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Generate HDL Code with Annotations or Comments

You can use the HDL Coder™ software to create text annotations to generated HDL code from the model annotations, text comments, or requirements in Simulink®.

Simulink Annotations

You can generate code with comments for blocks, including Inport and Outport blocks, by using Simulink annotations. When you add Simulink annotations to the model, HDL Coder renders the text from these annotations as plain text comments in generated code. The comments are generated at the same level in the model hierarchy as the subsystem that contains the annotations. The figure shows a model that has Simulink annotations for Inports, Outports, and other blocks.

Simulink Annotation for Inports,Outports,blocks.

This code snippet displays the generated VHDL® code, which includes text comments for the blocks.

USE IEEE.std_logic_1164.ALL;
USE IEEE.numeric_std.ALL;

  PORT( -- inport1 comment
        In1                               :   IN    std_logic;
        -- inport2 comment
        In2                               :   IN    std_logic;
         -- outport1 comment
        Out1                              :   OUT   std_logic;
        -- outport2 comment
        Out2                              :   OUT   std_logic

  -- Component Declarations
  COMPONENT Subsystem
    PORT( In1                             :   IN    std_logic;
          In2                             :   IN    std_logic;
          Out1                            :   OUT   std_logic
-- instantiated module comment
  u_Subsystem : Subsystem
    PORT MAP( In1 => Not_gate_out1,
              In2 => Logical_Operator_out1,
              Out1 => Subsystem_out1

  Logical_Operator_out1 <= In1 OR In2;
  Not_gate_out1 <=  NOT Logical_Operator_out1;
  Out1 <= Not_gate_out1;
  Out2 <= Subsystem_out1;

END rtl;

For Constant blocks, to convert annotations to comments in the HDL code, in the Configuration Parameters window:

For more information about annotations, See Annotate Models.

Signal Descriptions

When you provide a description for the signals in your Simulink model, the generated HDL code displays these descriptions as comments above the signal declaration statements. To specify a description for the signal, right-click the signal and select Properties to open the Signal Properties dialog box. Select the Documentation tab and enter a description for the signal in the Description field. For the signal description, use ASCII characters. Non-ASCII characters in the generated code may interfere with downstream synthesis and lint tools. In some cases, due to optimizations that act on the signals, the generated code may not translate all signal descriptions to HDL comments or may create replicas of HDL comments for certain signal descriptions.

Text Comments

If you enter text comments in your model by using a DocBlock block, HDL Coder renders the text from the DocBlock block in the generated code as plain text comments. The comments are generated at the same level in the model hierarchy as the subsystem that contains the DocBlock block.

Set the Document type parameter of the DocBlock block to Text. HDL Coder does not support the HTML or RTF options.

For more information about the DocBlockblock, see DocBlock.

Requirement Comments and Hyperlinks

By using Requirements Toolbox™, you can assign requirements to model elements and generate them as comments in the generated code. For example:

  1. Open the model hdlcoder_simple_up_counter using this command:


  2. Open the Requirements Editor (Requirements Toolbox) and create a new requirement set named Delay_requirements.slreqx.

    For more information, see Author Requirements in MATLAB or Simulink (Requirements Toolbox).

    Create requirements with these details:

    • Custom ID: 1

    • Summary: Requirement 1

    • Description: Sample text 1

    Similarly, create another requirement with these details:

    • Custom ID: 2

    • Summary: Requirement 2

    • Description: Sample text 2


    You can create links without leaving the Simulink Editor by using the Requirements Perspective. For more information, see View and Link Requirements in Simulink (Requirements Toolbox).

  3. Open the HDL_DUT subsystem.

  4. Right-click the Delay block named Delay. From the context menu, select Requirements > Open Outgoing Links dialog.

  5. Link the requirements. Click the New button. Set the Description to Requirements 1, set Document type to Requirements Toolbox, set Document to Delay_requirements.slreqx, and Location (Type/Identifier) to Requirement 1.

    Outgoing Links diaglog box with sample details filled out such as Description, Document type, Document, and Location.

    Repeat the steps 4 and 5 and link the Requirement 2 to the Subsystem.

  6. Click Apply and OK.

  7. To include requirements as hyperlinked comments in the generated code, in the Configuration Parameters dialog box:

    Alternatively, if you generate code from the command line, set the Traceability and RequirementComments properties using the makehdl function.


    The generated HDL code includes comments for the Delay block and HDL_DUT subsystem within the hdlcoder_simple_up_counter model, which contains links to the associated requirements for each block and subsystem from the original model.

    // Module: HDL_DUT
    // Source Path: hdlcoder_simple_up_counter/HDL_DUT
    // Hierarchy Level: 0
    // Model version: 9.2 
    //  Block requirements for <Root>/HDL_DUT 
    //  1. Delay_requirements.slreqx:Requirement 2
    // -------------------------------------------------------------
    module HDL_DUT
      input   clk;
      input   reset;
      input   clk_enable;
      input   [7:0] count_threshold;  // uint8
      input   Enable;
      output  ce_out;
      output  [7:0] out;  // uint8
      // Block requirements for <S1>/Delay1
       //  1. Delay_requirements.slreqx:Requirement 1
       always @(posedge clk or posedge reset)
        begin : Delay1_process
          if (reset == 1'b1) begin
            Delay1_out1 <= 8'b00000000;
          else begin
            if (enb) begin
              Delay1_out1 <= Switch1_out1;
      assign out = Delay1_out1;
      assign ce_out = clk_enable;
    endmodule  // HDL_DUT

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