scan chain can only receive its value from the corresponding next-state line. Thus, its state cannot be controlled directly by a scan operation. In the improved autoscan approach, we allow the inverted next-state line to drive the first flip-flop of a scan chain during scan operations. We refer to the improved autoscan approach as autoscan-invert. We describe a scan synthesis procedure appropriate for autoscan-invert and present experimental results.