Moore vs Mealy · RTL Design

Part 4 · FSM & Control · Intermediate

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12lessons

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Moore outputs depend on the state register alone. Mealy outputs depend on the state and the current inputs.

Model	Output depends on	Typical consequence
Moore	The current state only	No direct input-to-output path; decode can glitch near an edge; sometimes one extra state
Mealy	The current state and the current inputs	Reacts in the same cycle; fewer states; outputs can change when inputs do

Block diagram contrasting Moore and Mealy output paths.

Moore outputs are a function of the state register alone. A Mealy output logic also sees the inputs, shown by the dotted line.

Lessons in this topic

4 lessons, about 13 min

Behavioral DifferenceYou can predict the output timing of a Moore and a Mealy detector for the same input, and say why they are equivalent in function.3 min
Side-by-Side: A Sequence DetectorYou can code the two-ones detector as Mealy and Moore, trace both on the same samples, and count the flip-flops each needs.4 min
Registered Mealy OutputsYou can register a Mealy output, state the latency cost, and say what registration does and does not fix.3 min
Choosing and VerifyingYou can choose Moore, Mealy or registered outputs for a destination, and write a checklist that verifies the FSM.3 min

Check yourself Which model can respond to an input in the same clock cycle it arrives?

Mealy. Its outputs are combinational functions of the inputs, so they change as soon as an input does (before the next edge). A Moore output reacts one clock edge later, when the state has changed.

Revision sheet

The whole topic on one page, taken from each lesson's Remember box. Read it before an interview, or print it.

Behavioral Difference

A Mealy output uses the state and the live input, so it responds in the same cycle.
A Moore output uses the state only, so it follows the clock edge that enters the state.
A Mealy output feeding another block is a combinational path from your input to their logic.

Side-by-Side: A Sequence Detector

Read the output just before the rising edge, where the input is settled and the state has not yet updated.
Mealy y = (state == S1) && x; drop the && x and it signals only that the previous sample was 1.
Moore needs a state for 'two ones seen', so it costs three states against two.

Rule of thumb: the Moore output is the Mealy output delayed by one cycle.

Registered Mealy Outputs

Registering moves the decision from before the capture edge to after it.
A registered Mealy output is a Moore machine whose state includes the output bit.
Choose by the receiver's timing and latency contract. Source registration alone does not handle CDC.

Choosing and Verifying

Boundaries want registered outputs, and local fast paths want Mealy.
Reach every state through legal inputs rather than forcing the state register.
Check reset state, defaults, reachability, progress under the environment and illegal-state behaviour.

Practise it

Mini project: an SPI master, controller and datapath
Practice question: Types of FSMs and their difference
Practice question: How outputs are generated in Moore FSMs
Code lab: Sequence FSM
Practice question: FSM output encoding: registered vs combinational hazards
Interview: run a mock interview on this topic

Up nextBehavioral Difference

Practice this lesson Report an error

Model

Output depends on

Typical consequence

Moore

The current state only

No direct input-to-output path; decode can glitch near an edge; sometimes one extra state

Mealy

The current state and the current inputs

Reacts in the same cycle; fewer states; outputs can change when inputs do

Lessons in this topic

4 lessons, about 13 min

Behavioral DifferenceYou can predict the output timing of a Moore and a Mealy detector for the same input, and say why they are equivalent in function.3 min
Side-by-Side: A Sequence DetectorYou can code the two-ones detector as Mealy and Moore, trace both on the same samples, and count the flip-flops each needs.4 min
Registered Mealy OutputsYou can register a Mealy output, state the latency cost, and say what registration does and does not fix.3 min
Choosing and VerifyingYou can choose Moore, Mealy or registered outputs for a destination, and write a checklist that verifies the FSM.3 min

Check yourself Which model can respond to an input in the same clock cycle it arrives?

Mealy. Its outputs are combinational functions of the inputs, so they change as soon as an input does (before the next edge). A Moore output reacts one clock edge later, when the state has changed.

Revision sheet

The whole topic on one page, taken from each lesson's Remember box. Read it before an interview, or print it.

Behavioral Difference

A Mealy output uses the state and the live input, so it responds in the same cycle.

A Moore output uses the state only, so it follows the clock edge that enters the state.

A Mealy output feeding another block is a combinational path from your input to their logic.

Side-by-Side: A Sequence Detector

Read the output just before the rising edge, where the input is settled and the state has not yet updated.

Mealy y = (state == S1) && x; drop the && x and it signals only that the previous sample was 1.

Moore needs a state for 'two ones seen', so it costs three states against two.

Rule of thumb: the Moore output is the Mealy output delayed by one cycle.

Registered Mealy Outputs

Registering moves the decision from before the capture edge to after it.

A registered Mealy output is a Moore machine whose state includes the output bit.

Choose by the receiver's timing and latency contract. Source registration alone does not handle CDC.

Choosing and Verifying

Boundaries want registered outputs, and local fast paths want Mealy.

Reach every state through legal inputs rather than forcing the state register.

Check reset state, defaults, reachability, progress under the environment and illegal-state behaviour.