Exams/ece241 2013 q4

module top_module (
	input clk,
	input reset,
	input [3:1] s,
	output reg fr3,
	output reg fr2,
	output reg fr1,
	output reg dfr
);


	// Give state names and assignments. I'm lazy, so I like to use decimal numbers.
	// It doesn't really matter what assignment is used, as long as they're unique.
	// We have 6 states here.
	parameter A2=0, B1=1, B2=2, C1=3, C2=4, D1=5;
	reg [2:0] state, next;		// Make sure these are big enough to hold the state encodings.
	


    // Edge-triggered always block (DFFs) for state flip-flops. Synchronous reset.	
	always @(posedge clk) begin
		if (reset) state <= A2;
		else state <= next;
	end



    // Combinational always block for state transition logic. Given the current state and inputs,
    // what should be next state be?
    // Combinational always block: Use blocking assignments.    
	always@(*) begin
		case (state)
			A2: next = s[1] ? B1 : A2;
			B1: next = s[2] ? C1 : (s[1] ? B1 : A2);
			B2: next = s[2] ? C1 : (s[1] ? B2 : A2);
			C1: next = s[3] ? D1 : (s[2] ? C1 : B2);
			C2: next = s[3] ? D1 : (s[2] ? C2 : B2);
			D1: next = s[3] ? D1 : C2;
			default: next = 'x;
		endcase
	end
	
	
	
	// Combinational output logic. In this problem, a procedural block (combinational always block) 
	// is more convenient. Be careful not to create a latch.
	always@(*) begin
		case (state)
			A2: {fr3, fr2, fr1, dfr} = 4'b1111;
			B1: {fr3, fr2, fr1, dfr} = 4'b0110;
			B2: {fr3, fr2, fr1, dfr} = 4'b0111;
			C1: {fr3, fr2, fr1, dfr} = 4'b0010;
			C2: {fr3, fr2, fr1, dfr} = 4'b0011;
			D1: {fr3, fr2, fr1, dfr} = 4'b0000;
			default: {fr3, fr2, fr1, dfr} = 'x;
		endcase
	end
	
endmodule

错误代码 在研究ing

module top_module (
	input clk,
	input reset,
	input [3:1] s,
	output reg fr3,
	output reg fr2,
	output reg fr1,
	output reg dfr
);

    parameter A = 0, B=1,C=2,D=3;
    reg[2:0] state, next;
    reg[1:0] previous;
    
    //注意只有这里的always是用的clk，其他的都是星号。
   
    //这里就是状态转换，注意B1,B2,C1,C2的区别
    always@(*)begin
        case(state)
            A:next <= s[1]?B:A;
            B:next <= s[2]? C: (s[1]?previous:A);
            C:next <= s[3]? D:(s[2]?previous:A);
            D:next <= s[3]? D:C;
            default:next <= A;
        endcase
    end
	
always @(posedge clk) begin
	if (reset)
		state <= A;
else begin
		state <= next;
		previous <= state;
end
end
    //这里是输出结果，根据不同的状态开不同的阀门和赋值不同的dfr
    always@(*)begin
        case(state)
            A: {fr3,fr2,fr1,dfr} = 4'b1111;
            B: {fr3,fr2,fr1,dfr} = 4'b0110;       
            C: {fr3,fr2,fr1,dfr} = 4'b0010;
            D: {fr3,fr2,fr1,dfr} = 4'b0000;
            default: {fr3, fr2, fr1, dfr} = 'x;
        endcase
    end
      
endmodule

另一种

module top_module (
    input clk,
    input reset,
    input [3:1] s,
    output fr3,
    output fr2,
    output fr1,
    output dfr
);  
    reg [1:0] state , nstate;
    reg [2:0] outfr;
    parameter 
    t1 = 2'b00,
    t2 = 2'b01,
    t3 = 2'b10,
    t4 = 2'b11;
    assign {fr3,fr2,fr1} = outfr;
    always@(posedge clk)begin
     if(reset)
            state <= t1;
        else
            state <= nstate;
    end
    always@(*)begin
        nstate = t1;
        case(s)
            3'b000:nstate = t1;
            3'b001:nstate = t2;
            3'b011:nstate = t3;
            3'b111:nstate = t4;
            default:nstate = t1;
        endcase
    end
    always@(posedge clk)begin
        if(reset)
        	outfr <= 3'b111;     
        else begin
            case(nstate)
                t1:outfr <= 3'b111;
                t2:outfr <= 3'b011;
                t3:outfr <= 3'b001;
                t4:outfr <= 3'b000;
            endcase
        end   
    end
    always@(posedge clk)begin
        if(reset)
        	dfr <= 1;
        else begin 
            if(nstate < state)
                dfr <= 1;
            else if(nstate > state)
                dfr <= 0;
            else 
                dfr <= dfr;
        end
    end
endmodule