Parameters, localparam, and generate

Make a module reusable without turning its internal settings into hardware inputs.

A parameter is fixed before synthesis

A parameter configures the structure of a module when it is instantiated. It does not become a circuit input and cannot change while the circuit is running.

module masked_register #(
  parameter WIDTH = 8
) (
  input  wire             i_clk,
  input  wire             i_reset,
  input  wire             i_load,
  input  wire [WIDTH-1:0] i_data,
  input  wire [WIDTH-1:0] i_mask,
  output reg  [WIDTH-1:0] o_data
);
 
  always @(posedge i_clk) begin
    if

module pulse_every #(
  parameter PERIOD = 10
) (
  input  wire i_clk,
  input  wire i_reset,
  output reg  o_tick
);
 
  localparam COUNTER_WIDTH = 32;
  localparam LAST_COUNT = PERIOD - 1;
 
  reg [COUNTER_WIDTH-1:0] count;
 
  always @(posedge i_clk) begin
    if (i_reset) begin
      count  <= 0;
      o_tick <= 1'b0;
    end else if (count == LAST_COUNT) begin
      count  <= 0;
      o_tick <= 1'b1;
    end else begin
      count  <= count + 1'b1;
      o_tick <= 1'b0;
    end
  end
 
endmodule

module parity_per_byte #(
  parameter BYTE_COUNT = 4
) (
  input  wire [BYTE_COUNT*8-1:0] i_data,
  output wire [BYTE_COUNT-1:0]   o_parity
);
 
  genvar lane;
  generate
    for (lane = 0; lane < BYTE_COUNT; lane = lane + 1) begin : g_parity
      assign o_parity[lane] = ^i_data[lane*8 +: 8];
    end
  endgenerate
 
endmodule

module optional_output_register #(
  parameter WIDTH = 8,
  parameter REGISTER_OUTPUT = 1
) (
  input  wire             i_clk,
  input  wire             i_reset,
  input  wire [WIDTH-1:0] i_data,
  output wire [WIDTH-1:0] o_data
);
 
  generate
    if (REGISTER_OUTPUT) begin : g_registered
      reg [WIDTH-1:0] data_reg;
 
      always @(posedge i_clk) begin
        if (i_reset)
          data_reg <= {WIDTH{1'b0}};
        else
          data_reg <= i_data;
      end
 
      assign o_data = data_reg;
    end else begin : g_direct
      assign o_data = i_data;
    end
  endgenerate
 
endmodule

Parameters, localparam, and generate

A parameter is fixed before synthesis

Overriding a parameter at instantiation

Keeping internal calculations in localparam

Repeating a structure with generate

Selecting a structure during elaboration

Key points