Way to initialize synthesizable 2D array with constant values in Verilog

Question

in VHDL, I can easily do this:

constant    cmdbytes       : bytearray(0 to Total) := (x"05", x"00", x...};

I want synthesizable constants so that when the FPGA starts, this array has the data I supplied. These registers are wired to VCC or ground to represent 1 or 0. I can then use them to generate a waveform. Also I would like to have 2D byte array which is 3D in verilog world.

Eloiseloisa

Answer 1

If you're just using the array to pull out one value at a time, how about using a case statement? Granted, it's a long-winded way of doing it, but you could always write a script to write the RTL for you.

reg [7:0] value;
reg [7:0] i;

always @(posedge clk or negedge rst_n) begin
    if(!rst_n)
        i <= 8'd0;
    else
        i <= i + 1;
end

always @(*) begin
    case(i) 
        8'h00: value = 8'd0;
        8'h01: value = 8'd34;
        ...
    endcase
endcase

Another way is to use an initial statement. As far as I'm aware, FPGA synthesis tools will allow you to set initial values for arrays in the following manner. Again, a script to write this may be the way to go.

reg [0:35][7:0] my_array;

initial begin
    my_array[0] = 8'd45;
    my_array[1] = 8'd26;
    ...
end

And if your FGPA synthesis tools support some SystemVerilog, you'll be able to initialise the array like so:

reg [0:34][7:0] my_array = '{ 8'd90, 8'd34, ... }; // note the '{

Mellicent

Answer 2

module test (

   input [7:0]   p1_sa,            // i
   input [7:0]   p1_sb,            // i
   output [7:0]   p3,            // o
   output [7:0]   p3b            // o
);

logic [7:0] array2d [7:0] = {99,124,119,123,242,107,111,197};

   assign p3    = array2d[p1_sa];
   assign p3b   = array2d[p1_sb];

endmodule

I tried the above system verilog code and it is working fine in modelsim and Vivado.

Bostick

Answer 3

An alternative to the case statement approach is to use a function to access values.

Example for an "array" of size 4 containing 8 bit constants:

function [7:0] cmdbytes;
        input    [1:0] index;
        reg      [7:0] t[0:3];
    begin
        {   
             t[0],t[1],t[2],t[3]
        } = {   
             8'h05, 8'h00, 8'h5b, 8'h45
        };

        cmdbytes = t[index];
    end
endfunction

One can access the constants like so:

wire [7:0] value0;
wire [7:0] value1;
wire [7:0] value2;
wire [7:0] value3;

assign value0 = cmdbytes(2'd0);
assign value1 = cmdbytes(2'd1);
assign value2 = cmdbytes(2'd2);
assign value3 = cmdbytes(2'd3);

Mezereum

Answer 4

Verilog 2005 doesn't allow array initialization. Though your FPGA vendor should have a means to generate a ROM.

Illhumored