Question

In a Moore machine, if the output is determined solely by the current state, what is the exact number of cycles required for an input change to manifest at the output register compared to a Mealy machine?

Accepted Answer

In a Moore machine, the output is a function solely of the current state, meaning the output value changes only when the internal state transitions to a new value. Since state transitions in synchronous digital logic are triggered by a clock edge, an input change must first propagate through the combinational logic to update the state register, and then the updated state must propagate through the output logic to reach the output register. This process requires exactly one full clock cycle to manifest at the output. Conversely, in a Mealy machine, the output is a function of both the current state and the current input. Because the output logic is connected directly to the input, a change in the input signal can propagate through the combinational output logic to reach the output register instantly, without waiting for a clock cycle. Therefore, a Moore machine requires one additional clock cycle of latency compared to a Mealy machine to reflect an input change at the output.

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In a Moore machine, if the output is determined solely by the current state, what is the exact number of cycles required for an input change to manifest at the output register compared to a Mealy machine?