牛客进阶题目19：根据状态转移写状态机-三段式

普通三段式，根据状态转移图写即可。

`timescale 1ns/1ns

module fsm1(
	input wire clk  ,
	input wire rst  ,
	input wire data ,
	output reg flag
);
//*************code***********//
localparam	S0 = 2'b00	;
localparam	S1 = 2'B01	;
localparam	S2 = 2'B10	;
localparam	S3 = 2'b11	;

reg [1:0]	state,nxt_state	;

always @(posedge clk or negedge rst) begin
	if(!rst)
		state <= S0 ;
	else
		state <= nxt_state ;
end

always @(*) begin
	case(state) 
		S0 : nxt_state = data ? S1 : S0 ;
		S1 : nxt_state = data ? S2 : S1 ;
		S2 : nxt_state = data ? S3 : S2 ;
		S3 : nxt_state = data ? S0 : S3 ;
		default : nxt_state = 2'b00 ;
	endcase
end
always @(posedge clk or negedge rst) begin
	if(!rst)
		flag <= 1'b0 ;
	else if(state==S3 && nxt_state==S0)
		flag <= 1'b1 ;
	else
		flag <= 1'b0 ;
end


//*************code***********//
endmodule