Re: verilog- parameterized mux

Altera_Forum · ‎04-20-2010

For the life of me I cannot figure out how to generate a parameterized mux in verilog...

I understand how to generate, say, a 4:1 MUX of "N" bits wide... But I am trying to generate an N:1 mux, of only 1 bit wide.

I've been scouring the googleweb for weeks with no such luck (aside from hit after hit for a variable-width mux)...

Have I stumbled upon something impossible?

thanks,

..dane

Altera_Forum · ‎04-21-2010

I hope I'm not doing your homework for you. One of the shortcomings of Verilog is the inability to generate ports. So you have to bring the input in as a bus.

I haven't compiled this so there may be errors:

module mux (#parameter  WIDTH           = 8,
           # parameter  CHANNELS        = 4) (
    input         in_bus,
    input       sel,   
    output                   out
    );
genvar ig;
    
wire     input_array ;
assign  out = input_array;
generate
    for(ig=0; ig<CHANNELS; ig=ig+1) begin: array_assignments
        assign  input_array = in_bus;
    end
endgenerate
//define the clogb2 function
function integer clogb2;
  input depth;
  integer i,result;
  begin
    for (i = 0; 2 ** i < depth; i = i + 1)
      result = i + 1;
    clogb2 = result;
  end
endfunction
endmodule

Altera_Forum · ‎04-21-2010

--- Quote Start ---

I hope I'm not doing your homework for you. One of the shortcomings of Verilog is the inability to generate ports. So you have to bring the input in as a bus.

--- Quote End ---

Hahaha.. no, you're not doing your homework for me. Truth be told it's a project I've been working on for a customer for about three months, and this is the last piece to the puzzle (the rest of the project is all done and working just fine) that I just can't seem to crack.

I will admit to only a few years of experience with verilog, so your code snippet has a few curiosities in it that I'll ask below.


genvar ig;
    
wire     input_array +:CHANNELS-1];
assign  out = input_array;
generate
    for(ig=0; ig<CHANNELS; ig=ig+1) begin: array_assignments
        assign  input_array = in_bus+:WIDTH];
    end
endgenerate

I am not familiar with a "+" in the port width. A very quick google indicates maybe it's a polarity indicator, but that doesn't help me much -- can you explain what you're accomplishing by including it and how it would behave if it were excluded?

I see what you're doing with your generate statement now, and feel a bit silly for not thinking of that myself. I was trying to go about it a much different (and obviously more difficult) way...

I will mess around with your code snip and report back how it goes..

thanks again,

..dane

Altera_Forum · ‎04-21-2010

1 - That first "+" sign in the array declaration is a typo. See I told you there might be errors.

:).

2 - The second "+:" sign in the "assign" statement is legitimate and is called a "Variable Vector Part Select" operator.

For example, a statement like:

assign data1 = data[8+:8]

is equivalent to:

assign data1 = data[15:8];

Generically:

assign data1 = data[x+:y]

So the "+:" operator uses "x" as the starting index of the vector and uses "y" to determine how many bits of the vector to index. The "+:" operator adds "y" bits to "x" while the "-:" operand subtracts "y" bits from "x".

Jake

I will post some equivalent examples shortly.

Altera_Forum · ‎04-21-2010

Equivalent / alternative examples

1 - The one I already posted with a correction:

module mux (#parameter  WIDTH           = 8,
           # parameter  CHANNELS        = 4) (
    input         in_bus,
    input       sel,   
    output                   out
    );
genvar ig;
    
wire     input_array ;
assign  out = input_array;
generate
    for(ig=0; ig<CHANNELS; ig=ig+1) begin: array_assignments
        assign  input_array = in_bus;
    end
endgenerate
//define the clogb2 function
function integer clogb2;
  input depth;
  integer i,result;
  begin
    for (i = 0; 2 ** i < depth; i = i + 1)
      result = i + 1;
    clogb2 = result;
  end
endfunction
endmodule

2 - Similar to# 1 but without using the "Variable Vector Part Select" operator:

module mux (#parameter  WIDTH           = 8,
           # parameter  CHANNELS        = 4) (
    input         in_bus,
    input       sel,   
    output                   out
    );
genvar ig;
    
wire     input_array ;
assign  out = input_array;
generate
    for(ig=0; ig<CHANNELS; ig=ig+1) begin: array_assignments
        assign  input_array = in_bus;
    end
endgenerate
//define the clogb2 function
function integer clogb2;
  input depth;
  integer i,result;
  begin
    for (i = 0; 2 ** i < depth; i = i + 1)
      result = i + 1;
    clogb2 = result;
  end
endfunction
endmodule

3 - Equivalent logic without using a generate statement. I don't recommend this because some simulators don't work well with the "reg" type declaration for the array. Again, you could rewrite this to not use the "+:" operator if you wish.

module module mux (#parameter  WIDTH           = 8,
           # parameter  CHANNELS        = 4) (
    input         in_bus,
    input       sel,   
    output                   out
    );
integer i;
    
reg      input_array ;
assign  out = input_array;
always @*
    for(i=0; i<CHANNELS; i=i+1)
        input_array = in_bus;
//define the clogb2 function
function integer clogb2;
  input depth;
  integer i,result;
  begin
    for (i = 0; 2 ** i < depth; i = i + 1)
      result = i + 1;
    clogb2 = result;
  end
endfunction
endmodule

Jake

Altera_Forum · ‎04-23-2010

Thanks again Jake for your help.. While cleaning up some of the project I found an optimization opportunity that greatly simplified the problem and made the mux much simpler. I really do appreciate your help though, I've learned a great deal from this thread!

thank you!

..dane

Altera_Forum · ‎10-30-2012

Old post, but wondering if this ever synthesized? For me, Quartus synthesis errors out on the line:

input [clogb2(CHANNELS-1)-1:0] sel,

Stating:

Error (10140): Verilog HDL error at param_mux.sv(6): range index is not constant

(I just added another parameter for SEL_WIDTH, and used that, forcing the user to manually enter this value. It's easy enough to do, but not ideal...)

Altera_Forum · ‎10-31-2012

Hi Rysc,

jakobjones suggested:


module mux (#parameter  WIDTH           = 8,
           # parameter  CHANNELS        = 4) (
    input         in_bus,
    input       sel,   
    output                   out
    );

but I am implementing slightly differently. To continue jakobjones' example it would be:


module mux 
(
in_bus,
sel,
out
);
parameter WIDTH = 8;
parameter CHANNELS = 4;
input  in_bus;
input  sel;
output  out;
//define the clogb2 function
function integer clogb2;
  input depth;
  integer i,result;
  begin
    for (i = 0; 2 ** i < depth; i = i + 1)
      result = i + 1;
    clogb2 = result;
  end
endfunction

I haven't actually tried to synthesize jakobjones' example, so I cannot comment on its ability to be synthesized. However, my structure above which separates the port list declaration and their width definitions does synthesize. I will report however that I've been in the Xilinx world lately so I cannot comment on how the Quartus tools respond to the above example.

good luck, and let us know how it goes.

..dane