Verilog parameters with parametric width

Question

It isn't hard to agree that parametrized module design is a good practice and data width is a good starting point.

I have been defining constants 0 and 1 of required bus or operand widths for years. That to avoid compiler warnings and to explicitly communicate the intention. Typically using something like:

parameter   WIDTH = 16;
//  ...
parameter   ZERO  = {WIDTH{1'b0}};            // all zeroes
parameter   UNO   = {{WIDTH-1{1'b0}}, 1'b1};  // all zeroes except LSB

This is all fine until I want to define an arbitrary constant with given parametrized WIDTH. I certainly can write a constant with fixed width - but that's not what I want:

parameter   FULL   = 16'd57;

However, analogous construct using the parametric WIDTH fails with syntax error:

parameter   LEVEL   = WIDTH'd57;   // <== *ERROR*

What is the proper syntax - if there is one?

dave_59 · Accepted Answer · 2021-01-17 07:06:41Z

2

This was a problem in Verilog because the RHS of a parameter assignment was used as the self-determined width of the parameter. SystemVerilog addressed this by allowing you to specify a datatype as part of a parameter declaration

parameter   WIDTH = 16;
//  ...
parameter bit [WIDTH-1:0] ZERO  = '0;           // all zeroes
parameter bit [WIDTH-1:0] UNO   = 1;  // all zeroes except LSB
parameter bit [WIDTH-1:0] LEVEL = 57;

The datatype does not change when overriding.

Another way is using a sizing cast

parameter LEVEL = WIDTH'(56);

But if you do it this way and override the parameter, the datatype becomes the width of the overriding value's type.

answered Jan 17, 2021 at 7:06

dave_59

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2 Comments

Martin Vana Over a year ago

This is fantastic! Thanks. The datatype specification is a gem. However, I find the sizing cast relevant too if - as an example - I want to create dependent parameter and use division - as an example:

Martin Vana Over a year ago

parameter bit [WIDTH-1:0] LEVEL1 = WIDTH'(LEVEL/2-1); Note: For simple divide by 2 I may still use LEVEL>>1 without need for cast.

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