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一、序列检测

  序列检测器指的就是将一个指定的序列（以‘10010’为例）从数字码流中识别出来，是一个经典的数字电路实例，也是数字IC和FPGA笔试面试中常考的知识点。写法总共可分为三种：
写法：

• 移位寄存器
• Moore状态机
• Mealy状态机

  常考的题目类型有以下特点，可能取其一类型进行拷打，也可能多个类型进行结合。比如说输入非连续且并行输入，最终需要提取出某个非重叠序列。
题目类型：

• 输入连续/非连续，非连续输入会有使能信号valid
• 重叠/非重叠序列检测
• 含无关项/不含无关项，比如说检测输入信号a是否满足011XXX110序列（长度为9位数据，前三位是011，后三位是110，中间三位不做要求）
• 串行输入/并行输入，比如说并行输入2bit数据

二、牛客真题

1. 输入序列连续的序列检测（输入连续、重叠、不含无关项、串行输入）

  以牛客上比较简单的题目VL25 输入序列连续的序列检测，介绍三种写法。

写法一：移位寄存器

module sequence_detect(input clk,input rst_n,input a,output reg match);reg [7:0] shift_reg;always @(posedge clk or negedge rst_n)beginif(~rst_n)beginshift_reg <= 'd0;endelse beginshift_reg <= {shift_reg[6:0], a};endendalways @(posedge clk or negedge rst_n)beginif(~rst_n)beginmatch <= 1'b0;endelse if(shift_reg==8'b0111_0001)beginmatch <= 1'b1;endelse beginmatch <= 1'b0;endend
endmodule

写法二：Moore状态机

module sequence_detect(input clk,input rst_n,input a,output reg match);localparam idle  = 'd0;localparam s0    = 'd1;localparam s1    = 'd2;localparam s2    = 'd3;localparam s3    = 'd4;localparam s4    = 'd5;localparam s5    = 'd6;localparam s6    = 'd7;localparam detect= 'd8;reg [3:0] curr_state;reg [3:0] next_state;always @(posedge clk or negedge rst_n)beginif(~rst_n)curr_state <= idle;else curr_state <= next_state;endalways @(*)begincase(curr_state)idle    : next_state = (a==1'b0)?s0     :idle   ;s0      : next_state = (a==1'b1)?s1     :s0     ;s1      : next_state = (a==1'b1)?s2     :s0     ;s2      : next_state = (a==1'b1)?s3     :s0     ;s3      : next_state = (a==1'b0)?s4     :idle   ;s4      : next_state = (a==1'b0)?s5     :s1     ;s5      : next_state = (a==1'b0)?s6     :s1     ;s6      : next_state = (a==1'b1)?detect :s0     ;detect  : next_state = (a==1'b1)?s3     :s0     ;default : next_state = idle;endcaseendalways @(posedge clk or negedge rst_n)beginif(~rst_n)match <= 1'b0;else if(curr_state==detect)match <= 1'b1;elsematch <= 1'b0;endendmodule

写法三：Mealy状态机

  注意:牛客上仿真需要用的是Moore状态机,因此Mealy状态机仿真结果的match会提前一个周期到来.

module sequence_detect(input clk,input rst_n,input a,output reg match);localparam idle  = 'd0;localparam s0    = 'd1;localparam s1    = 'd2;localparam s2    = 'd3;localparam s3    = 'd4;localparam s4    = 'd5;localparam s5    = 'd6;localparam s6    = 'd7;reg [2:0] curr_state;reg [2:0] next_state;always @(posedge clk or negedge rst_n)beginif(~rst_n)curr_state <= idle;else curr_state <= next_state;endalways @(*)begincase(curr_state)idle    : next_state = (a==1'b0)?s0     :idle   ;s0      : next_state = (a==1'b1)?s1     :s0     ;s1      : next_state = (a==1'b1)?s2     :s0     ;s2      : next_state = (a==1'b1)?s3     :s0     ;s3      : next_state = (a==1'b0)?s4     :idle   ;s4      : next_state = (a==1'b0)?s5     :s1     ;s5      : next_state = (a==1'b0)?s6     :s1     ;s6      : next_state = (a==1'b1)?s1     :s0     ;default : next_state = idle;endcaseendalways @(posedge clk or negedge rst_n)beginif(~rst_n)match <= 1'b0;else if(curr_state==s6 && a==1'b1)match <= 1'b1;elsematch <= 1'b0;endendmodule