二段式fsm

 1 //2-paragraph method to describe FSM
 2 //Describe sequential state transition in 1 sequential always block
 3 //State transition conditions in the other combinational always block
 4 //Package state output by task. Then register the output
 5 //Westor Wang, Dec. 2006
 6 //Verilog Training -- How to write FSM better
 7 
 8 
 9 module state2 ( 
10                input nrst,
11                input clk,
12                input i1,
13                input i2,
14                output reg o1,
15                output reg o2,
16                output reg err
17                );
18 
19 reg    [2:0]   NS,CS;
20 
21 parameter [2:0]      //one hot with zero idle
22       IDLE   = 3‘b000,
23       S1     = 3‘b001,
24       S2     = 3‘b010,
25       ERROR  = 3‘b100;
26 
27 //first paragraph:sequential state transition
28 always @ (posedge clk or negedge nrst)
29       if (!nrst)            
30          CS <= IDLE;        
31       else                  
32          CS <=NS;           
33 
34 //second paragraph:combinational condition judgment
35 always @ (nrst or CS or i1 or i2)
36           begin
37                NS = 3‘bx;  //default state  x
38                ERROR_out;
39                case (CS)
40                     IDLE:     begin
41                                    IDLE_out;
42                                    if (~i1)           NS = IDLE;
43                                    if (i1 && i2)      NS = S1;
44                                    if (i1 && ~i2)     NS = ERROR;
45                               end
46                     S1:       begin
47                                    S1_out;
48                                    if (~i2)           NS = S1;
49                                    if (i2 && i1)      NS = S2;
50                                    if (i2 && (~i1))   NS = ERROR;
51                               end
52                     S2:       begin
53                                    S2_out;
54                                    if (i2)            NS = S2;
55                        if (~i2 && i1)     NS = IDLE;
56                                    if (~i2 && (~i1))  NS = ERROR;
57                               end
58                     ERROR:    begin
59                                    ERROR_out;
60                                    if (i1)            NS = ERROR;
61                                    if (~i1)           NS = IDLE;
62                               end
63                endcase
64          end
65 
66 
67 //package output with task
68 task IDLE_out;
69      {o1,o2,err} = 3‘b000;
70 endtask
71 
72 task S1_out;
73      {o1,o2,err} = 3‘b100;
74 endtask
75 
76 task S2_out;
77      {o1,o2,err} = 3‘b010;
78 endtask
79 
80 task ERROR_out;
81      {o1,o2,err} = 3‘b111;
82 endtask
83 
84 endmodule