在重置值的分配中确定参数值的大小

Question

The following code is correct as RESET_VAL is extended or truncated to WIDTH automagically:

parameter RESET_VAL = 5;
reg [WIDTH-1:0] data;

always @(posedge clk or negedge nres) begin
    if (!nres) begin
        data <= RESET_VAL;
    end
    ...

However, a synthesis tool is throwing a warning:

Width of left hand side 'data' [6] doesn't match the width of right hand side [32] in assignment

Can anyone help me with the correct syntax for sizing the parameter value?

I would like to pass parameters through hierarchy, specifying their (32-bit) value without having to dive into submodules for determining the correct size of the parameters.

Answer 1

The type of a parameter in Verilog/SV is inferred from its value.
The post specifies an unsized number literal which is an integer in Verilog whose default is 32 bits.
Change the number to be 6 bits this way, to form a size 6-bit decimal value of five:

parameter RESET_VAL = 6'd5;  

Example:

module testbench ();
  
  parameter RESET_VAL = 6'd5;
  
  initial
    $display("RESET_VAL = %b",RESET_VAL);
  
endmodule  

Produces:

RESET_VAL = 000101

Alternate Answer: If you want to have a 32 bit parameter, and eliminate the warning at the assignment, use one of these:

module testbench ();
  
  parameter RESET_VAL = 5;
  parameter WIDTH     = 6;
  
  initial begin
    $display("RESET_VAL = %b",WIDTH'(RESET_VAL));
    $display("RESET_VAL = %b",RESET_VAL[WIDTH - 1:0]);
  end
  
endmodule    

Referring to the alternate answer: Pass WIDTH in from the top of various modules to resize differently in different modules (answers the part of the question asked as additional question details in the comments).

Answer 2

Normally the type of a parameter is based on the type being assigned to it. The correct thing to do here is sizing the parameter to match the size of the variable it is to be used with.

parameter bit [WIDTH-1:0] RESET_VAL = 5;
logic [WIDTH-1:0] data;

always @(posedge clk or negedge nres) begin
    if (!nres) begin
        data <= RESET_VAL;
    end

RESET_VAL now always has a fixed type regardless of the value on the RHS or overridden,