This document is a tutorial introduction to Chisel (Constructing Hardware In a Scala Embedded Language). Chisel is a hardware construction language embedded in the high-level programming language Scala. At some point we will provide a proper reference manual, in addition to more tutorial examples. In the meantime, this document along with a lot of trial and error should set you on your way to using Chisel. Chisel is really only a set of special class definitions, predefined objects, and usage conventions within Scala, so when you write Chisel you are actually writing a Scala program that constructs a hardware graph. However, for the tutorial we don’t presume that you understand how to program in Scala. We will point out necessary Scala features through the Chisel examples we give, and significant hardware designs can be completed using only the material contained herein. But as you gain experience and want to make your code simpler or more reusable, you will find it important to leverage the underlying power of the Scala language. We recommend you consult one of the excellent Scala books to become more expert in Scala programming.
Through the tutorial, we format commentary on our design choices as in this paragraph. You should be able to skip the commentary sections and still fully understand how to use Chisel, but we hope you’ll find them interesting.
We were motivated to develop a new hardware language by years of struggle with existing hardware description languages in our research projects and hardware design courses. Verilog and VHDL were developed as hardware simulation languages, and only later did they become a basis for hardware synthesis. Much of the semantics of these languages are not appropriate for hardware synthesis and, in fact, many constructs are simply not synthesizable. Other constructs are non-intuitive in how they map to hardware implementations, or their use can accidentally lead to highly inefficient hardware structures. While it is possible to use a subset of these languages and still get acceptable results, they nonetheless present a cluttered and confusing specification model, particularly in an instructional setting.
However, our strongest motivation for developing a new hardware language is our desire to change the way that electronic system design takes place. We believe that it is important to not only teach students how to design circuits, but also to teach them how to design circuit generators —programs that automatically generate designs from a high-level set of design parameters and constraints. Through circuit generators, we hope to leverage the hard work of design experts and raise the level of design abstraction for everyone. To express flexible and scalable circuit construction, circuit generators must employ sophisticated programming techniques to make decisions concerning how to best customize their output circuits according to high-level parameter values and constraints. While Verilog and VHDL include some primitive constructs for programmatic circuit generation, they lack the powerful facilities present in modern programming languages, such as object-oriented programming, type inference, support for functional programming, and reflection.
Instead of building a new hardware design language from scratch, we chose to embed hardware construction primitives within an existing language. We picked Scala not only because it includes the programming features we feel are important for building circuit generators, but because it was specifically developed as a base for domain-specific languages.