Saturday, December 19, 2015

FuseSoC and VUnit

I recently improved the VHDL support in FuseSoC and since I've been using vunit a bit lately I thought it could be a fun experiment to see if I could combine the strengths of these projects.

For those not aware, FuseSoC is a package manager and build system for FPGA/ASIC systems. It handles dependencies between IP cores and has functionality to build FPGA images or run testbenches a single core or a complete SoC. Each core has a .core file to describe which source files it contains, which dependencies it has on other cores and lots of other things. The aim of FuseSoC is to make it easier to reuse components and create SoCs that can target different FPGA vendors and technologies. FuseSoC is Open Souce, written in Python and can be found at

VUnit is an open source unit testing framework for VHDL released under the terms of Mozilla Public License, v. 2.0. It features the functionality needed to realize continuous and automated testing of your VHDL code. VUnit doesn't replace but rather complements traditional testing methodologies by supporting a "test early and often" approach through automation.

VUnit has a lot of great functionality for writing unit tests in VHDL, but requires the users to set up the source tree with all dependecies and their libraries themselves

FuseSoC on the other hand has knowledge of each cores files and dependencies, but very little convenience functions for writing unit tests. The ones that are existing are mainly targeting verilog users.

Given these preconditions, my idea was to let FuseSoC collect all source code and give it to VUnit to run unit tests on them.

Let's get started

VUnit requires the user to write a small Python script that sets up simulation settings, collects all source files, put them into libraries and starts an external simulator. This script is then launched from the command line with options to decide which unit tests to run, which simulator to use and where the output should go among other things. Here's the example script from VUnit's user guide
# file:
from vunit import VUnit

# Create VUnit instance by parsing command line arguments
vu = VUnit.from_argv()

# Create library 'lib'
lib = vu.add_library("lib")

# Add all files ending in .vhd in current working directory

# Run vunit function

FuseSoC is also launched from the command line and expects to be instructed which core or system to use, if it should do synthesis+P&R or run a simulator and other options. FuseSoC however was always meant to be used as a library as well as a command-line tool, so to make these work together, we create a VUnit run script that imports the necessary functions from FuseSoC. Thankfully both tools are written in Python, or I would have given up at this point.

The first inconvenient difference between FuseSoC and VUnit is that the vunit run script need all source files that should be compiled for any testbench to run. FuseSoC on the other hand don't know which source files to use until we tell it which core to use as it's top-level core. To work around this I decided to look at the VUnit's -o option, which is used to tell VUnit which output directory to use. I simply peek at the output directory and use that as the FuseSoC top-level core name. We now got the first lines of the new script.

from vunit import VUnit
vu = VUnit.from_argv()
top_core = os.path.basename(vu._output_path)

Now we need to do some basic FuseSoC initialization. First we create a core manager, which is a singleton instance that is handling the database of cores and their dependencies.

from fusesoc.coremanager import CoreManager, DependencyError
cm = CoreManager()

Next step is to register a core library in the core manager. Normally FuseSoC picks up locations of core libraries from the fusesoc.conf file, which can be in the current directory or ~/.config/fusesoc, or from the --cores-root=/path/to/library command-line options.

We don't have any command-line options for this, but we can get fusesoc.conf by using the Config() singleton class.

from fusesoc.config import Config
config = Config()

We can also add any known directories directly with


The core manager will scan the added directories recursively and pick up any FuseSoC .core files it finds. (Note: If a .core file is found in a directory, its subdirectories will not be searched for other .core files).

It's now time to sort out the dependency chain of the top-level core we requested earlier

    cores = cm.get_depends(top_core)
except DependencyError as e:
    print("'{}' or any of its dependencies requires '{}', but this core was not found".format(top_core, e.value))

If a dependency is missing, we tell the user and exit. If all was well, we now have a sorted list of cores in the 'cores' variable. Each element is a FuseSoC Core class that contains all necessary information about the core.

With all the cores found, we can now start iterating over them in order to get all the source files and other information we need and hand it over to VUnit. Some notes to the code below:

1. 'usage' is a list of tags to look for in the filesets. Only look at filesets where any of these tags are present. FuseSoC itself looks for the names 'sim' and 'synth' to indicate if the files should be used for simulation and synthesis. We can also choose to only use a fileset with a certain tool by for example setting the tag 'modelsim' or 'icarus' instead of 'sim'
2. File sets in FuseSoC can be public or private. Default is public and indicates that other cores might find the files in there useful. This is for example the files for synthesis and testbench helper functions such as BFMs or bus monitors. Private filesets are used for things like core-specifc testbenches and target-specific top-level files.
3. Even though the most common way to use libraries is to have one library for each core, there's nothing stopping us from splitting a library into several FuseSoC cores, or let one core contain multiple libraries.

usage = ['sim']
libs = OrderedDict()
for core_name in cores:
    core = cm.get_core(core_name)
    basepath = core.files_root
    for fs in core.file_sets:
        if (set(fs.usage) & set(usage)) and ((core_name == top_core) or not fs.private):
            for file in fs.file:
                if file.is_include_file:
                    #TODO: incdirs not used right now
                    incdirs.add(os.path.join(basepath, os.path.dirname(
                    except KeyError:
                    vu.add_source_file(os.path.join(basepath,, file.logical_name)

With that we are done, and can safely leave it to VUnit to do the rest


It all works fine, and to make it more interactive, I set up a demo project at This contains a packet generator for a made-up frame format together with a testbench. The packet generator has dependencies on two utility libraries (libstorage and libaxis) that I wrote a while ago to handle some common tasks that I got fed up with rewriting everytime I started a new project. The packet generator test bench uses some VUnit functions to make the example a bit more educational.

I hope this will be useful for all people using, or thinking of using, VUnit. For all you others, you can still use FuseSoC without VUnit for simulating and building systems and cores. The FuseSoC standard core library that can be downloaded as part of the FuseSoC installation contains about 60 cores, and there are several more core libraries on the internet that can be combined with this.

Happy hacking!