题解 | #含有无关项的序列检测#

含有无关项序列检测和普通序列检测采用一样的方法。一般有两种办法，一种是利用移位寄存器来实现，这种方法比较简单。另一种是利用状态机来实现，难点在于状态转移图的绘制，下面的状态机对于重复的序列无法检测到。下面使用状态机来实现，

首先画出状态转移图

然后根据状态转移图，写出三段式状态机，代码如下。

`timescale 1ns/1ns
module sequence_detect(
	input clk,
	input rst_n,
	input a,
	output reg match
	);
parameter    S0 = 4'd0,
                S1 = 4'd1,
                S2 = 4'd2,
                S3 = 4'd3,
                S4 = 4'd4,
                S5 = 4'd5,
                S6 = 4'd6,
                S7 = 4'd7,
                S8 = 4'd8,
                S9 = 4'd9;
    
    reg [3:0] curr_state,next_state;
    //状态跳转
    always@(posedge clk or negedge rst_n)begin
        if(rst_n == 0)
            curr_state <= S0;
        else
            curr_state <= next_state;
    end
    //状态转移逻辑
    always@(*)begin
        case(curr_state)
            S0:next_state = a ? S0 : S1 ;
            S1:next_state = a ? S2 : S1 ;
            S2:next_state = a ? S3 : S1 ;
            S3:next_state = S4 ;
            S4:next_state = S5 ;
            S5:next_state = S6 ;
            S6:next_state = a ? S7 : S1 ;
            S7:next_state = a ? S8 : S1 ;
            S8:next_state = a ? S0 : S9 ;
            S9:next_state = a ? S0 : S1 ;
            default:next_state = S0;
        endcase
    end
    //输出，使用时序将match延后一个周期，不延后可以用assign  match = (curr_state == S9);
    //注意，使用assign需将match类型改为wire。
    always@(posedge clk or negedge rst_n) begin
        if(rst_n == 0)
            match <= 0;
        else
            match = (curr_state == S9);
    end
endmodule