【例 10.5】狀態機設計的例子

module FSM(clk,clr,out,start,step2,step3); 
input clk,clr,start,step2,step3; 
output[2:0] out;
reg[2:0] out;
reg[1:0] state,next_state;

parameter	
state0=2'b00,state1=2'b01,
state2=2'b11,state3=2'b10;
/*狀態編碼，采用格雷（Gray）編碼方式*/
always @(posedge clk or posedge clr)	/*該進程定義起始狀態*/

begin

if (clr) 
state <= state0;

else 
state <= next_state;

end


always @(state or start or step2 or step3)	/*該進程實現狀態的轉換*/

begin

case (state)

state0: 
  begin
    if (start)	
      next_state <=state1; 
    else	
      next_state <=state0; 
  end
state1: 
  begin
  end

state2: 
  begin
    next_state <= state2;
      if (step2)	
        next_state <=state3; 
      else	
	next_state <=state0; 
  end
state3: 
  begin
    if (step3)	
      next_state <=state0; 
    else	
      next_state <=state3; 
  end
default:	
  next_state <=state0;	/*default 語句*/

endcase 
end

always @(state)	/*該進程定義組合邏輯（FSM 的輸出）*/

begin 
case(state)
state0: out=3'b001; 
state1: out=3'b010; 
state2: out=3'b100; 
state3: out=3'b111;
default:out=3'b001;	/*default 語句，避免鎖存器的產生*/

endcase end


endmodule