Verilog Code for Crossbar Switch: Syntax and Example

A crossbar switch in Verilog connects multiple inputs to multiple outputs with configurable routing. You can implement it using nested for loops and multiplexers inside a module to select which input connects to each output.

Syntax

A crossbar switch module typically has parameters for the number of inputs and outputs, input and output data widths, and a control signal to select connections. Inside, for loops and case or multiplexers route inputs to outputs based on control signals.

Key parts include:

• Inputs: Data lines and control signals.
• Outputs: Routed data lines.
• Control: Selects which input connects to each output.

verilog

module crossbar_switch #(
    parameter N_INPUTS = 4,
    parameter N_OUTPUTS = 4,
    parameter DATA_WIDTH = 8
)(
    input  wire [N_INPUTS*DATA_WIDTH-1:0] inputs,
    input  wire [N_OUTPUTS*($clog2(N_INPUTS))-1:0] select,
    output wire [N_OUTPUTS*DATA_WIDTH-1:0] outputs
);

    genvar i;
    generate
        for (i = 0; i < N_OUTPUTS; i = i + 1) begin : output_loop
            wire [$clog2(N_INPUTS)-1:0] sel = select[i*$clog2(N_INPUTS) +: $clog2(N_INPUTS)];
            wire [DATA_WIDTH-1:0] out_data;

            assign out_data = inputs[sel*DATA_WIDTH +: DATA_WIDTH];
            assign outputs[i*DATA_WIDTH +: DATA_WIDTH] = out_data;
        end
    endgenerate
endmodule

Example

This example shows a 4x4 crossbar switch connecting four 8-bit inputs to four outputs. The select input chooses which input connects to each output. The testbench sets different select values to demonstrate routing.

verilog

module crossbar_switch #(
    parameter N_INPUTS = 4,
    parameter N_OUTPUTS = 4,
    parameter DATA_WIDTH = 8
)(
    input  wire [N_INPUTS*DATA_WIDTH-1:0] inputs,
    input  wire [N_OUTPUTS*($clog2(N_INPUTS))-1:0] select,
    output wire [N_OUTPUTS*DATA_WIDTH-1:0] outputs
);

    genvar i;
    generate
        for (i = 0; i < N_OUTPUTS; i = i + 1) begin : output_loop
            wire [$clog2(N_INPUTS)-1:0] sel = select[i*$clog2(N_INPUTS) +: $clog2(N_INPUTS)];
            wire [DATA_WIDTH-1:0] out_data;

            assign out_data = inputs[sel*DATA_WIDTH +: DATA_WIDTH];
            assign outputs[i*DATA_WIDTH +: DATA_WIDTH] = out_data;
        end
    endgenerate
endmodule

// Testbench
module tb_crossbar;
    reg [31:0] inputs;
    reg [7:0] select; // 4 outputs * 2 bits each
    wire [31:0] outputs;

    crossbar_switch #(.N_INPUTS(4), .N_OUTPUTS(4), .DATA_WIDTH(8)) uut (
        .inputs(inputs),
        .select(select),
        .outputs(outputs)
    );

    initial begin
        // Set inputs: 4 inputs of 8 bits each
        inputs = {8'hAA, 8'hBB, 8'hCC, 8'hDD}; // inputs[31:24]=AA, [23:16]=BB, [15:8]=CC, [7:0]=DD

        // Route inputs to outputs
        // Output0 selects input3 (11), Output1 selects input2 (10), Output2 selects input1 (01), Output3 selects input0 (00)
        select = {2'b11, 2'b10, 2'b01, 2'b00};
        #10;
        $display("Outputs: %h", outputs); // Expected: DDCCBBAA

        // Change routing
        select = {2'b00, 2'b01, 2'b10, 2'b11};
        #10;
        $display("Outputs: %h", outputs); // Expected: AACCBBDD

        $finish;
    end
endmodule

Output

Outputs: DDCCBBAA
Outputs: AACCBBDD

Common Pitfalls

Common mistakes when coding a crossbar switch in Verilog include:

• Incorrect bit slicing of inputs or outputs, causing wrong data routing.
• Using fixed select widths that don't match the number of inputs, leading to synthesis errors.
• Not using generate loops for scalable designs, making code hard to maintain.
• Forgetting to use $clog2 to calculate select signal width dynamically.

Always verify bit ranges carefully and test with different select values.

verilog

/* Wrong way: fixed select width 2 bits for 8 inputs (needs 3 bits) */
module wrong_crossbar(
    input wire [7:0] inputs,
    input wire [3:0] select, // Incorrect width
    output wire output_data
);
    assign output_data = inputs[select]; // Will cause error or wrong behavior
endmodule

/* Right way: use $clog2 for select width */
module correct_crossbar #(
    parameter N_INPUTS = 8
)(
    input wire [N_INPUTS-1:0] inputs,
    input wire [$clog2(N_INPUTS)-1:0] select,
    output wire output_data
);
    assign output_data = inputs[select];
endmodule

Quick Reference

• Use $clog2(N) to calculate select signal width for N inputs.
• Use generate loops for scalable crossbar designs.
• Slice input and output buses carefully with [start +: width] syntax.
• Test with different select values to verify routing.

Key Takeaways

Use parameterized modules with $clog2 to handle variable input/output sizes.

Implement crossbar routing with generate loops and careful bit slicing.

Verify select signal widths match the number of inputs to avoid errors.

Test your design with multiple select configurations to ensure correct routing.

Avoid hardcoding widths and indexes; use parameters for flexibility.