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Verilog基础https://blog.csdn.net/weixin_45791458/category_12263729.html
对于Verilog HDL而言,有限状态机(FSM)是一种重要而强大的模块,常见的有限状态机书写方式可以分为一段式,二段式和三段式,笔者强烈建议使用三段式因为这样能使状态机逻辑清晰且易于维护。
有限状态机有两种基本类型:Mealy机和Moore机。两者的区别在于:Mealy机的下一状态和输出都取决于当前状态和当前输入,而Moore机的下一状态取决于当前状态和当前输入,输出只取决于当前状态。这两类有限状态机的下一状态和输出都是组合逻辑的形式的(指输出不直接来自寄存器的输出),两类状态机的结构如图1、图2所示。
图1 Mealy型状态机
图2 Moore型状态机
下面以一个简单的例子说明三段式Moore型状态机的书写方式。图3是一个有两个状态的异步复位的Moore机。
图3 一个简单的Moore机
- module top_module (
- input clk,
- input in,
- input rst_n,
- output reg out
- );
- parameter A = 0;
- parameter B = 1;
- reg state, next_state; //定义寄存器变量保存状态信息
-
- //第一段,下一状态组合逻辑
- always@(*) begin
- case (state) //根据不同的状态和输入,决定下一时钟周期的状态
- A: next_state = in ? A : B;
- B: next_state = in ? B : A;
- endcase
- end
-
- //第二段,状态转移时序逻辑
- always @(posedge clk, negedge rst_n) begin
- if (!rst_n)
- state <= B; //异步复位到状态B
- else
- state <= next_state;
- end
-
- //第三段,输出组合逻辑
- always@(*) begin
- if(state == B)
- out = 1;
- else
- out = 0;
- end
-
- //因为输出比较简单,这里的第三段的输出组合逻辑也可以用assign连续赋值
- //但out此时不能定义为reg
- //assign out = (state == B);
-
- endmodule
对于Mealy型状态机,因为输出直接受输出影响,可能在某些情况下会出现毛刺(即不在时钟边沿的输出变化),所以可以使用寄存器采集输出。对于Moore型状态机,虽然没有输出毛刺的问题,但也可以使用寄存器采集输出以避免大段组合逻辑输出。图4和图5分别给出了寄存输出的Mealy型状态机和Moore型状态机的结构。
图4 寄存输出的Mealy型状态机
图5 寄存输出的Moore型状态机
上面两图不难理解,但是一个新的问题出现了,即输出会延后一个周期得到,如果既需要当前周期给出输出,又需要对输出寄存,就不能使用当前状态和输入确定输出,而是应该使用下一状态组合逻辑和输入确定输出。图6和图7给出了这种情况下的Mealy型状态机和Moore型状态机。
图6 寄存输出的Mealy型状态机(下一状态)
图7 寄存输出的Moore型状态机(下一状态)
对于Mealy状态机,因为需要状态转移和相应状态的输出同时出现,输出寄存器需要保存由下一状态组合逻辑和的输入推导的输出。对于Moore状态机,输出寄存器需要保存由下一状态组合逻辑推导的输出。
下面用一个更加复杂的有限状态机为例,说明寄存输出的具体写法。这是一个序列检测器,用于检测输入序列中三个连续的1信号,首先给出Moore型序列检测器的Verilog描述。
- module Seq_Rec_Moore(output reg D_out, D_out_r1, D_out_r2, input D_in, En, clk, rst_n);
- parameter S_idle = 3'd0;
- parameter S_0 = 3'd1;
- parameter S_1 = 3'd2;
- parameter S_2 = 3'd3;
- parameter S_3 = 3'd4;
-
- reg [2:0] state, next_state;
-
- //下一状态组合逻辑
- always@(*)begin
- case(state)
- S_idle: if((En == 1)&&(D_in == 1))
- next_state = S_1;
- else if((En == 1)&&(D_in == 0))
- next_state = S_0;
- else
- next_state = S_idle;
- S_0: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_1;
- else
- next_state = S_idle;
- S_1: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_2;
- else
- next_state = S_idle;
- S_2: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_3;
- else
- next_state = S_idle;
- S_3: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_3;
- else
- next_state = S_idle;
- default: next_state = S_idle;
- endcase
- end
-
- //状态转移时序逻辑
- always@(posedge clk, negedge rst_n)begin
- if(!rst_n)
- state <= S_idle;
- else
- state <= next_state;
- end
-
- //输出组合逻辑
- always@(*) begin
- D_out = (state == S_3);
- end
-
- //寄存输出
- always @(posedge clk, negedge rst_n) begin
- if(!rst_n)
- D_out_r1 <= 0;
- else
- D_out_r1 <= D_out;
- end
-
- //寄存输出(下一状态)
- always@(posedge clk, negedge rst_n)begin
- if(!rst_n)
- D_out_r2 <= 0;
- else
- D_out_r2 <= (next_state == S_3);
- end
- endmodule
一个简单的testbench如下所示,图8所示的仿真截图显示了三种不同形式的输出。
- `timescale 1ns / 1ns
- module t_Seq();
- reg D_in, clk, rst_n,En;
- wire D_out, D_out_r1, D_out_r2;
-
- Seq_Rec_Moore Seq_Rec_Moore_1 (.D_in(D_in), .clk(clk), .rst_n(rst_n), .D_out(D_out), .En(En), .D_out_r1(D_out_r1), .D_out_r2(D_out_r2));
-
- initial begin
- D_in = 0;
- clk = 0;
- rst_n = 1;
- En = 0;
- end
-
- initial begin
- #5 rst_n = 0;
- #4 rst_n = 1;
- end
-
- always begin
- #5 clk = 0;
- #5 clk = 1;
- end
-
- initial begin
- #5 En =1;
- end
-
- initial begin
- #5 D_in = 1;
- end
- endmodule
图8 Moore状态机仿真截图
下面是Mealy型序列检测器的Verilog描述。
- module Seq_Rec_Mealy(output reg D_out, D_out_r1, D_out_r2, input D_in, En, clk, rst_n);
- parameter S_idle = 3'd0;
- parameter S_0 = 3'd1;
- parameter S_1 = 3'd2;
- parameter S_2 = 3'd3;
- parameter S_3 = 3'd4;
-
- reg [2:0] state, next_state;
-
- //下一状态组合逻辑
- always@(*)begin
- case(state)
- S_idle: if((En == 1)&&(D_in == 1))
- next_state = S_1;
- else if((En == 1)&&(D_in == 0))
- next_state = S_0;
- else
- next_state = S_idle;
- S_0: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_1;
- else
- next_state = S_idle;
- S_1: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_2;
- else
- next_state = S_idle;
- S_2: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_3;
- else
- next_state = S_idle;
- S_3: if(D_in == 0)
- next_state = S_0;
- else if(D_in == 1)
- next_state = S_3;
- else
- next_state = S_idle;
- default: next_state = S_idle;
- endcase
- end
-
- //状态转移时序逻辑
- always@(posedge clk, negedge rst_n)begin
- if(!rst_n)
- state <= S_idle;
- else
- state <= next_state;
- end
-
- //输出组合逻辑
- always@(*) begin
- D_out = (state == S_3)&(D_in == 1);
- end
-
- //寄存输出
- always @(posedge clk, negedge rst_n) begin
- if(!rst_n)
- D_out_r1 <= 0;
- else
- D_out_r1 <= D_out;
- end
-
- //寄存输出(下一状态)
- always@(posedge clk, negedge rst_n)begin
- if(!rst_n)
- D_out_r2 <= 0;
- else
- D_out_r2 <= (next_state == S_3)&(D_in == 1);
- end
- endmodule
图9 Mealy状态机仿真截图
一个简单的testbench如下所示,图9的仿真截图显示了如果使用寄存输出,而输入无法维持到下个时钟沿,则会丢失寄存输出信号。
- `timescale 1ns / 1ns
- module t_Seq();
- reg D_in, clk, rst_n,En;
- wire D_out, D_out_r1, D_out_r2;
-
- Seq_Rec_Mealy Seq_Rec_Mealy_1 (.D_in(D_in), .clk(clk), .rst_n(rst_n), .D_out(D_out), .En(En), .D_out_r1(D_out_r1), .D_out_r2(D_out_r2));
-
- initial begin
- D_in = 0;
- clk = 0;
- rst_n = 1;
- En = 0;
- end
-
- initial begin
- #5 rst_n = 0;
- #4 rst_n = 1;
- end
-
- always begin
- #5 clk = 0;
- #5 clk = 1;
- end
-
- initial begin
- #5 En =1;
- end
-
- initial begin
- #5 D_in = 1;
- #30 D_in = 0;
- #10 D_in = 1;
- #37 D_in = 0;
- end
- endmodule