Verilog and Yosys¶

DigSim simulates Verilog designs by first turning them into a netlist, a list of gates and how they are connected. This is done with Yosys, an open-source Verilog synthesis tool.

DigSim ships with YoWASP, a portable WebAssembly build of Yosys, so nothing extra needs to be installed. If a native yosys is found on your PATH, it is used instead.

You don't always need to synthesize by hand: the GUI and YosysComponent both accept .v files and synthesize them on load. Creating a netlist yourself is useful when the design has several modules, or when the same design is loaded often (synthesis of large designs takes a while).

The synthesis helper¶

DigSim includes a small command-line helper:

python -m digsim.synth synth -i <file1.v> [<file2.v> ...] -o <netlist.json> -t <top_module>

Option	Description
`-i`, `--input-files`	One or more Verilog files
`-o`, `--output-file`	The JSON netlist to write
`-t`, `--top`	The name of the top module
`-s`, `--silent`	Hide the Yosys output

To list the modules found in a set of Verilog files:

python -m digsim.synth list -i <file1.v> [<file2.v> ...]

Synthesis from Python¶

from digsim.synth import Synthesis

synthesis = Synthesis(["counter.v"], "counter")
synthesis.synth_to_json_file("counter.json")

synth_to_dict() returns the netlist instead of writing it, see testbenches.

Running Yosys directly¶

Yosys is also available as the yowasp-yosys command. Start it interactively:

yowasp-yosys

The following script is what DigSim itself uses, and gives good results:

read -sv <verilog_file.v>
hierarchy -top <top_module>
proc; flatten
memory_dff
proc; opt; techmap; opt
synth -noabc -top <top_module>
write_json <netlist.json>

Save the commands in a file to run them as a script:

yowasp-yosys <script.ys>

Limitations¶

A Verilog file loaded directly into a Yosys component can contain only one module. Synthesize multi-module designs into a netlist first.
When reloading a component, its ports must keep the same names and widths.

See the Yosys documentation for everything else Yosys can do.