Simulation#

Hog supports all simulation software currently supported by Vivado (see the full list here), as well as standalone GHDL simulations.

Hog simulation has been tested with Vivado Simulator (also called Xsim), Mentor Graphics QuestaSim, Mentor Graphics ModelSim, and Aldec Riviera Pro.

The simulation setup is always handled by Vivado, as if the simulation button in the GUI was clicked; the simulation run is then performed with the selected simulation software.

Note

Currently, simulation is not supported under other IDEs. Complete support will be included in future releases.

Hog Simulation List Files#

Simulations are organized into “simulation sets.” Hog creates a simulation set for each .sim list file in the top project folder (Top/myproj/list).

Starting from Hog2025.2, .sim list files can include both the list of simulation source files and simulation properties. The new list files follow a TOML-like structure with the following sections:

  • [files]: List of simulation source files.

  • [properties]: Properties for the simulation set.

  • [generics]: Generics to be passed to the top testbench file.

  • [hog]: Hog-specific simulation properties.

An example simulation list file in the new format:

# Simulator xsim
# Generated by Hog on 2025-06-30 13:50:09

[properties]
ACTIVE=1
RUNTIME=10us
TOP=TB_fifo
XSIM.SIMULATE.RUNTIME=10us

[files]
sources/sim/tb_fifo.vhd
sources/sim/TB_example_wave.do
sources/sim/TB_example.udo

[generics]
MYGENERIC=1

[hog]
HOG_SIMPASS_STR="Simulation Passed"

It is also possible to have multiple simulation libraries in the same simulation file set. To do so, assign the lib=library_name property to the included file or sim list file in the .sim list file in the top simulation folder.

For example, if you have a simulation list file called mysim in your top project folder with the following content:

[files]
src/top_sim.vhd
src/lib1/lib1_sim.vhd lib=lib1
src/lib2/lib2.sim lib=lib2

This will create a simulation file set mysim with three libraries: mysim, lib1, and lib2.

Hog also allows you to generate .sim files automatically by clicking the Hog SIM Button in the Vivado GUI.

To set a particular simset as the active one, set the ACTIVE property to 1 in your .sim file under the relevant simset section. If no active simset is specified, Hog will set the last created simulation set as active by default.

Generics Section (Vivado Only)#

Starting from Hog2023.1, you can pass generic values to the top simulation file in your Vivado project by defining them in the [generics] section of the .sim file. For example:

[generics]
MY_GENERIC_INT=0
MY_GENERIC_STRING="a string"
MY_GENERIC_HEX=4'h7

The value of the generics should resemble the generic’s type in your top testbench file. E.g. for the above generics the definition should look like,

entity my_tb is
  generic (
    MY_GENERIC_INT    : integer;
    MY_GENERIC_STRING : string;
    MY_GENERIC_HEX    : std_logic_vector(3 downto 0) -- To use HEX, the length of the vector should be multiple of 4
  );
end entity top_module;

Warning

This feature is not yet supported in Quartus.

Hog Section (Vivado Only)#

From Hog2024.2, you can use the optional [hog] section to specify Hog directives valid for the simulation project.

Supported properties include:

  • HOG_SILENT_SIM: Runs the simulation with the -quiet option.

  • HOG_SIMPASS_STR: Defines a simulation pass string. Hog will parse the simulation log and return an error exit if the provided string is not found. Otherwise, the simulation will rely on the simulator exit code.

When to use this directive: In some simulators, a failure in the simulation is not considered an error, meaning no error exit code is returned by the simulator. As a result, when running a simulation in GitLab CI, the simulation job could be detected as successful even if a failure is present. Known simulators with this issue include: QuestaSim/ModelSim, Aldec Riviera, and Xcelium.

Examples of working simulation pass strings for different simulation libraries:

Simulator library

PASS string

UVVM (Universal VHDL Verification Methodology)

“>> Simulation SUCCESS:”

An example of the [hog] section defining HOG_SIMPASS_STR when using QuestaSim and UVVM:

[hog]
HOG_SIMPASS_STR=">> Simulation SUCCESS:"

Project Simulator#

The simulation software is set at the project level in Vivado. To change it, modify the target_simulator property in the main section of your hog.conf file.

Note

Vivado does not allow running different simulations with different simulators automatically. If you want to do this locally, you must manually switch the simulator in the simulation settings each time. The setting is valid at the project level for all simulation sets. If you change the simulator by clicking in the Vivado GUI, the change will not be propagated to the repository.

Simulation Library#

When using external simulation software, you must specify the path to the compiled simulation library. By default, Hog sets this path to repo_path/SimulationLib, where repo_path is the root folder of your git repository. If the HOG_SIMULATION_LIB_PATH environment variable is defined (as in Continuous Integration; see GitLab CI/CD setup and GitHub Actions), Hog will use that path.

You can also specify the simulation library path when creating the project using the --lib flag:

./Hog/Do CREATE <myproj> -lib <library_path>

The sim.conf File#

It is still possible to configure simulation properties using a file called ./Top/<my_project>/sim.conf, with a syntax similar to hog.conf. In this case, the .sim file should only include the list of simulation sources without TOML headers.

Warning

sim.conf files are expected to be phased out in Hog2026.1. We advise you to migrate to the new .sim list file format as soon as possible.

If you use the Hog SIM button to recreate your simulation configuration files, Hog will remove the sim.conf file and write all configuration inside the .sim list files.

Simulation properties can be set individually for each simulation set or globally for all sets. For example, to set the xsim.elaborate.rangecheck property for the simulation set sim_1, write:

[sim_1]
xsim.elaborate.rangecheck = true

Alternatively, to set this property for all your simulation sets:

[sim]
xsim.elaborate.rangecheck = true

Simulation in CI#

If the simulation job is configured in your project’s CI, Hog CI will automatically run the simulation sets.

The CI stage will fail if any simulation fails. Therefore, each simulation set should be designed to fail if any unwanted behavior is present.

Even though Vivado does not allow different simulators in the same project, in Hog CI you can specify a different software to be used in each set. This can be done by adding one of the following lines at the top of the .sim list file:

#Simulator xsim       # For Vivado Simulator
#Simulator questa     # For QuestaSim Simulator
#Simulator modelsim   # For ModelSim Simulator
#Simulator riviera    # For Aldec Riviera Simulator
#Simulator activehdl  # For Aldec Active-HDL Simulator
#Simulator ies        # For IES Simulator
#Simulator vcs        # For Synopsys VCS Simulator
#Simulator ghdl       # For GHDL Simulator

#Simulator skip_simulation # To skip the simulation in the CI

If you do not specify the simulator software, Hog will use ModelSim as the default.

If you do not want a specific simulation set to be simulated by the CI, use skip_simulation.

An example .sim list file:

#Simulator xsim
[files]
tb_source_dir/tb_for_lib1.vhd
wave_source_dir/wave_lib1.tcl
do_source_dir/dofile_lib1.do
tb_source_dir/another_file.vhd
[properties]
TOP=TB_FOR_LIB1

Simulating Without GUI#

If you want to run the simulation locally without using the GUI (as in CI), you can use the Hog/Do script:

./Hog/Do SIMULATE <project_name> [-lib_path <sim_lib_path>] [-simset <sim_sets>] [-quiet] [-recreate] [-check_syntax] [-scripts_only] [-verbose] [-compile_only]

This script will launch all the simulation sets in the specified project, using the software specified in the .sim file, and will skip the simulation if skip_simulation is specified in a .sim list file. More details can be found here.

GHDL Simulations#

Starting from Hog2025.2, it is possible to run GHDL simulations and create GHDL-only projects. To use GHDL, ensure that both ghdl and tcllib are installed on your machine.

To run a simulation set with GHDL, add #Simulator ghdl as the first line of your .sim set file. For example:

#Simulator ghdl
[files]
tb_source_dir/tb_for_lib1.vhd
[properties]
TOP=TB_FOR_LIB1
options="-frelaxed"
run_options="--assert-level=note"

In the [properties] section, you can specify the options and run options that Hog should pass to GHDL using the options and run_options properties, as shown above.

To launch the simulation, simply execute:

./Hog/Do SIM <project> [-simset <sim_sets>]

Replace <project> with the name of the project you want to simulate. Use the -simset option to specify the simulation sets to run.