BILBO registers with nonlinear feedback

Abstract

Built in logic block observation (BILBO) has become one of the most widely accepted techniques for self-testing of complex digital ICs. This technique is based on grouping the storage elements of the circuit in the two registers which give this technique its name. A BILBO register has four functional modes: with each of the stages acting as independent registers; as a generator of pseudorandom sequences; as analyser of multiple-input signatures; and reset of all stages. For a BILBO register to perform these four functions correctly the feedback from its state to its first stage must satisfy certain conditions. First, the pseudorandom sequence that it generates must be of maximum length, i.e. it must run through all possible states except zero. Secondly, when it is acting as a parallel signature analyser the number of stages must be large enough for the error escape probability to be negligible. Normally the feedback of a BILBO register is linear. We propose the use of nonlinear feedback, through Exclusive-Nor gates, which conserves the properties of linear feedback and provides the advantage that the fourth functional mode, reset, serves as a seed for the other modes. Additional logic is not required and a seed does not have to be introduced from an external source.