// These are the symbolic names for states
// 定义状态的符号名称
parameter[1:0]
S0 = 2'h0,
S1 = 2'h1,
S2 = 2'h2,
S3 = 2'h3;

// These are the current state and next state variables
// 定义当前状态和下一状态变量
reg [1:0] state;
reg [1:0] next_state;
// state_vector state
// 状态向量的转移关系
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

同样的状态机也可以用下面的代码以“One hot”编码方式实现。
// These are the symbolic names for states
// 定义状态的符号名称
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
// 定义当前状态和下一状态变量
reg [3:0] state;
reg [3:0] next_state;
// state_vector state
// 状态向量的转移关系
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