A framework for testing special-purpose memories

Current memory testing methods rely on fault models that are inadequate to accurately represent potential defects that occur in modern, often specialized, memories. To remedy this, the authors present a formal framework for modeling and testing special-purpose memories. Their approach uses three models: the transistor circuit, the event-sequence model, and finite-state machines. The methodology is explained using the example of a content-addressable memory (CAM). The fault model they describe comprises input stuck-at, transistor, and bridging faults. The authors show that functional tests can reliably detect all input stuck-at faults, most transistor faults (including all stuck-open faults), and about 50% of bridging faults. The remaining faults are detectable by parametric tests. A test of length 7n+2l+9 that detects all the reliably testable faults in an n-word by l-bit CAM is presented. A CAM test by Giles & Hunter is evaluated with respect to the input stuck-at faults. It is shown that this test fails to detect certain faults; it can be modified to achieve full coverage at the cost of increased length.