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.