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Arachne-pnr is not maintained anymore; use nextpnr instead, which is a complete functional replacement with major improvements.


2015-08-06: Interface change: Default seed is 1, can be randomized with -r option.

2015-07-18: New version. Release notes:

  • IceStorm and arachne-pnr now support the iCE40LP/HX8K
  • huge speed improvements (~50x)
  • arachne-pnr now prints the random seed on each run. The seed can be set with the -s option. With the same seed, arachne-pnr should be deterministic across platforms and C++ compilers.

What is arachne-pnr?

Arachne-pnr implements the place and route step of the hardware compilation process for FPGAs. It accepts as input a technology-mapped netlist in BLIF format, as output by the Yosys [0] synthesis suite for example. It currently targets the Lattice Semiconductor iCE40 family of FPGAs [1]. Its output is a textual bitstream representation for assembly by the IceStorm [2] icepack command. The output of icepack is a binary bitstream which can be uploaded to a hardware device.

Together, Yosys, arachne-pnr and IceStorm provide an fully open-source Verilog-to-bistream tool chain for iCE40 1K and 8K FPGA development.


This is experimental software! It might have bugs that cause it to produce bitstreams which could damage your FPGA! So when you buy an evaluation board, get a few.

We have done extensive verification-based testing (see tests/), but so far limited hardware-based testing. This will change.


Arachne-pnr uses a simulated annealing-based algorithm for placement and a multi-pass congestion-based router. Arachne-pnr supports all features documented by IceStorm, although the Block RAM has not been extensively tested. This should include everything on the chip except PLLs and timing information. Support for PLLs is in the works. We hope to support timing information as soon as it is documented by the IceStorm project.


Arachne-pnr is written in C++11. It has been tested under OSX and Linux with LLVM/Clang and GCC. It should work on Windows, perhaps with a little work. (Patches welcome.) It depends on IceStorm. You should also install Yosys to synthesize designs.

To install, just go to the arachne-pnr directory and run

$ make && sudo make install


Lattice has several low-cost breakout boards: iCEstick for the 1K [3] and iCE40-HX8K Breakout Board [4] for the 8K.

There is a simple example for the iCEstick evaluation board in example/rot which generates a rotating pattern on the user LEDs. The Verilog input is in rot.v and the physical (pin) constraints are in rot.pcf. To build it, just run make, which executes the following commands:

yosys -q -p "synth_ice40 -blif rot.blif" rot.v
../../bin/arachne-pnr -p rot.pcf rot.blif -o rot.txt
icepack rot.txt rot.bin

You can use the Lattice tools or iceprog from IceStorm to upload rot.bin onto the board.


Feedback, feature requests, bug reports and patches welcome. Please email the author, Cotton Seed, or submit a issue on Github.


Arachne-pnr would not have been possible without Clifford Wolf and Mathias Lasser's IceStorm project to reverse-engineer the iCE40 FPGAs and build supporting tools. Also, it would not be useful without Clifford Wolf's Yosys project to synthesize and technology map designs. Thanks to contributors Larry Doolittle, jhol, laanwj, Clifford Wolf (cliffordwolf) and zeldin.