Scala Other
Latest commit d7d658d Feb 9, 2017 @jackkoenig jackkoenig committed with aswaterman Fix mismatch between Chisel and Firrtl on UInt -& UInt
Fixes #501. Also added UIntOps test.


Chisel3 is a new Firrtl based chisel. It is currently in BETA VERSION, so some Chisel features may change in the coming months.

Please visit the Wiki for a more detailed description.

The ScalaDoc for Chisel3 is available at the API tab on the Chisel web site.


Chisel3 is much more modular than Chisel2, and the compilation pipeline looks like:

  • Chisel3 (Scala) to Firrtl (this is your "Chisel RTL").
  • Firrtl to Verilog (which then be passed into FPGA or ASIC tools).
  • Verilog to C++ for simulation and testing using Verilator.


This will walk you through installing Chisel and its dependencies:

  • sbt, which is the preferred Scala build system and what Chisel uses.
  • Firrtl, which compiles Chisel's IR down to Verilog. Separate installation of Firrtl is no longer required.
  • Verilator, which compiles Verilog down to C++ for simulation. The included unit testing infrastructure uses this.

(Ubuntu-like) Linux

  1. Install sbt, which isn't available by default in the system package manager:

    echo "deb /" | sudo tee -a /etc/apt/sources.list.d/sbt.list
    sudo apt-key adv --keyserver hkp:// --recv 642AC823
    sudo apt-get update
    sudo apt-get install sbt
  2. Install Verilator. As of November 2016, the version of Verilator included by in Ubuntu's default package repositories is too out of date, so it must be compiled from source.

    1. Install prerequisites (if not installed already):

      sudo apt-get install git make autoconf g++ flex bison
    2. Clone the Verilator repository:

      git clone
    3. In the Verilator repository directory, check out a known good version:

      git pull
      git checkout verilator_3_886
    4. In the Verilator repository directory, build and install:

      unset VERILATOR_ROOT # For bash, unsetenv for csh
      autoconf # Create ./configure script
      sudo make install

Arch Linux

yaourt -S firrtl-git verilator sbt


TODO: write me. If you really want to see this happen, let us know by filing a bug report!

Mac OS X

  1. Install sbt:

    brew cask install sbt
  2. Install Verilator:

    brew install verilator

Getting Started

If you are migrating to Chisel3 from Chisel3, please visit Chisel3 vs Chisel2

Data Types Overview

These are the base data types for defining circuit wires (abstract types which may not be instantiated are greyed out):


Chisel Tutorial

For Hardware Engineers

This section describes how to get started using Chisel to create a new RTL design from scratch.

Project Setup


TODO: toy example


The simulation unit testing infrastructure is still a work in progress.

See src/test/scala/chiselTests for example unit tests. Assert.scala is a pretty bare-bones unittest which also somewhat verifies the testing system itself.

Unit tests are written with the ScalaTest unit testing framework, optionally with ScalaCheck generators to sweep the parameter space where desired.

BasicTester-based tests run a single circuit in simulation until either the test finishes or times out after a set amount of cycles. After compilation, there is no communication between the testdriver and simulation (unlike Chisel2's Tester, which allowed dynamic peeks and pokes), so all testvectors must be determined at compile time.

The circuits run must subclass BasicTester, which is a Module with the addition of a stop function which finishes the testbench and reports success. Any asserts that trigger (in either the BasicTester or a submodule) will cause the test to fail. printfs will forward to the console.

To write a test using the BasicTester that integrates with sbt test, create a class that extends either ChiselFlatSpec (BDD-style testing) or ChiselPropSpec (ScalaCheck generators). In the test content, use

assert(execute{ new MyTestModule })

where MyTestModule is your top-level test circuit that extends BasicTester.

A more Chisel2-like tester may come in the future.

Compiling to Simulation

TODO: commands to compile project to simulation

TODO: running testbenches

For Chisel Developers

This section describes how to get started developing Chisel itself, including how to test your version locally against other projects that pull in Chisel using sbt's managed dependencies.

Compiling and Testing Chisel

In the Chisel repository directory, run:

sbt compile

to compile the Chisel library. If the compilation succeeded, you can then run the included unit tests by invoking:

sbt test

Running Projects Against Local Chisel

Chisel3 is still undergoing rapid development and we haven't pusblished a stable version to the Nexus repository. You will need to build from source and publish-local. The repo version can be found in the build.sbt file. At last check it was:

  version := "3.1-SNAPSHOT",

To publish your version of Chisel to the local Ivy (sbt's dependency manager) repository, run:

sbt publish-local

PROTIP: sbt can automatically run commands on a source change if you prefix the command with ~. For example, the above command to publish Chisel locally becomes sbt ~publish-local.

sbt's manual recommends that you use a SNAPSHOT version suffix to ensure that the local repository is checked for updates. Since the current default is a SNAPSHOT, and the version number is already incremented compared to the currently published snapshot, you dont need to change version.

The compiled version gets placed in ~/.ivy2/local/. You can nuke the relevant subfolder to un-publish your local copy of Chisel.

In order to have your projects use this version of Chisel, you should update the libraryDependencies setting in your project's build.sbt file to:

libraryDependencies += "edu.berkeley.cs" %% "chisel3" % "3.1-SNAPSHOT"

The version specifier in libraryDependencies in the project's build.sbt should match the version string in your local copy of Chisel's build.sbt.

Technical Documentation

Chisel3 Architecture Overview

The Chisel3 compiler consists of these main parts:

  • The frontend, chisel.*, which is the publicly visible "API" of Chisel and what is used in Chisel RTL. These just add data to the...
  • The Builder, chisel.internal.Builder, which maintains global state (like the currently open Module) and contains commands, generating...
  • The intermediate data structures, chisel.firrtl.*, which are syntactically very similar to Firrtl. Once the entire circuit has been elaborated, the top-level object (a Circuit) is then passed to...
  • The Firrtl emitter, chisel.firrtl.Emitter, which turns the intermediate data structures into a string that can be written out into a Firrtl file for further processing.

Also included is:

  • The standard library of circuit generators, chisel.util.*. These contain commonly used interfaces and constructors (like Decoupled, which wraps a signal with a ready-valid pair) as well as fully parameterizable circuit generators (like arbiters and muxes).
  • Driver utilities, chisel.Driver, which contains compilation and test functions that are invoked in the standard Verilog generation and simulation testing infrastructure. These can also be used as part of custom flows.