Assign a synthesizable initial value to a reg in Verilog

Question

I m an FPGA noob trying to learn Verilog   How can I  assign  a value to a reg in an always block  either as an initial value  or as a constant   I m trying to do something like this in the code below   I get an error because the 8 bit constant doesn t count as input   I also don t want to trigger the always off of a clock   I just want to assign a register to a specific value   As I want it to be synthesisable I can t use an initial block   Thanks a lot    module top       input wire clk      output wire  7 0  led          reg  7 0  data reg    always    begin     data reg   8 b10101011  end  assign led   data reg   endmodule

User · Answer

When a chip gets power all of it s registers contain random values  It s not possible to have an an initial value  It will always be random   This is why we have reset signals  to reset registers to a known value  The reset is controlled by something off chip  and we write our code to use it   always   posedge clk  begin     if  reset    1  begin    For an active high reset         data reg   8 b10101011      end else begin         data reg   next data reg      end end

User · Answer

The always    would never trigger as no Right hand arguments change  Why not use a wire with assign   module top       input wire clk      output wire  7 0  led        wire  7 0  data reg    assign data reg     8 b10101011  assign led          data reg   endmodule   If you actually want a flop where you can change the value  the default would be in the reset clause   module top       input        clk      input        rst n      input   7 0  data      output  7 0  led         reg  7 0  data reg    always   posedge clk or negedge rst n  begin   if   rst n      data reg  lt   8 b10101011    else     data reg  lt   data    end  assign led   data reg   endmodule   Hope this helps

User · Answer

The other answers are all good   For Xilinx FPGA designs  it is best not to use global reset lines  and use initial blocks for reset conditions for most logic   Here is the white paper from Ken Chapman  Xilinx FPGA guru   http   japan xilinx com support documentation white papers wp272 pdf

User · Answer

You can combine the register declaration with initialization   reg  7 0  data reg   8 b10101011    Or you can use an initial block  reg  7 0  data reg  initial data reg   8 b10101011

User · Answer

You should use what your FPGA documentation recommends  There is no portable way to initialize register values other than using a reset net  This has a hardware cost associated with it on most synthesis targets

[verilog] Assign a synthesizable initial value to a reg in Verilog

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