Moore machine vs Mealy machine in Verilog - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output behavior of a Moore machine

Consider this Verilog code snippet of a Moore machine. What is the output y when the input x changes from 0 to 1?

Verilog

module moore_example(input clk, input x, output reg y);
  reg state;
  always @(posedge clk) begin
    case(state)
      0: if (x) state <= 1;
      1: if (!x) state <= 0;
    endcase
  end
  always @(state) begin
    case(state)
      0: y = 0;
      1: y = 1;
    endcase
  end
endmodule

A Output y changes immediately when x changes at clock edge.

B Output y never changes regardless of input x.

C Output y changes only after the clock edge following the input change.

D Output y changes asynchronously with input x.

💡 Hint

Remember that in a Moore machine, outputs depend only on the current state, not directly on inputs.

In a Moore machine, outputs depend solely on the current state. The state updates on the clock edge based on input x. Therefore, output y changes only after the state updates at the clock edge, not immediately when x changes.

❓ Predict Output

intermediate

Output behavior of a Mealy machine

Given this Verilog code snippet of a Mealy machine, what is the output y when input x changes from 0 to 1?

Verilog

module mealy_example(input clk, input x, output reg y);
  reg state;
  always @(posedge clk) begin
    case(state)
      0: if (x) state <= 1;
      1: if (!x) state <= 0;
    endcase
  end
  always @(*) begin
    case(state)
      0: y = x;
      1: y = ~x;
    endcase
  end
endmodule

A Output y changes only when state changes.

B Output y changes immediately when input x changes.

C Output y changes only after the clock edge following the input change.

D Output y never changes regardless of input x.

💡 Hint

In a Mealy machine, outputs depend on both state and inputs.

In a Mealy machine, outputs depend on the current state and the current inputs. Since output y is assigned in a combinational block sensitive to inputs, it changes immediately when input x changes.

🧠 Conceptual

advanced

Difference in output timing between Moore and Mealy machines

Which statement correctly describes the difference in output timing between Moore and Mealy machines?

A Moore machine outputs change only on clock edges; Mealy machine outputs can change asynchronously with inputs.

B Moore machine outputs change asynchronously with inputs; Mealy machine outputs change only on clock edges.

C Both Moore and Mealy machine outputs change only on clock edges.

D Both Moore and Mealy machine outputs change asynchronously with inputs.

💡 Hint

Think about when outputs are updated in each machine type.

Moore machine outputs depend only on the state, which updates on clock edges, so outputs change only at clock edges. Mealy machine outputs depend on state and inputs, so outputs can change immediately when inputs change.

🔧 Debug

advanced

Identify the error in this Mealy machine Verilog code

This Verilog code is intended to implement a Mealy machine. What error will it cause?

Verilog

module mealy_bug(input clk, input x, output reg y);
  reg state;
  always @(posedge clk) begin
    case(state)
      0: if (x) state <= 1;
      1: if (!x) state <= 0;
    endcase
  end
  always @(state) begin
    case(state)
      0: y = x;
      1: y = ~x;
    endcase
  end
endmodule

A No error; code works as intended.

B Output y does not update immediately when input x changes.

C Syntax error due to missing initial state assignment.

D Output y is not updated correctly because it depends on input x but is only sensitive to state.

💡 Hint

Check the sensitivity list of the output logic block.

The output y depends on input x but the always block is sensitive only to state changes. This means y will not update immediately when x changes, causing incorrect behavior for a Mealy machine.

🚀 Application

expert

Design choice for output stability in FSM

You need to design a finite state machine in Verilog where output glitches must be avoided and outputs should be stable during the clock cycle. Which machine type is best suited and why?

A Moore machine, because outputs depend only on state and change synchronously with clock edges.

B Mealy machine, because outputs depend on inputs and can change immediately, reducing latency.

C Mealy machine, because outputs are stable during the clock cycle.

D Moore machine, because outputs change asynchronously with inputs.

💡 Hint

Consider output stability and timing in your choice.

Moore machines produce outputs based only on the current state, which changes synchronously on clock edges. This ensures outputs are stable and free from glitches during the clock cycle, making Moore machines suitable for designs requiring stable outputs.