Parameters set the interface

A parameter is an input to elaboration. The instantiating parent sets it with a named override.

Use parameters for externally configurable values like width or depth. Defaults in the declaration keep the common case short.

Localparams derive the rest

A localparam cannot be overridden from outside. It computes dependent dimensions in one place, so bus widths cannot mismatch.

Width math belongs here. One formula feeds every use, and the N equal to 1 case needs a guard:

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// snippet
// Width math with the N = 1 guard ($clog2(1) is 0, so the
// unguarded range [$clog2(N)-1:0] would become [-1:0]).
localparam int AW = (DEPTH > 1) ? $clog2(DEPTH) : 1;
input logic [AW-1:0] addr;

Common mistakes

Testbench

The lane array plus guarded DUT select ports for N=1 and N=5. The finite bench checks actual elaborated port sizes, every legal selection, and the defined zero result for spare binary codes. N and WIDTH must be positive.

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module lane_array #(
  parameter int LANES = 4,
  parameter int WIDTH = 16
) (
  input  logic [LANES*WIDTH-1:0] in_flat,
  output logic [LANES*WIDTH-1:0] out_flat
);
  genvar i;
  generate
    for (i = 0; i < LANES; i++) begin : GEN_LANE
      assign out_flat[i*WIDTH +: WIDTH] = in_flat[i*WIDTH +: WIDTH];
    end
  endgenerate
endmodule

module lane_select #(
  parameter int N = 1,
  parameter int WIDTH = 8,
  localparam int SW = (N > 1) ? $clog2(N) : 1
) (
  input logic [N*WIDTH-1:0] lanes,
  input logic [SW-1:0] sel,
  output logic [WIDTH-1:0] y
);
  always_comb begin
    y = '0;
    if (sel < N) y = lanes[sel*WIDTH +: WIDTH];
  end
endmodule

Related topics

• Generate loops and conditionals

• Avoid defparam overrides