The concept of applying partial fencing to logic built-in self test (LBIST) hardware structures for the purpose of using partially good chips is well known in the chip design industry. Deceptively difficult though is the task of verifying that any particular implementation of partial fencing logic actually provides the desired behavior of blocking down-stream impact of all signals from fenced interfaces, and also ensuring that the partial fencing does not inadvertently preclude any common logic from being fully tested. In this paper we discuss a case study for a verification method which exploits the power of formal verification to prove that any given partial fencing design satisfies all behavioral expectations. We describe the details of the verification method and discuss the benefits of using this approach versus using traditional simulation methods. We also discuss the testbenches created as part of applying this new method. Furthermore, we discuss the formal verification algorithms that were employed during application of the method along with the tuning that was done to enable efficient completion of the verification tasks at hand.
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