Corner Cases and Verification Thinking

A corner case is a boundary or unusual operating condition where a design is more likely to fail. Good verification is not about collecting the largest possible number of tests; it is about making each test probe a distinct promise.

For a small counter, high-value cases include:

• reset from a nonzero state;
• the first transition after reset;
• hold while enable is inactive;
• the transition into the maximum value; and
• an attempted increment while already at the maximum.

Those cases cover reset, control, ordinary state movement, and the saturation boundary. A second ordinary middle-range increment may add little after the same behavior has already passed.

For each proposed test, write one sentence: “This case could expose ___.” If two tests complete that sentence in exactly the same way, keep the clearer or cheaper one and look for an uncovered boundary.

A compact suite should be deterministic, quick to run, and varied enough that different incorrect implementations fail for different reasons.