Verilog/SV conditional variable definition

Question

Is there a way to manipulate variable instantiation depending on a parameter?

For example, here if I were to put just bit [WIDTH-1:0] a; and set DEPTH == 1, WIDTH would be 0 and bit [-1:0] a; would not make sense.

When I code it like in the example below I get an error on the second $display: "Undeclared identifier: a". Is there a way to achieve this in Verilog/SV or is there an error in my code?

module test #(
  parameter DEPTH = 2,
  parameter WIDTH = $clog2(DEPTH)
)();

  generate
    if (WIDTH == 0) begin
      bit             a;
    end else begin
      bit [WIDTH-1:0] a;
    end
  endgenerate

  initial begin
    $display("WIDTH: %d", WIDTH);
    $display("Bit width of a: %d", $bits(a));
  end
endmodule: test

dave_59 · Accepted Answer · 2016-08-31 16:13:07Z

3

All you need to do is

bit [(WIDTH>0 ? WIDTH-1 : 0):0] a;

answered Aug 31, 2016 at 16:13

dave_59

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Rich Maes · Accepted Answer · 2016-08-31 18:06:16Z

0

Which version of Verilog are you using? $clog2 was introduced in Verilog-2005. Prior implementations, can give odd results.

See below, I did a loop to show incrementing depth versus result of clog2.

CLOG2(0) =           0
CLOG2(1) =           0
CLOG2(2) =           1
CLOG2(3) =           2
CLOG2(4) =           2

To represent the value of 1, for instance, you need 1 bit, not 0 To represent the value of 2, you need 2 bits...not 1. To represent the value of 4, you need 3 bits. CLOG2 is always 1 short at the transition points. The prior solution will not account for that as you grow.

So if you say

WIDTH = $clog(DEPTH+1);

I think you will automatically get the results you want.

edited Aug 31, 2016 at 18:06

answered Aug 31, 2016 at 17:59

Rich Maes

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1 Comment

evilpascal Over a year ago

I'm using SystemVerilog. Unfortunately, I want the result of CLOG2(1) and CLOG2(2) to be both 1, so this is not what I need.

Kirk Weedman · Accepted Answer · 2018-12-16 19:14:15Z

I made & use this on a regular basis. It determines the width in bits of an integer value. Special case of 0 will return a value of 1 (you still need 1 bit to hold the value). Let's say you need to define an index variable based on memory that has 256 addresses

parameter NUM_ADDR 256

localparam A_SZ = bit_size(NUM_ADDR-1); // example: 255 (highest address) should return 8

logic [A_SZ-1:0] index; // example: [7:0] index

Then All I ever need to change is NUM_ADDR

function integer bit_size;
input integer value;
reg [31:0] shifted;
integer res;
begin
  if (value != 0)
  begin
       shifted = value;
       for (res=0; (shifted != 0); res=res+1)
           shifted = shifted >> 1;
       bit_size = res;
  end
  else
     bit_size = 1; // minimum size, even for a value of 0
end
endfunction

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