【Verilog】HDLBits题解——Verification: Reading Simulations

Finding bugs in code

• Mux

  题目链接

module top_module (
    input sel,
    input [7:0] a,
    input [7:0] b,
    output [7:0] out  );

    assign out = ({
    8{
    sel}} & a) | ({
    8{
    ~sel}} & b);
    
endmodule

• NAND

  题目链接

module top_module (input a, input b, input c, output out);//
	wire out_reverse;
    andgate inst1 (out_reverse, a, b, c, 1, 1);
	assign out = ~out_reverse;
endmodule

• Mux

  题目链接

module top_module (
    input [1:0] sel,
    input [7:0] a,
    input [7:0] b,
    input [7:0] c,
    input [7:0] d,
    output [7:0] out  ); //

    wire [7:0] mux0, mux1;
    mux2 mux2_0 ( sel[0],    a,    b, mux0 );
    mux2 mux2_1 ( sel[0],    c,    d, mux1 );
    mux2 mux2_2 ( sel[1], mux0, mux1,  out );

endmodule

• Add/Sub

  题目链接

// synthesis verilog_input_version verilog_2001
module top_module ( 
    input do_sub,
    input [7:0] a,
    input [7:0] b,
    output reg [7:0] out,
    output reg result_is_zero
);//

    always @(*) begin
        case (do_sub)
          0: out = a+b;
          1: out = a-b;
        endcase

        if (&(~out))
            result_is_zero = 1;
        else
            result_is_zero = 0;
    end

endmodule

• Case Statement

  题目链接

module top_module (
    input [7:0] code,
    output reg [3:0] out,
    output reg valid=1 );//

     always @(*)
        case (code)
            8'h45: begin out <= 0; valid <= 1; end
            8'h16: begin out <= 1; valid <= 1; end
            8'h1e: begin out <= 2; valid <= 1; end
            8'h26: begin out <= 3; valid <= 1; end
            8'h25: begin out <= 4; valid <= 1; end
            8'h2e: begin out <= 5; valid <= 1; end
            8'h36: begin out <= 6; valid <= 1; end
            8'h3d: begin out <= 7; valid <= 1; end
            8'h3e: begin out <= 8; valid <= 1; end
            8'h46: begin out <= 9; valid <= 1; end
            default: begin out <= 0; valid <= 0; end
        endcase

endmodule

Build a circuit from a simulation waveform

• Combinational circuit 1

  题目链接

module top_module (
    input a,
    input b,
    output q );//

    assign q = a & b; // Fix me

endmodule

• Combinational circuit 2

  题目链接

module top_module (
    input a,
    input b,
    input c,
    input d,
    output q );//

    assign q = ~(a ^ b ^ c ^ d); // Fix me

endmodule

• Combinational circuit 3

  题目链接

module top_module (
    input a,
    input b,
    input c,
    input d,
    output q );//

    assign q = (c | d) & (a | b); // Fix me

endmodule

• Combinational circuit 4

  题目链接

module top_module (
    input a,
    input b,
    input c,
    input d,
    output q );//

    assign q = c | b; // Fix me

endmodule

• Combinational circuit 5

  题目链接

module top_module (
    input [3:0] a,
    input [3:0] b,
    input [3:0] c,
    input [3:0] d,
    input [3:0] e,
    output [3:0] q );
    reg [3:0] q_reg;
    assign q = q_reg;
    always @ (*) begin
        case(c)
            0: q_reg = b;
            1: q_reg = e;
            2: q_reg = a;
            3: q_reg = d;
            default: q_reg = 'hf;
        endcase
    end
endmodule

• Combinational circuit 6

  题目链接

module top_module (
    input [2:0] a,
    output [15:0] q ); 
	
    reg [15:0] q_reg;
    assign q = q_reg;
    always @ (*) begin
        case(a)
            0: q_reg = 'h1232;
            1: q_reg = 'haee0;
            2: q_reg = 'h27d4;
            3: q_reg = 'h5a0e;
            4: q_reg = 'h2066;
            5: q_reg = 'h64ce;
            6: q_reg = 'hc526;
            7: q_reg = 'h2f19;
        endcase
    end
endmodule

• Sequential circuit 7

  题目链接

module top_module (
    input clk,
    input a,
    output q );
    always @ (posedge clk) begin
        q <= ~a;
    end
endmodule

• Sequential circuit 8

  题目链接

module top_module (
    input clock,
    input a,
    output p,
    output q );
    reg q_reg = 0;
    reg p_reg = 0;
    assign p = p_reg;
    assign q = q_reg;
    always @ (*) begin
        if(clock)
        	p_reg <= a;
    end
    
    always @ (negedge clock) begin
        q_reg <= a;
    end
endmodule

• Sequential circuit 9

  题目链接

module top_module (
    input clk,
    input a,
    output [3:0] q );
    reg [3:0] q_reg;
    assign q = q_reg;
    always @ (posedge clk) begin
        if (a) begin
            q_reg <= 4;
        end
        else begin
            q_reg <= (q_reg + 1) % 7;
        end
    end
endmodule

• Sequential circuit 10

  题目链接

module top_module (
    input clk,
    input a,
    input b,
    output q,
    output state  );
    reg state_reg;
    assign state = state_reg;
	assign q = a ^ b ^ state;
    always @ (posedge clk) begin
        state_reg <= (a & b) | (a & state_reg) | (b & state_reg);
    end
endmodule