狀態機舉例

你可以指定狀態寄存器和狀態機的狀態。以下是一個有四種狀態的普通狀態機。

// These are the symbolic names for states
// 定義狀態的符號名稱
parameter? [1:0]
? S0 = 2'h0,
? S1 = 2'h1,
? S2 = 2'h2,
? S3 = 2'h3;

// These are the current state and next state variables
// 定義當前狀態和下一狀態變量
reg [1:0] state;
reg [1:0] next_state;


// state_vector state
// 狀態向量的轉移關系
always @ (state or y or x)
begin
? next_state = state;
? case (state)????????????????
??? S0: begin
????? if (x) begin
??????? next_state = S1;
????? end
????? else begin
??????? next_state = S2;
????? end
??? end
??? S1: begin
????? if (y) begin
??????? next_state = S2;
????? end
????? else begin
??????? next_state = S0;
????? end
??? end
??? S2: begin
????? if (x & y) begin
??????? next_state = S3;
????? end
????? else begin
??????? next_state = S0;
????? end
??? end
??? S3: begin
????? next_state = S0;
??? end
? endcase
end


always @ (posedge clk or posedge reset)
begin
? if (reset) begin
??? state <= S0;
? end
? else begin
??? state <= next_state;
? end
end


　　同樣的狀態機也可以用下面的代碼以“One hot”編碼方式實現。

// These are the symbolic names for states
// 定義狀態的符號名稱
parameter  [1:0]
  S0 = 2'h0,
  S1 = 2'h1,
  S2 = 2'h2,
  S3 = 2'h3;
 
parameter  [3:0] 
  s0 = 4'h1,
  s1 = 4'h2,
  s2 = 4'h4,
  s3 = 4'h8;
  
// These are the current state and next state variables
// 定義當前狀態和下一狀態變量
reg [3:0] state;
reg [3:0] next_state;
 
 
// state_vector state
// 狀態向量的轉移關系
always @ (state or y or x)
begin 
  next_state = state;
  case (1)                 
    state[S0]: begin 
      if (x) begin 
        next_state = 1 << S1;
      end
      else begin 
        next_state = 1 << S2;
      end
    end
    state[S1]: begin 
      if (y) begin 
        next_state = 1 << S2;
      end
      else begin 
        next_state = 1 << S0;
      end
    end
    state[S2]: begin 
      if (x & y) begin 
        next_state = 1 << S3;
      end
      else begin 
        next_state = 1 << S0;
      end
    end
    state[S3]: begin 
      next_state = 1 << S0;
    end
  endcase
end
always @ (posedge clk or posedge reset)
begin 
  if (reset) begin 
    state <= 1 << S0;
  end
  else begin 
    state <= next_state;
  end
end