Moore vs Mealy State Machines – Which One Should I Use?

The key difference between Moore and Mealy state machines lies in how they generate their outputs. In a Moore machine, the output depends only on the current state, whereas in a Mealy machine, the output depends on both the current state and the input. This results in different timing behaviors: Moore outputs change only on state transitions (i.e., clock edges), while Mealy outputs can respond immediately to input changes without waiting for a state transition.

In practice, this means that Moore machines are more stable and less prone to glitches, making them easier to simulate and debug. However, they may require more states and often have a one-clock-cycle delay in response. On the other hand, Mealy machines can be more efficient, often requiring fewer states and providing faster responses, but they can suffer from glitches if the inputs aren’t properly synchronized.

For example, in a vending machine design, a Moore FSM might only assert a "dispense item" output after reaching a dedicated "dispense" state. In contrast, a Mealy FSM could assert the output as soon as the right coin combination is detected, resulting in faster response without needing an extra state.

When choosing between the two, use Moore when you need clean, stable outputs and can tolerate a slight delay. Use Mealy when output speed matters and you’re optimizing for state count or hardware efficiency — just be cautious about potential glitches.

In summary, both models are valid. Moore FSMs are simpler and safer, while Mealy FSMs are more responsive and compact. Your choice depends on the specific needs of your digital design — whether you value timing stability or reaction speed more.