Mealy vs. Moore Machines
Moore: Output depends on current state
only.
Mealy: Output depends on current state
and input.
For Mealy Output:- Output is function of state n inputs. Specifies state on transition arc.
For Moore Output:- Output is only function of State. Specifies state bubble in state diagram.
MEALY STATE MACHINE EXAMPLE:
module ED_Mealy (
input reset,
input clk,
input in,
output reg out
);
parameter A = 3'b000; //0
parameter B = 3'b001; //1
parameter C = 3'b010; //2
reg [2:0] state; // Current state
reg [2:0] nxtState; // Next state
always @(posedge clk)
begin
if (reset)
state <= A; // Initial state
else
state <= nxtState;
end
always @(*)
begin
nxtState = state;
out = 0;
case (state)
A :
if (in)
nxtState = C;
else
nxtState = B;
B :
if (in)
begin
out = 1;
nxtState = C;
end
C :
if (~in)
begin
out = 1;
nxtState = B;
end
default :
begin
out = 1'bx;
nxtState = 3'bx;
end
endcase
end
endmodule
MOORE STATE MACHINE:
module ED_Moore (
input reset,
input clk,
input in,
output reg out
);
parameter A = 3'b000; //0
parameter B = 3'b001; //1
parameter C = 3'b010; //2
parameter D = 3'b100; //4
parameter E = 3'b101; //5
reg [2:0] state; // Current state
reg [2:0] nxtState; // Next state
always @(posedge clk)
begin
if (reset)
state <= A; // Initial state
else
state <= nxtState;
end
always @(*)
begin
nxtState = state;
out = 0;
case (state)
A :
if (in)
nxtState = C;
else
nxtState = B;
B :
if (in)
nxtState = D;
C :
if (~in)
nxtState = E;
D :
begin
out = 1;
if (in)
nxtState = C;
else
nxtState = E;
end
E :
begin
out = 1;
if (in)
nxtState = D;
else
nxtState = B;
end
default :
begin
out = 1'bx;
nxtState = 3'bx;
end
endcase
end
endmodule
STIMULUS:
module ED_tb;
reg reset, clk, in;
wire out;
integer i;
ED_Mealy ED_Mealy_inst (
.reset (reset ),
.clk (clk ),
.in (in ),
.out (out )
);
always
#5 clk = ~clk;
initial
begin
clk = 0;
reset = 1;
in = 0;
repeat(5)
@(posedge clk);
reset = 0;
@(posedge clk);
for (i=0;i<=20;i=i+1)
begin
in = $random;
@(posedge clk);
end
$stop;
end
endmodule
