Chisel3

Chisel3 is a new FIRRTL based chisel. It is currently in ALPHA VERSION, so many Chisel features may change in the coming months.

TODO: A better description, perhaps lifted off Chisel2's README

Chisel2 Migration

For those moving from Chisel2, there were some backwards incompatible changes and your RTL needs to be modified to work with Chisel3. The required modifications are:

Wire declaration style:

val wire = Bits(width = 15)

becomes (in Chisel3):

val wire = Wire(Bits(width = 15))

Sequential memories:

val addr = Reg(UInt())
val mem = Mem(UInt(width=8), 1024, seqRead = true)
val dout = when(enable) { mem(addr) }

becomes (in Chisel3):

val addr = UInt()
val mem = SeqMem(1024, UInt(width=8))
val dout = mem.read(addr, enable)

Notice the address register is now internal to the SeqMem(), but the data will still return on the subsequent cycle.

Please refer to the Chisel3 compatibility section for instructions on preparing your Chisel2 designs for Chisel3.

Overview

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.

Installation

This will walk you through installing Chisel and its dependencies:

sbt, which is the preferred Scala build system and what Chisel uses.
FIRRTL, which compile Chisel's IR down to Verilog. A alpha version of FIRRTL written in Scala is available.
- FIRRTL is currently a separate repository but may eventually be made available as a standalone program through system package managers and/or included in the Chisel source tree.
- FIRRTL has known issues compiling under JDK 8, which manifests as an infinite recursion / stack overflow exception. Instructions for selecting JDK 7 are included.
Verilator, which compiles Verilog down to C++ for simulation. The included unit testing infrastructure uses this.

(Ubuntu-like) Linux

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

echo "deb https://dl.bintray.com/sbt/debian /" | sudo tee -a /etc/apt/sources.list.d/sbt.list
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 642AC823
sudo apt-get update
sudo apt-get install sbt

Install FIRRTL.
Clone the FIRRTL repository:

```
git clone git@github.com:ucb-bar/firrtl.git
```

Build Scala-FIRRTL. In the cloned FIRRTL repository:

```
make build-scala
```
* This compiles FIRRTL into a JAR and creates a wrapper script `firrtl` to
  make the JAR executable. The generated files are in `firrtl/utils/bin`.
* If this fails with an infinite recursion / stack overflow exception, this
  is a known bug with JDK 8. You can either increase the stack size by
  invoking:

  ```
  JAVA_OPTS=-Xss256m make build-scala
  ```
* Or, revert to JDK 7:
  1. Install JDK 7 (if not installed already):

    ```
    sudo apt-get install openjdk-7-jdk
    ```
  2. Select JDK 7 as the default JDK:

    ```
    sudo update-alternatives --config java
    ```

Add the FIRRTL executable to your PATH. One way is to add this line to your .bashrc:

```
export PATH=$PATH:<path-to-your-firrtl-repository>/utils/bin
```

Install Verilator. As of February 2016, the version of Verilator included by in Ubuntu's default package repositories are too out of date, so it must be compiled from source.
Install prerequisites (if not installed already):

```
sudo apt-get install git make autoconf g++ flex bison
```

Clone the Verilator repository:

```
git clone http://git.veripool.org/git/verilator
```

In the Verilator repository directory, check out a known good version:

```
git pull
git checkout verilator_3_880
```

In the Verilator repository directory, build and install:

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

Arch Linux

```
yaourt -S firrtl-git verilator sbt
```

Windows

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

Mac OS X

Install sbt:

brew cask install sbt

Install FIRRTL:
Clone the FIRRTL repository:

```
git clone git@github.com:ucb-bar/firrtl.git
```

Build Scala-FIRRTL. In the cloned FIRRTL repository:

```
make build-scala
```
* This compiles FIRRTL into a JAR and creates a wrapper script `firrtl` to
  make the JAR executable. The generated files are in `firrtl/utils/bin`.
* If this fails with an infinite recursion / stack overflow exception, this
  is a known bug with JDK 8. You can either increase the stack size by
  invoking

  ```
  JAVA_OPTS=-Xss256m make build-scala`
  ```
* Or, revert to JDK 7:

  ```
  brew install caskroom/versions/java7
  ```

Add the FIRRTL executable to your PATH. One way is to add this line to your .bashrc:

```
export PATH=$PATH:<path-to-your-firrtl-repository>/utils/bin
```

Install Verilator:

brew install verilator

Getting Started

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

TODO: quick howto for running chisel-tutorial, once chisel-tutorial exists

For Hardware Engineers

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

Project Setup

TODO: recommended sbt style, project structure

RTL

TODO: toy example

Verification

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 := "3.0",

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. Change the version string in build.sbt to:

  version := "3.0-SNAPSHOT"

and re-execute sbt publish-local to accomplish this.

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" %% "chisel" % "3.0-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.

shunshou/chisel3