三、Circuits

Combinational logic-Multiplexers

1、2-to-1 Multiplexer

Problem Statement：

Create a one-bit wide, 2-to-1 multiplexer. When sel=0, choose a. When sel=1, choose b.

module top_module( 
    input a, b, sel,
    output out 
);
    always@(*)begin
    	case(sel)
       		0 : out = a;
       		1 : out = b;
    	endcase
    end

endmodule

2、2-to-1 bus Multiplexer 

Problem Statement：

Create a 100-bit wide, 2-to-1 multiplexer. When sel=0, choose a. When sel=1, choose b.

module top_module( 
    input [99:0] a, b,
    input sel,
    output [99:0] out 
);
    always@(*)begin
        case(sel)
            0 : out = a;
            1 : out = b;
        endcase
    end

endmodule

3、9-to-1 Multiplexer 

Problem Statement：

Create a 16-bit wide, 9-to-1 multiplexer. sel=0 chooses a, sel=1 chooses b, etc. For the unused cases (sel=9 to 15), set all output bits to '1'.

module top_module( 
    input [15:0] a, b, c, d, e, f, g, h, i,
    input [3:0] sel,
    output [15:0] out 
);
    always@(*)begin
        case(sel)
            4'd0 : out = a;
            4'd1 : out = b;
            4'd2 : out = c;
            4'd3 : out = d;
            4'd4 : out = e;
            4'd5 : out = f;
            4'd6 : out = g;
            4'd7 : out = h;
            4'd8 : out = i;
            default : out = 16'd65535;
        endcase
    end

endmodule

4、256-to-1 Multiplexer 

Problem Statement：

Create a 1-bit wide, 256-to-1 multiplexer. The 256 inputs are all packed into a single 256-bit input vector. sel=0 should select in[0], sel=1 selects bits in[1], sel=2 selects bits in[2], etc.

module top_module( 
    input [255:0] in,
    input [7:0] sel,
    output out 
);
    
    assign out = in[sel];

endmodule

5、256-to-1 4-bit Multiplexer  

Problem Statement：

Create a 4-bit wide, 256-to-1 multiplexer. The 256 4-bit inputs are all packed into a single 1024-bit input vector. sel=0 should select bits in[3:0], sel=1 selects bits in[7:4], sel=2 selects bits in[11:8], etc.

module top_module( 
    input [1023:0] in,
    input [7:0] sel,
    output [3:0] out 
);

    assign out = in[sel * 4 +: 4];
    
endmodule