Casting packed array to unpacked array for arrays that are used as parameters

Question

Is it possible to cast a packed array to an unpacked array and use the unpacked array as a parameter in a module instantiation? The packed array is defined as a localparam.

Here is an illustration of what I am trying to do

localparam [7:0] packed         = '0;
localparam      unpacked [3:0]  = packed[3:0]; <-- Needs to be casted to an unpacked array

module1 #(unpacked) myModule1(...); <--- The parameter here needs to be of unpacked type

Have you mixed up you packed and unpacked arrays? localparam packed[7:0] is an unpacked array; localparam [3:0] unpacked is a packed array. — Matthew
– Matthew, Commented Feb 1, 2017 at 15:19
And, BTW: packed is a reserved word in System-Verilog, so you cannot use it yourself. — Matthew
– Matthew, Commented Feb 1, 2017 at 15:19
There was a mistake in the example. The type of array should match the names. I have corrected the code to reflect this. — Eivind
– Eivind, Commented Feb 2, 2017 at 12:47

Matthew · Accepted Answer · 2017-02-02 09:32:23Z

5

You can convert a packed array to an unpacked array using an assignment pattern, but it's not very flexible/extensible:

unpacked_array = '{ packed_array[0], packed_array[1], ... , packed_array[...]};

eg

localparam [7:0] packed_array   = '{default: 0};
localparam unpacked_array [3:0] = '{ packed_array[0], packed_array[1], packed_array[2], packed_array[3]}; 

module1 #(unpacked_array) myModule1();

https://www.edaplayground.com/x/3v8V

You can convert an unpacked array to a packed array using a streaming operator:

packed_array = { << { unpacked_array }};

eg

localparam       unpacked_array[7:0] = '{default: 0};
localparam [3:0] packed_array        = { << { unpacked_array[3:0] }}; 

module1 #(packed_array) myModule1();

https://www.edaplayground.com/x/5Q5b

edited Feb 2, 2017 at 9:32

answered Feb 1, 2017 at 15:18

Matthew

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2 Comments

dave_59 Over a year ago

A better solution to the streaming operator when using fixed sized arrays is a bit-stream cast. A little more work because you need to create typedefs for the cast, but safer as it will not pad if the number of bits does not match.

Eivind Over a year ago

I did a mistake when I wrote the example. I did mean converting from packed to unpacked array, I just placed the brackets on the wrong side of the parameter name. So the question is: is it possible to do the conversion the opposite direction from what you showed me? From packed to unpacked.

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