I am trying to implement Fast Fourier Transform in Verilog for 32-point sample and have written the following butterfly module:

module bffly(
input wire signed [15:0] xa_r,
input wire signed [15:0] xa_i,
input wire signed [15:0] xb_r,
input wire signed [15:0] xb_i,
input wire signed [15:0] w_r,
input wire signed [15:0] w_i,
output reg signed [15:0] ya_r,
output reg signed [15:0] ya_i,
output reg signed [15:0] yb_r,
output reg signed [15:0] yb_i,
output reg ready,
input wire clk,
input wire enable

// ya = xa + xb*w
// yb = xa - xb*w

// ya_r = xa_r + xb_r*w_r - xb_i*w_i
// ya_i = xa_i + xb_r*w_i + xb_i*w_r

// yb_r = xa_r - xb_r*w_r + xb_i*w_i
// yb_i = xa_i - xb_r*w_i - xb_i*w_r

wire signed [31:0] temp_r_32;
wire signed [31:0] temp_i_32;

wire signed [15:0] temp_r_16;
wire signed [15:0] temp_i_16;

assign temp_r_32 = xb_r*w_r - xb_i*w_i;
assign temp_i_32 = xb_r*w_i + xb_i*w_r;

assign temp_r_16 = {temp_r_32[31], temp_r_32[28:14]};
assign temp_i_16 = {temp_i_32[31], temp_i_32[28:14]};

always@(posedge clk)
 if (enable)
    ya_r <= xa_r + temp_r_16; 
    ya_i <= xa_i + temp_i_16;
    yb_r <= xa_r - temp_r_16;
    yb_i <= xa_i - temp_i_16;
    ready <= 1;

 else ready<= 0;


The butterfly module is working properly. I have checked it with th test fixture.

In the fft module, I have made several calls to the butterfly module. Here is a code snippet of the fft module:

module fft(
 input enable1,
 input clk,
     input [15:0] a0,...,input [15:0] a31,
     output [15:0] br0,..., output [15:0] br31,
     output [15:0] bi0,..., output [15:0] bi31, 
     output wand ready,
     output wan enable3
wire [15:0] wr0, etc. stores the twiddle factors.
reg [15:0] tr0;...,reg [15:0] tr31;
reg [15:0] ti0,...,reg [15:0] ti31;

bffly z12(.ready(enable2),.clk(clk),.enable(enable1),.xa_r(a3),.xa_i(16'b0),.xb_r(a19),.xb_i(16'b0), .w_r(wr0), .w_i(wi0),.ya_r(tr24), .ya_i(ti24),.yb_r(tr25), .yb_i(ti25));
bffly z13 (.ready(enable2),.clk(clk),.enable(enable1),.xa_r(a11),.xa_i(16'b0),.xb_r(a27),.xb_i(16'b0), .w_r(wr0), .w_i(wi0),.ya_r(tr26),.ya_i(ti26), .yb_r(tr27), .yb_i(ti27));

For all such calls to the butterfly there is an error as follows:

Target tr24 of concurrent assignment or output port connection should be a net type.

Target tr26 of concurrent assignment or output port connection should be a net type., respectively

When the target is assigned as a wire and not as a reg, then it works fine.

What do I do in this case ??

  • 1
    \$\begingroup\$ You do not make 'calls' you 'instantiate'. You are representing blocks of physical hardware. I am not too picky about the words but sounds like you might be approaching from a software angle rather than hardware. \$\endgroup\$ Commented Mar 4, 2014 at 10:51

1 Answer 1


When the target is assigned as a wire and not as a reg, then it works fine

When you say assigned I assume you mean changing (in the fft module):

reg [15:0] tr0;...,reg [15:0] tr31;
reg [15:0] ti0,...,reg [15:0] ti31;


wire [15:0] tr0;...,wire [15:0] tr31;
wire [15:0] ti0,...,wire [15:0] ti31;

Which is the correct thing todo. The types reg, wire only apply in the current module and are not carried over port connections.

Remember the choice of wire or reg is for the simulator not indicative of the hardware. In SystemVerilog the majority of wire/reg can be replaced with logic. The only place this does not work is for tristate busses then you should use tri.


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