A characteristic table predicts the next state from the present state and inputs. An excitation table works backwards: it lists which input settings can produce a required state transition. This distinction is useful when designing counters or converting one storage-element type into another.
Worked example
For a T flip-flop, next Q = current Q XOR T. To move from Q = 0 to next Q = 1, choose T = 1. To keep Q = 1 at one, choose T = 0. A D flip-flop is simpler: D must equal the required next Q.
What to watch for
An excitation-table don't-care means multiple input settings can achieve the transition. It should not be confused with an unknown physical voltage.
Try it yourself
What T input is needed to move from Q = 1 to next Q = 0?
Answer: T = 1, because the desired transition changes the stored bit.
How to study this topic
Write the old state, input values and next state in separate columns. Mark the active clock edge or enable interval. This prevents a stored value from being confused with an input that is merely waiting to be captured.
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Digital Electronics