三种不同状态机写法

一段式状态机：

 1 reg[3:0]  cs, ns;
 2 always @(posedge clk or negedge rst_n) begin
 3     if (!rst_n) begin
 4         cs    <=  IDLE;
 5         cmd <=  3'b111;
 6     end
 7     else begin
 8         case (cs)
 9             IDLE :   if (wr_req) begin cs <= WR_S1; cmd <= 3'b011; end
10                 else if (rd_req) begin cs <= RD_S1; cmd <= 3'b011; end
11                 else             begin cs <= IDLE;  cmd <= 3'b111; end
12             WR_S1:               begin cs <= WR_S2; cmd <= 3'b101; end
13             WR_S2:               begin cs <= IDLE;  cmd <= 3'b111; end
14             RD_S1:   if (wr_req) begin cs <= WR_S2; cmd <= 3'b101; end
15                      else        begin cs <= RD_S2; cmd <= 3'b110; end
16             RD_S2:   if (wr_req) begin cs <= WR_S1; cmd <= 3'b011; end
17                      else        begin cs <= IDLE;  cmd <= 3'b111; end
18             default :                  cs <= IDLE;
19         endcase
20     end
21 end

两段式状态机：

 1 reg[3:0]  cs, ns;
 2 //----------   时序逻辑   ------------------
 3 always @(posedge clk or negedge rst_n) begin
 4     if (!rst_n)
 5         cs    <=  IDLE;
 6     else 
 7         cs    <=  ns;
 8 end
 9 //----------   组合逻辑   ------------------
10 always @(*) begin
11     case (cs)
12         IDLE :   if (wr_req) begin cs <= WR_S1; cmd <= 3'b011; end
13             else if (rd_req) begin cs <= RD_S1; cmd <= 3'b011; end
14             else             begin cs <= IDLE;  cmd <= 3'b111; end
15         WR_S1:               begin cs <= WR_S2; cmd <= 3'b101; end
16         WR_S2:               begin cs <= IDLE;  cmd <= 3'b111; end
17         RD_S1:   if (wr_req) begin cs <= WR_S2; cmd <= 3'b101; end
18                  else        begin cs <= RD_S2; cmd <= 3'b110; end
19         RD_S2:   if (wr_req) begin cs <= WR_S1; cmd <= 3'b011; end
20                  else        begin cs <= IDLE;  cmd <= 3'b111; end
21         default :                  cs <= IDLE;
22     endcase
23 end

三段式状态机：

 1 reg[3:0]  cs, ns;
 2 //----------   时序逻辑   ------------------
 3 always @(posedge clk or negedge rst_n) begin
 4     if (!rst_n)
 5         cs    <=  IDLE;
 6     else 
 7         cs    <=  ns;
 8 end
 9 //----------   组合逻辑   ------------------
10 always @(*) begin
11     case (cs)  //现态
12         IDLE :   if (wr_req) begin cs <= WR_S1; end
13             else if (rd_req) begin cs <= RD_S1; end
14             else             begin cs <= IDLE;  end
15         WR_S1:               begin cs <= WR_S2; end
16         WR_S2:               begin cs <= IDLE;  end
17         RD_S1:   if (wr_req) begin cs <= WR_S2; end
18                  else        begin cs <= RD_S2; end
19         RD_S2:   if (wr_req) begin cs <= WR_S1; end
20                  else        begin cs <= IDLE;  end
21         default :                  cs <= IDLE;
22     endcase
23 end
24 //----------   时序逻辑   ------------------
25 always @(posedge clk or negedge rst_n) begin
26     if (!rst_n)
27         cmd    <=  3'b011;
28     else begin
29         case (ns)  //次态
30             IDLE :   if (wr_req) begin cmd <= 3'b011; end
31                 else if (rd_req) begin cmd <= 3'b011; end
32                 else             begin cmd <= 3'b111; end
33             WR_S1:               begin cmd <= 3'b101; end
34             WR_S2:               begin cmd <= 3'b111; end
35             RD_S1:   if (wr_req) begin cmd <= 3'b101; end
36                      else        begin cmd <= 3'b110; end
37             RD_S2:   if (wr_req) begin cmd <= 3'b011; end
38                      else        begin cmd <= 3'b111; end
39             default : ;
40         endcase 
41     end
42 end

三种写法对比：

    （1）一段式状态机不利于维护（简单状态机可以用）；

    （2）两段式状态机是常见写法，时序逻辑进行状态切换，时序逻辑实现各个输入、输出以及状态判断，利于维护，不过组合逻辑容易出现毛刺等常见问题；

    （3）三段式状态机推荐写法，代码易维护，时序逻辑输出解决了两段式写法种组合逻辑的毛刺问题，但是耗费资源多一些且三段式从输入到输出比一段式和两段式会延时一个时钟周期。

转载：http://www.cnblogs.com/yllinux/p/8641634.html