Array as module parameter

Question

How to pass a constant array as a module parameter?

I want to build a shift register width different shift widths. The possible shift widths should be definable via a module parameter. I tried something like the following, but this not working.

module ShiftReg
#(
  SHIFT_WIDTH = '{1, 2, 4, 8},
  WIDTH = $clog2($size(SHIFT_WIDTH))
)
(
  ...
  input  logic [WIDTH-1:0] shift_rate_i,
  ...
);
  ...
endmodule

This results in following error message:

** Error: shift_reg.sv(3): Illegal concatenation of an unsized constant.

Is such a generic construction of a shift register with different widths possible in SystemVerilog?

Greg · Accepted Answer · 2015-12-01 19:24:50Z

1

Not every simulator supports arrayed parameters. For those that do, the array needs to be defined with an array identifier (ex: [], [SIZE]) and a bit width for the entries; int SHIFT_WIDTH [] = '{1, 2, 4, 8} should work.

I tried different combinations on EDAplaygroud. Queued arrays ([$]) was not accepted by any simulator. VCS supported [] arrayed parameters, but it does not accept it with $size() or .size() in a parameter definition. A fixed array size does work on VCS and Riviera-PRO. Declaring the size is an extra step, but it works.

module ShiftReg
#(
  SIZE = 4,
  int SHIFT_WIDTH [SIZE] = '{1, 2, 4, 8},
  WIDTH = $clog2(SIZE)
//WIDTH = $clog2($size(SHIFT_WIDTH)) // <-- this also works if SHIFT_WIDTH is a fixed size
)
(
  ...
  input  logic [WIDTH-1:0] shift_rate_i,
  ...
);
  ...
endmodule

answered Dec 1, 2015 at 19:24

Greg

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3 Comments

sharvil111 Over a year ago

Just a side question. Will this be synthesizable? I doubt that.

Greg Over a year ago

@sharvil111, it could synthesize if the synthesizer implemented it. Most synthesizers probably do not support it right now, but I cannot see any reason why it shouldn't. Just a matter of demand for the feature and time.

sharvil111 Over a year ago

Yaa I agree. This should be made synthesizable. But obviously dynamic/queues arrays must not be accepted. Thanks for the clarification.

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