Verilog generate genvar in an always block

Question

I m trying to get a module to pass the syntax check in ISE 12 4  and it gives me an error I don t understand  First a code snippet   parameter ROWBITS   4   reg  ROWBITS-1 0  temp   genvar c  generate     always   posedge sysclk  begin         for  c   0  c  lt  ROWBITS  c   c   1  begin  test             temp c   lt   1 b0          end     end endgenerate   When I try a syntax check  I get the following error message      ERROR HDLCompiler 731 -  test v  Line 46  Procedural assignment to a   non-register  lt c gt  is not permitted    I really don t understand why it s complaining   c  isn t a wire  it s a genvar  This should be the equivalent of the completely legal syntax   reg  3 0  temp   always   posedge sysclk  begin     temp 0   lt   1 b0      temp 1   lt   1 b0      temp 2   lt   1 b0      temp 3   lt   1 b0  end   Please  no comments about how it d be easier to write this without the generate  This is a reduced example of a much more complex piece of code involving multiple ifs and non-blocking assignments to  temp   Also  don t just tell me there are newer versions of ISE  I already know that  OTOH  if you know it s fixed in a later version of ISE  please let me know which version you know works

User · Answer

You need to reverse the nesting inside the generate block:

genvar c;
generate
    for (c = 0; c < ROWBITS; c = c + 1) begin: test
        always @(posedge sysclk) begin
            temp[c] <= 1'b0;
        end
    end
endgenerate

Technically, this generates four always blocks:

always @(posedge sysclk) temp[0] <= 1'b0;
always @(posedge sysclk) temp[1] <= 1'b0;
always @(posedge sysclk) temp[2] <= 1'b0;
always @(posedge sysclk) temp[3] <= 1'b0;

In this simple example, there's no difference in behavior between the four always blocks and a single always block containing four assignments, but in other cases there could be.

The genvar-dependent operation needs to be resolved when constructing the in-memory representation of the design (in the case of a simulator) or when mapping to logic gates (in the case of a synthesis tool). The always @posedge doesn't have meaning until the design is operating.

Subject to certain restrictions, you can put a for loop inside the always block, even for synthesizable code. For synthesis, the loop will be unrolled. However, in that case, the for loop needs to work with a reg, integer, or similar. It can't use a genvar, because having the for loop inside the always block describes an operation that occurs at each edge of the clock, not an operation that can be expanded statically during elaboration of the design.

User · Answer

Within a module  Verilog contains essentially two constructs  items and statements  Statements are always found in procedural contexts  which include anything in between begin  end  functions  tasks  always blocks and initial blocks  Items  such as generate constructs  are listed directly in the module  For loops and most variable constant declarations can exist in both contexts   In your code  it appears that you want the for loop to be evaluated as a generate item but the loop is actually part of the procedural context of the always block  For a for loop to be treated as a generate loop it must be in the module context  The generate  endgenerate keywords are entirely optional some tools require them  and have no effect  See this answer for an example of how generate loops are evaluated     Compiler sees this parameter ROWBITS   4  reg  ROWBITS-1 0  temp  genvar c       always   posedge sysclk    Procedural context starts here     begin         for  c   0  c  lt  ROWBITS  c   c   1  begin  test             temp c   lt   1 b0    Still a genvar         end     end

User · Answer

You don t need a generate bock if you want all the bits of temp assigned in the same always block   parameter ROWBITS   4  reg  ROWBITS-1 0  temp  always   posedge sysclk  begin     for  integer c 0  c lt ROWBITS  c c 1  begin  test         temp c   lt   1 b0      end end   Alternatively  if your simulator supports IEEE 1800  SytemVerilog   then   parameter ROWBITS   4  reg  ROWBITS-1 0  temp  always   posedge sysclk  begin         temp  lt    0     fill with 0     end end

User · Answer

To put it simply  you don t use generate inside an always process  you use generate to create a parametrized process or instantiate particular modules  where you can combine if-else or case  So you can move this generate and crea a particular process or an instantiation e g    module    parameter XLEN   64  parameter USEIP   0      input clk  input rstn  input  XLEN-1 0  opA  input  XLEN-1 0  opB  input  XLEN-1 0  opR  input en     generate  case USEIP  0 begin always   posedge clk or negedge rstn  begin if  rstn  begin  opR  lt     default 0   end else begin if en   opR  lt   opA opB  else opR  lt     default 0   end end end 1 begin   superAdder    XLEN XLEN    adder  clk clk   rstm rstn    opA opA    opB opB    opR opR    en en    end endcase  endmodule

User · Answer

for verilog just do  parameter ROWBITS   4  reg  ROWBITS-1 0  temp  always   posedge sysclk  begin   temp  lt    ROWBITS 1 b0       fill with 0 end

User · Answer

If you do not mind having to compile generate the file then you could use a pre processing technique  This gives you the power of the generate but results in a clean Verilog file which is often easier to debug and leads to less simulator issues   I use RubyIt to generate verilog files from templates using ERB  Embedded Ruby    parameter ROWBITS    lt    ROWBITS   gt    always   posedge sysclk  begin    lt    0   ROWBITS  each do  addr  -  gt      temp  lt    addr   gt    lt   1 b0     lt   end -  gt  end   Generating the module name v file with      ruby it --parameter ROWBITS 4 --outpath    --file   module name rv   The generated module name v  parameter ROWBITS   4   always   posedge sysclk  begin   temp 0   lt   1 b0    temp 1   lt   1 b0    temp 2   lt   1 b0    temp 3   lt   1 b0  end

[verilog] Verilog generate/genvar in an always block

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