An excitation table answers the design question: which inputs do I need? You know where Q is now and where you want it after the edge, and the table tells you what to put on the flip-flop's inputs.
It is the characteristic table read backwards. The characteristic table goes from inputs to the next state; the excitation table goes from a transition to the inputs.
The four transitions (X = don't care, either value works):
- JK: 0 → 0 needs J = 0, K = X. 0 → 1 needs J = 1, K = X. 1 → 0 needs J = X, K = 1. 1 → 1 needs J = X, K = 0.
- T: T = 1 exactly when Q must change, so T = , where N is the wanted next state.
- D: D is just the wanted next state, D = N.
- SR: 0 → 0 needs S = 0, R = X. 0 → 1 needs S = 1, R = 0. 1 → 0 needs S = 0, R = 1. 1 → 1 needs S = X, R = 0.
Why the X's: for 0 → 1 on a JK, both set (J = 1, K = 0) and toggle (J = 1, K = 1) work, so K doesn't matter. Those don't-cares make K-maps smaller when you design counters and state machines.
The design recipe: list each present state and next state, look up the inputs each flip-flop needs, then simplify each input as a function of the present state.
| D | T | ||
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 0 |