FFT implemetation in Verilog: Assigning Wire input to Register type array - fft

Question

I am trying to implement butterfly FFT algorithm in verilog.
I create K(Here 4) butterfly modules . I create modules like this.
localparam K = 4;
genvar i;
generate
for(i=0;i<N/2;i=i+1)
begin:BN
butterfly #(
.M_WDTH (3 + 2*1),
.X_WDTH (4)
)
bf (
.clk(clk),
.rst_n(rst_n),
.m_in(min),
.w(w[i]),
.xa(IN[i]),
.xb(IN[i+2]),
.x_nd(x_ndd),
.m_out(mout[i]),
.ya(OUT[i]),
.yb(OUT[i+2]),
.y_nd(y_nddd[i])
);
end
Each level I have to change input Xa and Xb for each Module (Here Number of level 3).
So I try to initialize reg type "IN"array and assign the array to input Xa and Xb. When I initialize "IN" array manually, it works perfectly.
The problem I face now, I couldn't assign Main module input X to register type "IN" array.
Main module input X ,
input wire signed [N*2*X_WDTH-1:0] X,
I have to assign this X into array "IN",
reg signed [2*X_WDTH-1:0] IN [0:N-1];
I assigned like this,
initial
begin
IN[0]= X[2*X_WDTH-1:0];
IN[1]=X[4*X_WDTH-1:2*X_WDTH];
IN[2]=X[6*X_WDTH-1:4*X_WDTH];
IN[3]= X[8*X_WDTH-1:6*X_WDTH];
IN[4]= X[10*X_WDTH-1:8*X_WDTH];
IN[5]=X[12*X_WDTH-1:10*X_WDTH];
IN[6]=X[14*X_WDTH-12*X_WDTH];
IN[7]= X[16*X_WDTH-1:14*X_WDTH];
end
I have gone through many tutorials and forums. No luck.
Can't we assign wire type to reg type array? If so how I can solve this problem.
Here is the Main module where I initialize Butterfly modules,
module Network
#(
// N
parameter N = 8,
// K.
parameter K = 3,
parameter M_WDTH=5,
parameter X_WDTH=4
)
(
input wire clk,
input wire rst_n,
// X
input wire signed [N*2*X_WDTH-1:0] X,
//Y
output wire signed [N*2*X_WDTH-1:0] Y,
output wire signed [K-1:0] y_ndd
);
wire y_nddd [K-1:0];
assign y_ndd ={y_nddd[1],y_nddd[0]};
reg [4:0] min=5'sb11111;
wire [4:0] mout [0:K-1];
reg x_ndd;
reg [2:0] count=3'b100;
reg [2*X_WDTH-1:0] w [K-1:0];
reg [2*X_WDTH-1:0] IN [0:N-1];
wire [2*X_WDTH-1:0] OUT [0:N-1];
assign Y = {OUT[3],OUT[2],OUT[1],OUT[0]};
reg [3:0] a;
initial
begin
//TODO : Here is the problem. Assigning Wire to reg array. Synthesize ok. In Simulate "red" output.
IN[0]= X[2*X_WDTH-1:0];
IN[1]=X[4*X_WDTH-1:2*X_WDTH];
IN[2]=X[6*X_WDTH-1:4*X_WDTH];
IN[3]= X[8*X_WDTH-1:6*X_WDTH];
IN[4]= X[10*X_WDTH-1:8*X_WDTH];
IN[5]=X[12*X_WDTH-1:10*X_WDTH];
IN[6]=X[14*X_WDTH-12*X_WDTH];
IN[7]= X[16*X_WDTH-1:14*X_WDTH];
//TODO :This is only a random values
w[0]=8'sb01000100;
w[1]=8'sb01000100;
w[2]=8'sb01000100;
w[3]=8'sb01000100;
end
/* levels */
genvar i;
generate
for(i=0;i<N/2;i=i+1)
begin:BN
butterfly #(
.M_WDTH (3 + 2*1),
.X_WDTH (4)
)
bf (
.clk(clk),
.rst_n(rst_n),
.m_in(min),
.w(w[i]),
.xa(IN[i]),
.xb(IN[i+N/2]),
.x_nd(x_ndd),
.m_out(mout[i]),
.ya(OUT[2*i]),
.yb(OUT[2*i+1]),
.y_nd(y_nddd[i])
);
end
endgenerate
always # (posedge clk)
begin
if (count==3'b100)
begin
count=3'b001;
x_ndd=1;
end
else
begin
count=count+1;
x_ndd=0;
end
end
always# (posedge y_ndd[0])
begin
//TODO
//Here I have to swap OUT-->IN
end
endmodule
Any help is appreciated.
Thanks in advance.

Answer 1

"Output is red", this likely means it is x this could be due to multiple drivers or an uninitialized value. If it was un-driven it would be z.
The main Issue I believe is that you do this :
initial begin
IN[0] = X[2*X_WDTH-1:0];
IN[1] = X[4*X_WDTH-1:2*X_WDTH];
...
The important part is the initial This is only evaluated once, at time 0. Generally everything is x at time zero. To make this an equivalent of the assign IN[0] = ... for a wire use always #* begin this is a combinatorial block which will update the values for IN when ever X changes.
always #* begin
IN[0] = X[2*X_WDTH-1:0];
IN[1] = X[4*X_WDTH-1:2*X_WDTH];
...
I am not sure why you do not just connect your X to your butterfly .xa and .xb ports directly though?
Other pointers
X is a bad variable name verilog as a wire or reg can hold four values 1,0,x or z.
In always #(posedge clk) you should be using non-blocking (<=) assignments to correctly model the behaviour of a flip-flop.
y_ndd is k bits wide but only the first 2 bits are assigned.
output signed [K-1:0] y_ndd
assign y_ndd = {y_nddd[1],y_nddd[0]};
Assignments should be in terms of their parameter width/size. For example IN has N entries but currently exactly 8 entries are assigned. There will been an issue when N!=8. Look into Indexing vectors and arrays with +:. Example:
integer idx;
always #* begin
for (idx=0; idx<N; idx=idx+1)
IN[idx] = X[ idx*2*X_WDTH +: 2*X_WDTH];
end
genvar gidx;
generate
for(gidx=0; gidx<N; gidx=gidx+1) begin
assign Y[ gidx*2*X_WDTH +: 2*X_WDTH] = OUT[gidx];
end
endgenerate