Qflow 1.3: An Open-Source Digital Synthesis Flow

Table of Contents

A Digital Synthesis Flow using Open Source EDA Tools
Required Components of the Tool Chain

A Digital Synthesis Flow using Open Source EDA Tools

A digital synthesis flow is a set of tools and methods used to turn a circuit design written in a high-level behavioral language like verilog or VHDL into a physical circuit, which can either be configuration code for an FPGA target like a Xilinx or Altera chip, or a layout in a specific fabrication process technology, that would become part of a fabricated circuit chip. Several digital synthesis flows targeting FPGAs are available, usually from the FPGA manufacturers, and while they are typically not open source, they are generally distributed for free (presumably on the sensible assumption that more people will be buying more FPGA hardware).

The OpenMSP430 microprocessor from OpenCores, placed and routed by qflow.

Qflow is a complete tool chain for synthesizing digital circuits starting from verilog source and ending in physical layout for a specific target fabrication process. In the world of commercial electronics, digital synthesis with a target application of a chip design is usually bundled into large EDA software systems like Cadence or Synopsys. As commercial electronics designers need to maintain cutting-edge performance, these commercial toolchains get more and more expensive, and have largely priced themselves out of all but the established integrated circuit manufacturers. This leaves an unfortunate gap where startup companies and small businesses cannot afford to do any sort of integrated circuit design. The algorithms for digital synthesis are all tied up in closed-source software, and development is controlled by a few EDA software vendors.

From a long time ago, bits and pieces of open source software for digital synthesis have been developed. The oldest of these are probably VIS and SIS, two software tools developed at Berkeley for verilog parsing, logic verification, and mapping of logic onto a digital standard cell library. They perform the task of logic optimization and cell mapping admirably. The main drawback of these tools is that they lack sophistication in dealing with complicated structures in the verilog source code. The principal need for the synthesis tool is that it must be able to map a logic circuit to a standard cell library. For this reason, a number of open-source parsers like icarus and cver, which target either FPGAs or executable computer code, or both, have not been considered for qflow (however, they are both useful for functional verification of both the source code and the gate-level netlist; see the tutorial page for details on running icarus verilog for simulation).

After a thorough investigation of alternatives, I settled on two good choices for front-end synthesis: The first is a combination of the open-source verilog parser Odin-II and optimizer/mapper abc to anchor the digital synthesis flow. They have the advantage that they are part of an existing open-source FPGA synthesis flow called vtr ("verilog-to-routing"). The second is a recent entry (2013) into the field: The extraordinarily capable yosys. This is a synthesis tool able to handle every type of Verilog-2005 syntax, and can synthesize large projects off of Open Cores. Due to the versatility and capability of yosys, I have chosen it as the default frontend. Odin-II and ABC can be specified as an alternative frontend.

Digital standard cell libraries are a major component of the flow. While synthesis tools can make use of proprietary digital standard cells provided by various vendors (usually the fabrication facility, such as X-Fab, or IBM, or TSMC, but sometimes by 3rd-party vendors), the problem with proprietary standard cell libraries is that they cannot be distributed, and cannot therefore be used for examples and posted on public websites. Fortunately, there have been a few sets of open-source standard cells for popular processes. Some of these are based on the "scalable CMOS rules" from MOSIS, a set of design rules that are more conservative than the vendor rules and are allowed by the vendors to be distributed openly. Physical layout generated using the scalable CMOS rules may be distributed as open-source, and can be fabricated in the processes for which the rules were designed. For the first distribution of qflow, I am using the Oklahome State University (OSU) open-source standard cell library. In the near future, I hope to expand the distribution to include support for the open-source libraries from VLSI Technology (vlsitechnology.org), as well as the rest of the OSU standard cell libraries.

After mapping onto a standard cell library, a design needs to be placed and routed. The placement stage determines a rough estimate of the routing required, and tries to put all the cells into a block, ordering them to minimize the total amount of wiring connecting all the pins together. A placement tool known as graywolf was developed (under the name "TimberWolf") at Yale University, and was distributed as open source for a time until it was taken commercial. The last open-source version of this tool does not perform detail routing, but is a professional-grade placement tool.

The final step in creating a digital standard-cell layout is the detail route, describing exactly how the physical wiring should be generated to connect together all the pins in the design. An open source detail router has been pointedly missing. Eventually, knowing that an open source digital synthesis tool flow for chip design would never be created without one, and deciding that lack of cutting-edge performance should not be an impediment to the creation of a working flow, I coded up a moderately capable detail router, called qrouter, which has now become the final link in the open source synthesis chain.

It should not be assumed that the qflow tool chain can be used to create the next generation of multi-gigahertz microprocessors. But the qflow tool chain is perfectly capable of handling digital subsystems needed by many chips, including host-to-device communications (SPI and I2C, for example), signal processing (digital filters, sigma-delta modulators), arithmetic logic units, and so forth. Early versions of the qflow digital flow were used to create digital circuits used in high-performance commercial integrated circuits. It's real, and it works!

Qflow is an evolving set of tools that makes use of the best components that are available, are open-source, and that can be made to work within the flow. When better alternatives to any component exist, I will incorporate them into the flow.

Click on Reference here or on the button in the menu on the left to see how to use qflow.

A short video clip of a demonstration program being run on my Raven microprocessor, a RISC-V processor designed using qflow version 1.3 on the efabless.com "Open Galaxy" (CentOS) platform where it was set up for the X-Fab XH018 fabrication process (video from initial testing on February 5, 2019).

This small 32-bit processor was designed for embedded mixed-signal applications and has a 100MHz internal clock. The RISC-V processor core has over 20,000 gates, synthesized by yosys, placed and routed by graywolf and qrouter, and validated by vesta static timing analysis. It was a first-time silicon success triumph for open-source EDA tools.

The Raven chip is the one on the right with the white dot in the upper left corner. It is packaged in a 48-pin QFN (leadless) package. The chip below it to the left is an SPI flash external memory. The larger chip to the left is an FTDI communications chip to interface the board to a host computer.

The Raven RISC-V microprocessor from efabless. The digital core was placed and routed by qflow and verified with vesta; the top-level assembly was done with Magic and verified with Netgen.

Required Components of the Tool Chain

Qflow itself is a framework for managing a digital synthesis flow. It contains a number of tools unique to itself, but also relies on a number of tools that must be obtained elsewhere.

Click on Install here or on the button in the menu on the left to get installation instructions for the base qflow tool chain manager.

Required components of qflow: Note that qflow can make use of both front-end synthesis tools, yosys and Odin_II, but only one of them is manadatory, and yosys is the preferred front-end. abc is used by both yosys and Odin_II, but different versions need to be compiled for each tool; see details below.

Qflow

The qflow package contains all the scripts and most of the tools necessary for the open-source digital synthesis flow. It also comes with some of the files from the OSU (Oklahoma State University) 0.35um standard cell library, to provide a default technology.

Verilog source

You can write your own, or you can download example Verilog source from the OSU site (see below) or opencores.org

Warning: Most of the target applications on Open Cores make extensive use of all the features of Verilog-2005; they are all easily handled by yosys. The alternative Odin-II frontend will usually fail to synthesize the Open Cores sources without some editing of the source code.

Digital standard cell technology

For purposes of experimenting with the flow, qflow provides a set of files for the OSU 0.35um open-source standard cell set. This is an excellent source for LEF and GDS standard cells. The cells are all compatible with the MOSIS SCMOS rules for the various processes available through MOSIS (mostly TSMC and AMI, 0.18um to 0.5um). The standard cell set is available from vlsiarch.ecen.okstate.edu and is available for download, free of charge (see also the Flows main page). Look for the "osu_soc" tarball; as of this writing, the current tarball is osu_soc_v2.7.tar.gz. Be aware that this tarball does not extract into its own directory, so it is recommended to create a directory called "osu_soc_v2.7", place the tarball into it, and extract it there.

See the Reference page for instructions on how to use qflow with a different technology.

The important components to have (all of which are in the OSU standard cell sets) are:

1. A LEF-format file with all of the macros defined
2. A LEF-format file describing routing resources
3. A GDS file with all of the standard cell layouts
4. Timing parameters, preferably in Liberty format
5. A compatible technology file for Magic

Verilog parser, high-level synthesis, and logic optimization and verification (yosys)

This extraordinarily capable verilog parser and logic synthesizer was written by Clifford Wolf. It is an industrial-grade tool that will synthesize everything from decoders up to microprocessors with ease and accuracy. The code can be obtained from the Yosys website. Click on the "Download" link on the left, and follow instructions to compile and install. More detailed instructions are available in the README file in the distribution.

Some previous comments on this page about difficulties in compiling and installing yosys have been dealt with over a few version iterations. As of this writing (April 2015), yosys is in version 0.5, and I have no problems at all compiling it and all its dependencies. Note that the default configuration uses the clang compiler; on a Linux system with a stock gcc compiler, you will want to first do "make config-gcc" before doing "make". Otherwise, yosys should compile without complaint.

Note that yosys makes use of the logic optimizer "abc" (see below), but will download and compile the appropriate version as part of its make script.

Verilog parser and logic verification (Odin-II)

This Verilog parser, written by Peter Jamieson, was introduced at FCCM 2010 and continues to undergo development as part of the FPGA synthesis tool chain VTR. Obtain Odin-II from Google Code. You may want to follow the link to the VTR distribution (also see below), which may contain the most recent code base for Odin-II.

As of August 2, 2013, the Odin-II developers fixed a problem that made the Odin-II output incompatible with qflow. For a while, I posted a patch, but now I recommend that you make sure that your version of Odin-II post-dates the change. An additional patch was posted in October 2013.

As of May, 2015, Odin-II is not supported by the qflow development version 1.1.

Logic optimization (abc)

This tool takes the BLIF format description and creates a netlist representation using a set of standard cells described in the "genlib" format, performing logic optimization along the way. Obtain this tool from EECS at U.C. Berkeley. Here you will find a pointer to the latest code download on BitBucket.

The complete FPGA synthesis flow vtr contains both Odin-II and abc, and can be obtained from Google Code. In general, this is the best place to get the most recent sources for both Odin-II and abc.

Note that yosys also uses abc for combinatorial logic optimization, but yosys will automatically download and compile the correct version of abc with the compile-time options needed by yosys.

Placement (graywolf)

Obtain this tool from GitHub graywolf. Specifically, this can be cloned from the git repository using the command "git clone git://github.com/rubund/graywolf". Graywolf is the placement tool formerly known as TimberWolf. It is currently maintained by Ruben Undheim in a much streamlined form. "cmake" is required to build graywolf. Graywolf should compile cleanly using "cmake ." and "make", and install with "make install" to a standard directory (default /usr/local/).

Detail Router (qrouter 1.4)

Obtain Qrouter 1.4 from OpenCircuitDesign: Qrouter home page
This is a work in progress, but is a full-featured multi-layer, sea-of-gates maze router. For a very very long time, a good open-source maze router was not available anywhere, and it had been on my list of important things to do for ages. Finally, in June of 2011, I finally sat down and wrote it. Although as a starting point it was based on router code by Steve Beccue, it is totally overhauled and can be considered written from scratch for all practical purposes. It is a command-line-only tool, taking standard format (LEF and DEF) files as input and generating an annotated DEF format file as output.

Static Timing Analysis (vesta)

Vesta is a static timing analysis tool that is part of the qflow package.

Layout viewer (Magic 8.1)

Obtain Magic 8.1 from OpenCircuitDesign: Magic home page

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Last updated: February 20, 2019 at 8:35pm