Moore machine vs Mealy machine in Verilog - Visual Side-by-Side Comparison

Concept Flow - Moore machine vs Mealy machine

Input Signal

↓

State Update

↓

Output depends

↓

only on state

↓

Output Signal

Shows how Moore and Mealy machines process input and produce output differently based on state and input.

Execution Sample

Verilog

module moore_example(input clk, input x, output reg y);
  reg state;
  always @(posedge clk) begin
    state <= x;
    y <= state;
  end
endmodule

A simple Moore machine example where output y depends only on the current state.

Execution Table

Step	clk edge	Input x	State before	State after	Output y
1	rising	0	0	0	0
2	rising	1	0	1	0
3	rising	1	1	1	1
4	rising	0	1	0	1
5	rising	0	0	0	0

💡 Simulation stops after 5 clock cycles showing state and output changes.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
state	0	0	1	1	0	0
y	0	0	0	1	1	0
x	0	0	1	1	0	0

Key Moments - 2 Insights

Why does the output y in the Moore machine lag behind the input x by one clock cycle?

Because in the Moore machine output depends only on the current state, which updates on the clock edge after input changes. See execution_table rows 2 and 3 where output y changes after state updates.

How would output change if this was a Mealy machine?

In a Mealy machine, output depends on both state and input immediately, so output can change in the same clock cycle as input changes, unlike the Moore machine shown.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output y at step 3?

A 1

B 0

C Undefined

D Same as input x

💡 Hint

Check the 'Output y' column at step 3 in the execution_table.

At which step does the state first become 1?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'State after' column in the execution_table.

If this was a Mealy machine, how would output y behave compared to the Moore machine?

A Output y would never change

B Output y would change only after state updates

C Output y would change immediately with input x

D Output y would be random

💡 Hint

Recall Mealy output depends on state and input, unlike Moore output which depends only on state.

Concept Snapshot

Moore machine: output depends only on state, changes on clock edge.
Mealy machine: output depends on state and input, can change immediately.
Moore outputs lag input by one cycle.
Mealy outputs react faster to input changes.
Use Moore for simpler timing, Mealy for faster response.

Full Transcript

This visual execution compares Moore and Mealy machines using a simple Verilog Moore example. The Moore machine updates its state on each clock rising edge based on input x, and output y depends only on the current state. The execution table shows how state and output change step-by-step over 5 clock cycles. Output y lags input x by one cycle because it depends on the updated state. Key moments clarify why output lags in Moore machines and how Mealy machines differ by reacting immediately to input changes. The quiz tests understanding of output timing and state changes. The snapshot summarizes the main differences: Moore outputs depend only on state and update on clock edges, while Mealy outputs depend on both state and input and can change faster.

Practice

1. Which statement correctly describes the output behavior of a Moore machine compared to a Mealy machine?

easy

A. Moore machine output depends only on the current state, while Mealy machine output depends on state and input.

B. Moore machine output depends on input only, Mealy machine output depends on current state only.

C. Both Moore and Mealy machines output depend only on the current input.

D. Moore machine output changes immediately with input, Mealy output changes only on clock edges.

Solution

1. Step 1: Understand Moore machine output dependency

   Moore machine outputs depend solely on the current state, not directly on inputs.

2. Step 2: Understand Mealy machine output dependency

   Mealy machine outputs depend on both the current state and the current input, allowing immediate output changes.

3. Final Answer:

   Moore machine output depends only on the current state, while Mealy machine output depends on state and input. -> Option A

4. Quick Check:

   Moore = state-only output, Mealy = state + input output [OK]

Hint: Moore = state only; Mealy = state plus input [OK]

Common Mistakes:

• Confusing output dependency on input for Moore machine
• Thinking Mealy output changes only on clock edges
• Assuming both machines output depend only on input

2. Which Verilog code snippet correctly models a Moore machine output?

easy

A. always @(input or state) output = input & state;

B. always @(posedge clk) output <= state;

C. always @(posedge clk or input) output <= input;

D. always @(input) output = state;

Solution

1. Step 1: Identify Moore machine output timing

   Moore machine outputs update only on clock edges, so output assignment must be inside a posedge clk block.

2. Step 2: Check code snippets for correct output assignment

   always @(posedge clk) output <= state; uses 'always @(posedge clk)' and assigns output from state, matching Moore behavior.

3. Final Answer:

   always @(posedge clk) output <= state; -> Option B

4. Quick Check:

   Moore output updates on clock edge only [OK]

Hint: Moore output inside posedge clk block [OK]

Common Mistakes:

• Using input sensitivity in Moore output block
• Assigning output directly from input in Moore machine
• Using combinational always block for Moore output

3. Given the following Verilog snippet for a Mealy machine output, what is the output when state = 1 and input = 0?

always @(state or input) begin
  if (state == 1)
    output = input;
  else
    output = 1;
end

medium

A. 1

B. Undefined

C. 0

D. Z (high impedance)

Solution

1. Step 1: Analyze condition when state = 1

   The if condition 'state == 1' is true, so output = input.

2. Step 2: Substitute input value

   Input is 0, so output = 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Mealy output depends on state and input, output = 0 [OK]

Hint: Check state condition first, then assign output [OK]

Common Mistakes:

• Ignoring input value when state is 1
• Assuming output is always 1
• Confusing Mealy output with Moore output

4. Identify the error in this Verilog code snippet intended to model a Moore machine output:

always @(input or state) begin
  output = state;
end

medium

A. Output assignment should use non-blocking '<=' operator.

B. Output should depend on input, not state.

C. The sensitivity list should include only input, not state.

D. Output should be assigned only on clock edge, not on input or state changes.

Solution

1. Step 1: Recall Moore machine output timing

   Moore machine outputs update only on clock edges, not on input or state changes directly.

2. Step 2: Check sensitivity list and assignment timing

   The sensitivity list includes input and state, causing output to update asynchronously, which is incorrect for Moore.

3. Final Answer:

   Output should be assigned only on clock edge, not on input or state changes. -> Option D

4. Quick Check:

   Moore output updates on clock edge only [OK]

Hint: Moore output must update only on clock edge [OK]

Common Mistakes:

• Using combinational always block for Moore output
• Ignoring clock in sensitivity list
• Confusing blocking and non-blocking assignments

5. You want to design a state machine in Verilog where the output must change immediately when the input changes, but also depend on the current state. Which machine type should you choose and why?

hard

A. Mealy machine, because output depends on state and input and can change immediately.

B. Moore machine, because it is simpler to implement and faster to respond.

C. Moore machine, because output depends only on state and changes on clock edges.

D. Neither, because outputs cannot depend on inputs in state machines.

Solution

1. Step 1: Understand output timing requirements

   The output must change immediately when input changes, so it cannot wait for clock edges.

2. Step 2: Match machine type to output behavior

   Mealy machine outputs depend on both state and input and can change immediately with input changes, fitting the requirement.

3. Final Answer:

   Mealy machine, because output depends on state and input and can change immediately. -> Option A

4. Quick Check:

   Immediate output change with input = Mealy machine [OK]

Hint: Immediate output change with input means Mealy machine [OK]

Common Mistakes:

• Choosing Moore machine for immediate output change
• Thinking Moore machine outputs depend on input
• Believing outputs cannot depend on inputs in state machines