Frequently Asked Questions

1. How to report bugs and request support?

If possible, do not mail the author directly with bug reports or support questions. Instead, post them on GitHub Issues (for bugs) or Discussions (for questions), so others can profit from your question and the posted answers as well.

A good bug report contains a minimal, complete, and verifiable code example that demonstrates the problem you are running into. Complete means not just a code snippet, but an entire top verilog module that can be synthesized. Also include the yosys script or command line. If your problem is with one of the flows (e.g. the open source IceStorm flow), also include scripts or command line for the other tools in the flow and auxiliary files needed to reporduce the problem (such as placement constraint files used for place and route). It should be possible to reproduce the problem you are seeing within seconds using the information you provided! Every file of line of code someone trying to reproduce your problem has to write themself will increase the effort someome has to put into reproducing your problem and at the same time will decrease the chances of your problem being reproduced correctly!

Always consider that someone reading your bug report only has the information you provided. Before clicking the send button on your bug report and/or support request, double check that you have provided all the information necessary to easily and quickly reproduce the problem you are seeing.

2. What synthesis targets are supported by Yosys?

Yosys is retargetable and adding support for additional targets is not very hard. At the moment, Yosys ships with mature flows targeting Lattice iCE40 and ECP5 FPGAs as well as Xilinx 7-Series FPGAs, experimental flows for many others, and support for ASIC synthesis from liberty cell library files.

Note that in all these cases Yosys only performs synthesis. Yosys's sister project nextpnr can be used for place and route for several FPGA families.

For a complete open source ASIC flow using Yosys see Qflow or the OpenROAD Project's OpenLane, for a complete open source iCE40 flow see Project IceStorm.

3. Does Yosys support timing driven synthesis?

In general no.

For ASIC synthesis there is timing driven logic optimization and technology mapping available via ABC: See options -constr and -D to the abc command (help abc). This options will also make ABC generate a post-synthesis timing report.

For iCE40 synthesis there is timing analysis available via the icetime program that is bundled with the IceStorm tools.

4. Does Yosys support multi-output cells (e.g. full adders)?

Multi-output logic cells are simply ignored by ABC at the moment. However, it is possible to write dedicated mapping rules for things like adders, that simply map arithmetic operations in the design to e.g. full-adder cells. See e.g. inference of SB_CARRY+SB_LUT blocks for arithmetic operations in the iCE40 flow.

5. The Verilog front-end does not create nice errors for invalid code!

Not really a question, but yes, this is correct. The focus so far for development of the Yosys Verilog front-end was on features and processing valid input, not good error reporting for invalid code. In some cases it will even accept invalid code! Use a simulatior (such as Icarus Verilog or Verilator) to check your code for syntax errors and similar problems. Verilator even has a --lint-only mode for just checking code without compiling it.

On a side note: I usually get this question from people who do not simulate their designs. Don't be this person! Regardless of what tools you are using, you should always simulate your designs first before going into synthesis!