Project IceStorm

What is Project IceStorm?

Project IceStorm aims at documenting the bitstream format of Lattice iCE40 FPGAs and providing simple tools for analyzing and creating bitstream files. The IceStorm flow (Yosys, Arachne-pnr, and IceStorm) is a fully open source Verilog-to-Bitstream flow for iCE40 FPGAs.

The focus of the project is on the iCE40 LP/HX 1K/4K/8K chips. (Most of the work was done on HX1K-TQ144 and HX8K-CT256 parts.) The iCE40 UltraPlus parts are also supported, including DSPs, oscillators, RGB and SPRAM. iCE40 LM, Ultra and UltraLite parts are not yet supported.

2018-01-30: Released support for iCE40 UltraPlus devices.
2017-03-13: Released support for LP384 chips (in all package variants).
2016-02-07: Support for all package variants of LP1K, LP4K, LP8K and HX1K, HX4K, and HX8K.
2016-01-17: First release of IceTime timing analysis. Video:
2015-12-27: Presentation of the IceStorm flow at 32C3 (Video on Youtube).
2015-07-19: Released support for 8k chips. Moved IceStorm source code to GitHub.
2015-05-27: We have a working fully Open Source flow with Yosys and Arachne-pnr! Video:
2015-04-13: Complete rewrite of IceUnpack, added IcePack, some major documentation updates
2015-03-22: First public release and short YouTube video demonstrating our work:

Project IceStorm is an open-source project initiated by Clifford Wolf that aims to provide a comprehensive set of tools for working with the Lattice iCE40 series of FPGAs (Field-Programmable Gate Arrays). The iCE40 series of FPGAs are known for their low cost and low power consumption, making them particularly appealing for hobbyists, makers, and researchers.

Project IceStorm specifically targets enabling a fully open-source FPGA toolchain, which is a significant departure from the proprietary toolchains offered by most FPGA manufacturers. The project's toolchain includes several key components:

  1. IceStorm – The core of the project, IceStorm is the part that handles the bitstream documentation and provides the tools for bitstream analysis and creation for iCE40 FPGAs.
  2. Arachne-PNR – A place-and-route tool initially used in conjunction with IceStorm (later replaced by nextpnr).
  3. Nextpnr – An improved and versatile place-and-route tool that has largely supplanted Arachne-PNR in the open-source FPGA community. It is designed to be a vendor-neutral, timing-driven place and route tool.
  4. Yosys – An open-source synthesis tool for converting Verilog (a hardware description language) code into a gate-level representation that can be used with the place-and-route tools. Yosys is highly flexible and supports various synthesis tasks.

By providing an open-source alternative to the traditionally closed-source environment of FPGA design, Project IceStorm has enabled hobbyists and small-scale developers to more easily explore and innovate with FPGA technology. The ability to inspect and modify every aspect of the toolchain is powerful for education, research, and the development of highly customized hardware solutions.

Why the Lattice iCE40?

It has a very minimalistic architecture with a very regular structure. There are not many different kinds of tiles or special function units. This makes it both ideal for creating bitstream documentations and as a reference platform for general purpose FPGA tool development.

Also, with the Lattice iCEstick there is a cheap and easy to use development platform available, which makes the part interesting for all kinds of projects. (The iCEstick features an HX1K device. Lattice also sells an iCE40-HX8K Breakout Board featuring an HX8K chip.)

What is the Status of the Project?

We are pretty confident that we have the 1K and 8K devices completely documented. For example, it seems we can create correct functional Verilog models for all bitstreams generated by Lattice iCEcube2 for the iCE40 HX1K-TQ144 and the iCE40 HX8K-CT256 using our icebox_vlog tool.

Here is a list of currently supported parts and the corresponding options for arachne-pnr (place and route) and icetime (timing analysis):

PartPackagePin SpacingI/Osnextpnr optsarachne-pnr optsicetime opts
iCE40-LP1K-SWG16TR16-ball WLCSP (1.40 x 1.48 mm)0.35 mm10–lp1k –package swg16tr-d 1k -P swg16tr-d lp1k
iCE40-UP3K-UWG3030-ball WLCSP (2.15 x 2.55 mm)0.40 mm21–up5k –package uwg30-d 5k -P uwg30-d up5k
iCE40-UP5K-UWG3030-ball WLCSP (2.15 x 2.55 mm)0.40 mm21–up5k –package uwg30-d 5k -P uwg30-d up5k
iCE40-LP384-CM3636-ball ucBGA (2.5 x 2.5 mm)0.40 mm25–lp384 –package cm36-d 384 -P cm36-d lp384
iCE40-LP1K-CM3636-ball ucBGA (2.5 x 2.5 mm)0.40 mm25–lp1k –package cm36-d 1k -P cm36-d lp1k
iCE40-LP384-CM4949-ball ucBGA (3 x 3 mm)0.40 mm37–lp384 –package cm49-d 384 -P cm49-d lp384
iCE40-LP1K-CM4949-ball ucBGA (3 x 3 mm)0.40 mm35–lp1k –package cm49-d 1k -P cm49-d lp1k
iCE40-LP1K-CM8181-ball ucBGA (4 x 4 mm)0.40 mm63–lp1k –package cm81-d 1k -P cm81-d lp1k
iCE40-LP4K-CM8181-ball ucBGA (4 x 4 mm)0.40 mm63–lp8k –package cm81:4k-d 8k -P cm81:4k-d lp8k
iCE40-LP8K-CM8181-ball ucBGA (4 x 4 mm)0.40 mm63–lp9k –package cm81-d 8k -P cm81-d lp8k
iCE40-LP1K-CM121121-ball ucBGA (5 x 5 mm)0.40 mm95–lp1k –package cm121-d 1k -P cm121-d lp1k
iCE40-LP4K-CM121121-ball ucBGA (5 x 5 mm)0.40 mm93–lp8k –package cm121:4k-d 8k -P cm121:4k-d lp8k
iCE40-LP8K-CM121121-ball ucBGA (5 x 5 mm)0.40 mm93–lp8k –package cm121-d 8k -P cm121-d lp8k
iCE40-LP4K-CM225225-ball ucBGA (7 x 7 mm)0.40 mm167–lp8k –package cm225:4k-d 8k -P cm225:4k-d lp8k
iCE40-LP8K-CM225225-ball ucBGA (7 x 7 mm)0.40 mm178–lp8k –package cm225-d 8k -P cm225-d lp8k
iCE40-HX8K-CM225225-ball ucBGA (7 x 7 mm)0.40 mm178–hx8k –package cm225-d 8k -P cm225-d hx8k
iCE40-LP384-QN3232-pin QFN (5 x 5 mm)0.50 mm21–lp384 –package qn32-d 384 -P qn32-d lp384
iCE40-UP5K-SG4848-pin QFN (7 x 7 mm)0.50 mm39–up5k –package sg48-d 5k -P sg48-d up5k
iCE40-LP1K-QN8484-pin QFNS (7 x 7 mm)0.50 mm67–lp1k –package qn84-d 1k -P qn84-d lp1k
iCE40-LP1K-CB8181-ball csBGA (5 x 5 mm)0.50 mm62–lp1k –package cb81-d 1k -P cb81-d lp1k
iCE40-LP1K-CB121121-ball csBGA (6 x 6 mm)0.50 mm92–lp1k –package cb121-d 1k -P cb121-d lp1k
iCE40-HX1K-CB132132-ball csBGA (8 x 8 mm)0.50 mm95–hx1k –package cb132-d 1k -P cb132-d hx1k
iCE40-HX4K-CB132132-ball csBGA (8 x 8 mm)0.50 mm95–hx8k –package cb132:4k-d 8k -P cb132:4k-d hx8k
iCE40-HX8K-CB132132-ball csBGA (8 x 8 mm)0.50 mm95–hx8k –package cb132-d 8k -P cb132-d hx8k
iCE40-HX1K-VQ100100-pin VQFP (14 x 14 mm)0.50 mm72–hx1k –package vq100-d 1k -P vq100-d hx1k
iCE40-HX1K-TQ144144-pin TQFP (20 x 20 mm)0.50 mm96–hx1k –package tq144-d 1k -P tq144-d hx1k
iCE40-HX4K-TQ144144-pin TQFP (20 x 20 mm)0.50 mm107–hx8k –package tq144:4k-d 8k -P tq144:4k-d hx8k
iCE40-HX4K-BG121121-ball caBGA (9 x 9 mm)0.80 mm93–hx8k –package bg121:4k-d 8k -P bg121:4k-d hx8k
iCE40-HX8K-BG121121-ball caBGA (9 x 9 mm)0.80 mm93–hx8k –package bg121-d 8k -P bg121-d hx8k
iCE40-HX8K-CT256256-ball caBGA (14 x 14 mm)0.80 mm206–hx8k –package ct256-d 8k -P ct256-d hx8k

Current work focuses on further improving our timing analysis flow.

How do I use the Fully Open Source iCE40 Flow?

Synthesis for iCE40 FPGAs can be done with Yosys. Place-and-route can be done with arachne-pnr. Here is an example script for implementing and programming the rot example from arachne-pnr (this example targets the iCEstick development board):

yosys -p "synth_ice40 -blif rot.blif" rot.v
arachne-pnr -d 1k -p rot.pcf rot.blif -o rot.asc
icepack rot.asc rot.bin
iceprog rot.bin

A simple timing analysis report can be generated using the icetime utility:

icetime -tmd hx1k rot.asc

Where are the Tools? How to install?

Installing prerequisites (this command is for Ubuntu 14.04):

sudo apt-get install build-essential clang bison flex libreadline-dev \
                     gawk tcl-dev libffi-dev git mercurial graphviz   \
                     xdot pkg-config python python3 libftdi-dev \
                     qt5-default python3-dev libboost-all-dev cmake libeigen3-dev

On Fedora 24 the following command installs all prerequisites:

sudo dnf install make automake gcc gcc-c++ kernel-devel clang bison \
                 flex readline-devel gawk tcl-devel libffi-devel git mercurial \
                 graphviz python-xdot pkgconfig python python3 libftdi-devel \
                 qt5-devel python3-devel boost-devel boost-python3-devel eigen3-devel

Note: All tools will be installed relative to /usr/local

Installing the IceStorm Tools (icepack, icebox, iceprog, icetime, chip databases):

git clone icestorm
cd icestorm
make -j$(nproc)
sudo make install

Installing Arachne-PNR (place&route tool, predecessor to NextPNR):

git clone arachne-pnr
cd arachne-pnr
make -j$(nproc)
sudo make install

Installing NextPNR (place&route tool, Arachne-PNR replacement):

git clone nextpnr
cd nextpnr
cmake -DARCH=ice40 -DCMAKE_INSTALL_PREFIX=/usr/local .
make -j$(nproc)
sudo make install

Installing Yosys (Verilog synthesis):

git clone yosys
cd yosys
make -j$(nproc)
sudo make install

Both place and route tools (Arachne-PNR & NextPNR) convert the IceStorm text chip databases into the respective PNR binary chip databases during build. Always rebuild the PNR tools after updating your IceStorm installation.

Notes for Linux: Create a file /etc/udev/rules.d/53-lattice-ftdi.rules with the following line in it to allow uploading bit-streams to a Lattice iCEstick and/or a Lattice iCE40-HX8K Breakout Board as unprivileged user:

ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6010", MODE="0660", GROUP="plugdev", TAG+="uaccess"

Notes for Archlinux: just install icestorm-gitarachne-pnr-git and yosys-git from the Arch User Repository (no need to follow the install instructions above).

Notes for OSX: Please follow the additional instructions for OSX to install on OSX.

Please file an issue on github if you have additional notes to share regarding the install procedures on the operating system of your choice.

What are the IceStorm Tools?

The IceStorm Tools are a couple of small programs for working with iCE40 bitstream files and our ASCII representation of it. The complete Open Source iCE40 Flow consists of the IceStorm ToolsArachne-PNR, and Yosys.


The iceunpack program converts an iCE40 .bin file into the IceStorm ASCII format that has blocks of 0 and 1 for the config bits for each tile in the chip. The icepack program converts such an ASCII file back to an iCE40 .bin file. All other IceStorm Tools operate on the ASCII file format, not the bitstream binaries.


The icetime program is an iCE40 timing analysis tool. It reads designs in IceStorm ASCII format and writes times timing netlists that can be used in external timing analysers. It also includes a simple topological timing analyser that can be used to create timing reports.


A python library and various tools for working with IceStorm ASCII files and accessing the device database. For example icebox_vlog converts our ASCII file dump of a bitstream into a Verilog file that implements an equivalent circuit.


A small driver program for the FTDI-based programmer used on the iCEstick and HX8K development boards.


A tool for packing multiple bitstream files into one iCE40 multiboot image file.


A small program for calculating iCE40 PLL configuration parameters.


A small program for swapping the BRAM contents in IceStorm ASCII files. E.g. for changing the firmware image in a SoC design without re-running synthesis and place&route.


The IceStorm Makefile builds and installs two files: chipdb-1k.txt and chipdb-8k.txt. This files contain all the relevant information for arachne-pnr to place&route a design and create an IceStorm ASCII file for the placed and routed design.

IcePack/IceUnpack, IceBox, IceProg, IceTime, and IcePLL are written by Claire Wolf. IcePack/IceUnpack is based on a reference implementation provided by Mathias Lasser. IceMulti is written by Marcus Comstedt.

Where do I get support or meet other IceStorm users?

If you have a question regarding the IceStorm flow, use the yosys tag on stackoverflow to ask your question. If your question is a general question about Verilog HDL design, please consider using the verilog tag on stackoverflow instead.

For general discussions go to the Yosys Subreddit or #yosys on freenode IRC.

If you have a bug report please file an issue on github. (IceStorm Issue TrackerYosys Issue TrackerArachne-PNR Issue Tracker)

Where is the Documentation?

Recommended reading: Lattice iCE40 LP/HX Family Datasheet, Lattice iCE Technology Library (Especially the three pages on „Architecture Overview“, „PLB Blocks“, „Routing“, and „Clock/Control Distribution Network“ in the Lattice iCE40 LP/HX Family Datasheet. Read that first, then come back here.)

The FPGA fabric is divided into tiles. There are IO, RAM and LOGIC tiles.

  • LOGIC Tile Documentation
  • IO Tile Documentation
  • RAM Tile Documentation
  • The Bitstream File Format
  • The iCE40 HX1K Bit Docs
  • The iCE40 HX8K Bit Docs
  • Notes on UltraPlus features

The iceunpack program can be used to convert the bitstream into an ASCII file that has a block of 0 and 1 characters for each tile. For example:

.logic_tile 12 12

This bits are referred to as By[x] in the documentation. For example, B0 is the first line, B0[0] the first bit in the first line, and B15[53] the last bit in the last line.

The icebox_explain program can be used to turn this block of config bits into a description of the cell configuration:

.logic_tile 12 12
LC_7 0101010110101010 0000
buffer local_g0_2 lutff_7/in_3
buffer local_g1_4 lutff_7/in_0
buffer sp12_h_r_18 local_g0_2
buffer sp12_h_r_20 local_g1_4

IceBox contains a database of the wires and configuration bits that can be found in iCE40 tiles. This database can be accessed via the IceBox Python API. But IceBox is a large hack. So it is recommended to only use the IceBox API to export this database into a format that fits the target application. See icebox_chipdb for an example program that does that.

The recommended approach for learning how to use this documentation is to synthesize very simple circuits using Yosys and Arachne-pnr, run the icestorm tool icebox_explain on the resulting bitstream files, and analyze the results using the HTML export of the database mentioned above. icebox_vlog can be used to convert the bitstream to Verilog. The output file of this tool will also outline the signal paths in comments added to the generated Verilog code.

For example, consider the following Verilog and PCF files:

// example.v
module top (input a, b, output y);
  assign y = a & b;

# example.pcf
set_io a 1
set_io b 10
set_io y 11

And run them through Yosys, Arachne-PNR and IcePack:

$ yosys -p 'synth_ice40 -top top -blif example.blif' example.v
$ arachne-pnr -d 1k -o example.asc -p example.pcf example.blif
$ icepack example.asc example.bin

We would get something like the following icebox_explain output:

$ icebox_explain example.asc
Reading file 'example.asc'..
Fabric size (without IO tiles): 12 x 16

.io_tile 0 10
IoCtrl IE_0
IoCtrl IE_1
IoCtrl REN_0
buffer local_g0_5 io_1/D_OUT_0
buffer logic_op_tnr_5 local_g0_5

.io_tile 0 14
IoCtrl IE_1
IoCtrl REN_0
buffer io_1/D_IN_0 span4_vert_b_6

.io_tile 0 11
IoCtrl IE_0
IoCtrl REN_1
routing span4_vert_t_14 span4_horz_13

.logic_tile 1 11
LC_5 0001000000000000 0000
buffer local_g0_0 lutff_5/in_1
buffer local_g3_0 lutff_5/in_0
buffer neigh_op_lft_0 local_g0_0
buffer sp4_h_r_24 local_g3_0

And something like the following icebox_vlog output:

$ icebox_vlog -p example.pcf example.asc
// Reading file 'example.asc'..

module chip (output y, input b, input a);

wire y;
// io_0_10_1
// (0, 10, 'io_1/D_OUT_0')
// (0, 10, 'io_1/PAD')
// (0, 10, 'local_g0_5')
// (0, 10, 'logic_op_tnr_5')
// (0, 11, 'logic_op_rgt_5')
// (0, 12, 'logic_op_bnr_5')
// (1, 10, 'neigh_op_top_5')
// (1, 11, 'lutff_5/out')
// (1, 12, 'neigh_op_bot_5')
// (2, 10, 'neigh_op_tnl_5')
// (2, 11, 'neigh_op_lft_5')
// (2, 12, 'neigh_op_bnl_5')

wire b;
// io_0_11_0
// (0, 11, 'io_0/D_IN_0')
// (0, 11, 'io_0/PAD')
// (1, 10, 'neigh_op_tnl_0')
// (1, 10, 'neigh_op_tnl_4')
// (1, 11, 'local_g0_0')
// (1, 11, 'lutff_5/in_1')
// (1, 11, 'neigh_op_lft_0')
// (1, 11, 'neigh_op_lft_4')
// (1, 12, 'neigh_op_bnl_0')
// (1, 12, 'neigh_op_bnl_4')

wire a;
// io_0_14_1
// (0, 11, 'span4_horz_13')
// (0, 11, 'span4_vert_t_14')
// (0, 12, 'span4_vert_b_14')
// (0, 13, 'span4_vert_b_10')
// (0, 14, 'io_1/D_IN_0')
// (0, 14, 'io_1/PAD')
// (0, 14, 'span4_vert_b_6')
// (0, 15, 'span4_vert_b_2')
// (1, 11, 'local_g3_0')
// (1, 11, 'lutff_5/in_0')
// (1, 11, 'sp4_h_r_24')
// (1, 13, 'neigh_op_tnl_2')
// (1, 13, 'neigh_op_tnl_6')
// (1, 14, 'neigh_op_lft_2')
// (1, 14, 'neigh_op_lft_6')
// (1, 15, 'neigh_op_bnl_2')
// (1, 15, 'neigh_op_bnl_6')
// (2, 11, 'sp4_h_r_37')
// (3, 11, 'sp4_h_l_37')

assign y = /* LUT    1 11  5 */ b ? a : 0;



Links to related projects. Contact me if you have an interesting and relevant link.

iCE40 Boards

Lectures and Tutorials

Other FPGA bitstream documentation projects

In papers and reports, please refer to Project IceStorm as follows: Claire Wolf, Mathias Lasser. Project IceStorm., e.g. using the following BibTeX code:

	author = {Claire Wolf and Mathias Lasser},
	title = {Project IceStorm},
	howpublished = "\url{}"

Documentation mostly by Claire Wolf in 2015. Based on research by Mathias Lasser and Claire Wolf.
Buy an iCEstick or iCE40-HX8K Breakout Board from Lattice and see what you can do with the tools and information provided here.

Project IceStorm is an open-source effort that provides a fully open-source toolkit for working with Lattice iCE40 FPGAs (Field Programmable Gate Arrays). The iCE40 family of FPGAs from Lattice Semiconductor is popular due to its low cost and low power usage, making it ideal for hobbyists, researchers, and developers working on smaller-scale or budget-sensitive FPGA projects.

The name „IceStorm“ itself is indicative of the project's goal to provide transparency and ‚break the ice‘ on proprietary FPGA toolchains, which traditionally have been closed source. By creating a fully open-source toolkit, Project IceStorm allows for a deeper understanding and greater control over the entire FPGA development process.

Key Components of Project IceStorm:

  1. icepack: A tool that converts the textual intermediate representation (asc) to the iCE40 binary configuration file format (bin).
  2. iceunpack: A tool that performs the reverse of icepack; it converts the binary file back to a human-readable text format.
  3. iceprog: A simple programmer application to upload the bitstream to the iCE40 FPGA.
  4. icetime: A timing analysis tool that estimates the delay times of the bitstream.
  5. Chip Database: Project IceStorm also includes a database that documents the bitstream format for various parts of the iCE40 series of FPGAs, which is essential for the other tools to function correctly.

Features and Benefits of Project IceStorm:

  • Transparency: Open-source tools give insight into the bitstream composition and FPGA architecture, which is beneficial for educational purposes and research into FPGA design.
  • Community-Driven: Contributions from the open-source community enhance and maintain the toolkit, staying recent with developers‘ needs.
  • Flexibility: Developers have the freedom to modify and adapt the toolkit to fit specific project requirements without the restrictions often imposed by proprietary solutions.
  • Cost-Efficiency: Project IceStorm provides a free alternative to commercial tools, lowering the entry barrier for experimenting with and deploying FPGA designs.

Applications using Project IceStorm:

Project IceStorm enables various applications to benefit from its open-source FPGA toolkit:

  1. Educational Projects and Tutorials: It provides an accessible platform for teaching digital logic design and FPGA architecture without requiring expensive software licenses.
  2. Hobbyist Prototypes: Enthusiasts can develop and test their custom digital circuits on a physical FPGA at a lower cost.
  3. Research and Development: Academics and researchers can perform experiments with FPGA designs without the constraints of commercial license agreements, facilitating innovative breakthroughs.
  4. Startup Prototyping: Startups working on new hardware solutions can use IceStorm in their initial R&D phase to keep costs minimal while iterating on their designs.
  5. Automation and Robotics: Open hardware initiatives can use IceStorm for the development of custom logic circuits for automated systems and robots.

In summary, Project IceStorm represents a significant advancement in the democratization of FPGA programming, providing the community with tools that were once limited to proprietary software. This not only promotes learning and experimentations with iCE40 FPGAs but also supports the larger movement toward open-source hardware innovation.