A controller redesign technique to enhance testability of controller-data path circuits

We study the effect of the controller on the testability of sequential circuits composed of controllers and data paths. We show that even when all the loops of the circuit have been broken by using scan flip-flops (FF´s) and the control and data path parts are individually 100% testable, the composite circuit may not be easily testable by gate-level sequential automatic test pattern generation (ATPG). Analysis shows that a primary problem in test pattern generation of combined controller-data path circuits is the correlation of control signals due to implications imposed by the controller specification. A design-for-testability (DFT) technique is developed to redesign the controller such that the implications which may produce conflicts during test pattern generation are eliminated. The DFT technique involves adding extra control vectors to the controller. Experimental results show the ability of the controller DFT technique to produce highly testable controller-data path circuits, with nominal hardware overhead