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Chisel properties represent information about the design that is not hardware. This is useful to capture domain-specific knowledge and design intent alongside the hardware description within the same generator.


Properties are under active development and are not yet considered stable.

Property Types

The core primitive for using properties is the Property type.

Property types work similarly to the other Chisel Data Types, but rather than specifying the type of values held in state elements or flowing through wires in the circuit, properties never flow through or affect the generated hardware. Instead, they flow through the hierarchy as ports that can be connected.

What makes Property types useful is their ability to express non-hardware information that is present in the generated hierarchy, and can be composed to create domain-specific data models that are tightly coupled to the design. An input port with Property type represents a part of the data model that must be supplied when its module is instantiated. An output port with Property type represents a part of the data model that may be accessed when its module is instantiated. As the complete design is generated, an arbitrary data model can be generated alongside it.

The following are legal Property types:

  • Property[Int]
  • Property[Long]
  • Property[BigInt]
  • Property[String]
  • Property[Boolean]
  • Property[Seq[A]] (where A is itself a Property)

Using Properties

The Property functionality can be used with the following imports:

import chisel3._

The subsections below show example uses of Property types in various Chisel constructs.

Property Ports

The legal Property types may be used in ports. For example:

class PortsExample extends RawModule {
// An Int Property type port.
val myPort = IO(Input(Property[Int]()))

Property Connections

The legal Property types may be connected using the := operator. For example, an input Property type port may be connected to an output Property type port:

class ConnectExample extends RawModule {
val inPort = IO(Input(Property[Int]()))
val outPort = IO(Output(Property[Int]()))
outPort := inPort

Connections are only supported between the same Property type. For example, a Property[Int] may only be connected to a Property[Int]. This is enforced by the Scala compiler.

Property Values

The legal Property types may be used to construct values by applying the Property object to a value of the Property type. For example, a Property value may be connected to an output Property type port:

class LiteralExample extends RawModule {
val outPort = IO(Output(Property[Int]()))
outPort := Property(123)

Property Sequences

Similarly to the primitive Property types, sequences of Properties may also be for creating ports and values and they may also be connected:

class SequenceExample extends RawModule {
val inPort = IO(Input(Property[Int]()))
val outPort1 = IO(Output(Property[Seq[Int]]()))
val outPort2 = IO(Output(Property[Seq[Int]]()))
// A Seq of literals can by turned into a Property
outPort1 := Property(Seq(123, 456))
// Property ports and literals can be mixed together into a Seq
outPort2 := Property(Seq(inPort, Property(789)))

Property Expressions

Expressions can be built out of Property values for certain Property types. This is useful for expressing design intent that is parameterized by input Property values.

Integer Arithmetic

The integral Property types, like Property[Int], Property[Long] and Property[BigInt], can be used to build arithmetic expressions in terms of Property values.

In the following example, an output address port of Property[Int] type is computed as the addition of an offset Property[Int] value relative to an input base Property[Int] value.

class IntegerArithmeticExample extends RawModule {
val base = IO(Input(Property[Int]()))
val address = IO(Output(Property[Int]()))
val offset = Property(1024)
address := base + offset

The following table lists the possible arithmetic operators that are supported on integral Property typed values.

+Perform addition as defined by FIRRTL spec section Integer Add Operation
*Perform multiplication as defined by FIRRTL spec section Integer Multiply Operation
>>Perform shift right as defined by FIRRTL spec section Integer Shift Right Operation

Classes and Objects

Classes and Objects are to Property types what modules and instances are to hardware types. That is, they provide a means to declare hierarchies through which Property typed values flow. Class declares a hierarchical container with input and output Property ports, and a body that contains Property connections and Objects. Objects represent the instantiation of a Class, which requires any input Property ports to be assigned, and allows any output Property ports to be read.

This allows domain-specific data models to be built using the basic primitives of an object-oriented programming language, and embedded directly in the instance graph Chisel is constructing. Intuitively, inputs to a Class are like constructor arguments, which must be supplied to create an Object. Similarly, outputs from a Class are like fields, which may be accessed from an Object. This separation allows Class declarations to abstract over any Objects created in their body--from the outside, the inputs must be supplied and only the outputs may be accessed.

The graphs represented by Class declarations and Object instantiations coexist within the hardware instance graph. Object instances can exist within hardware modules, providing domain-specific information, but hardware instances cannot exist within Class declarations.

Objects can be referenced, and references to Objects are a special kind of Property[ClassType] type. This allows the data model captured by Class declarations and Object instances to form arbitrary graphs.

To understand how Object graphs are represented, and can ultimately be queried, consider how the hardware instance graph is elaborated. To build the Object graph, we first pick an entrypoint module to start elaboration. The elaboration process works according to the Verilog spec's definition of elaboration--instances of modules and Objects are instantiated in-memory, with connections to their inputs and outputs. Inputs are supplied, and outputs may be read. After elaboration completes, the Object graph is exposed in terms of the output ports, which may contain any Property types, including references to Objects.

To illustrate how these pieces come together, consider the following examples:

import chisel3.experimental.hierarchy.{instantiable, public, Definition, Instance}

// An abstract description of a CSR, represented as a Class.
class CSRDescription extends Class {
// An output Property indicating the CSR name.
val identifier = IO(Output(Property[String]()))
// An output Property describing the CSR.
val description = IO(Output(Property[String]()))
// An output Property indicating the CSR width.
val width = IO(Output(Property[Int]()))

// Input Properties to be passed to Objects representing instances of the Class.
@public val identifierIn = IO(Input(Property[String]()))
@public val descriptionIn = IO(Input(Property[String]()))
@public val widthIn = IO(Input(Property[Int]()))

// Simply connect the inputs to the outputs to expose the values.
identifier := identifierIn
description := descriptionIn
width := widthIn

The CSRDescription is a Class that represents domain-specific information about a CSR. This uses @instantiable and @public so the Class can work with the Definition and Instance APIs.

The readable fields we want to expose on Objects of the CSRDescription class are a string identifier, a string description, and an integer bitwidth, so these are output Property type ports on the Class.

To capture concrete values at each Object instantiation, we have corresponding input Property type ports, which are connected directly to the outputs. This is how we would represent something like a Scala case class using Class.

// A hardware module representing a CSR and its description.
class CSRModule(
csrDescDef: Definition[CSRDescription],
width: Int,
identifierStr: String,
descriptionStr: String)
extends Module {
override def desiredName = identifierStr

// Create a hardware port for the CSR value.
val value = IO(Output(UInt(width.W)))

// Create a property port for a reference to the CSR description object.
val description = IO(Output(csrDescDef.getPropertyType))

// Instantiate a CSR description object, and connect its input properties.
val csrDescription = Instance(csrDescDef)
csrDescription.identifierIn := Property(identifierStr)
csrDescription.descriptionIn := Property(descriptionStr)
csrDescription.widthIn := Property(width)

// Create a register for the hardware CSR. A real implementation would be more involved.
val csr = RegInit(0.U(width.W))

// Assign the CSR value to the hardware port.
value := csr

// Assign a reference to the CSR description object to the property port.
description := csrDescription.getPropertyReference

The CSRModule is a Module that represents the (dummy) hardware for a CSR, as well as a CSRDescription. Using a Definition of a CSRDescription, an Object is created and its inputs supplied from the CSRModule constructor arguments. Then, a reference to the Object is connected to the CSRModule output, so the reference will be exposed to the outside.

// The entrypoint module.
class Top extends Module {
// Create a Definition for the CSRDescription Class.
val csrDescDef = Definition(new CSRDescription)

// Get the CSRDescription ClassType.
val csrDescType = csrDescDef.getClassType

// Create a property port to collect all the CSRDescription object references.
val descriptions = IO(Output(Property[Seq[csrDescType.Type]]()))

// Instantiate a couple CSR modules.
val mcycle = Module(new CSRModule(csrDescDef, 64, "mcycle", "Machine cycle counter."))
val minstret = Module(new CSRModule(csrDescDef, 64, "minstret", "Machine instructions-retired counter."))

// Assign references to the CSR description objects to the property port.
descriptions := Property(Seq(,

The Top module represents the entrypoint. It creates the Definition of the CSRDescription, and creates some CSRModules. It then takes the description references, collects them into a list, and outputs the list so it will be exposed to the outside.

While it is not required to use the Definition API to define a Class, this is the "safe" API, with support in Chisel for working with Definitions and Instances of a Class. There is also an "unsafe" API. See DynamicObject for more information.

To illustrate what this example generates, here is a listing of the FIRRTL:

FIRRTL version 4.0.0
circuit Top :
class CSRDescription :
output identifier : String
output description : String
output width : Integer
input identifierIn : String
input descriptionIn : String
input widthIn : Integer

propassign identifier, identifierIn
propassign description, descriptionIn
propassign width, widthIn

module mcycle :
input clock : Clock
input reset : Reset
output value : UInt<64>
output description : Inst<CSRDescription>

object csrDescription of CSRDescription
propassign csrDescription.identifierIn, String("mcycle")
propassign csrDescription.descriptionIn, String("Machine cycle counter.")
propassign csrDescription.widthIn, Integer(64)
regreset csr : UInt<64>, clock, reset, UInt<64>(0h0)
connect value, csr
propassign description, csrDescription

module minstret :
input clock : Clock
input reset : Reset
output value : UInt<64>
output description : Inst<CSRDescription>

object csrDescription of CSRDescription
propassign csrDescription.identifierIn, String("minstret")
propassign csrDescription.descriptionIn, String("Machine instructions-retired counter.")
propassign csrDescription.widthIn, Integer(64)
regreset csr : UInt<64>, clock, reset, UInt<64>(0h0)
connect value, csr
propassign description, csrDescription

public module Top :
input clock : Clock
input reset : UInt<1>
output descriptions : List<Inst<CSRDescription>>

inst mcycle of mcycle
connect mcycle.clock, clock
connect mcycle.reset, reset
inst minstret of minstret
connect minstret.clock, clock
connect minstret.reset, reset
propassign descriptions, List<Inst<CSRDescription>>(mcycle.description, minstret.description)

To understand the Object graph that is constructed, we will consider an entrypoint to elaboration, and then show a hypothetical JSON representation of the Object graph. The details of how we go from IR to an Object graph are outside the scope of this document, and implemented by related tools.

If we elaborate Top, the descriptions output Property is our entrypoint to the Object graph. Within it, there are two Objects, the CSRDescriptions of the mcycle and minstret modules:

"descriptions": [
"identifier": "mcycle",
"description": "Machine cycle counter.",
"width": 64
"identifier": "minstret",
"description": "Machine instructions-retired counter.",
"width": 64

If instead, we elaborate one of the CSRModules, for example, minstret, the description output Property is our entrypoint to the Object graph, which contains the single CSRDescription object:

"description": {
"identifier": "minstret",
"description": "Machine instructions-retired counter.",
"width": 64

In this way, the output Property ports, Object references, and choice of elaboration entrypoint allow us to view the Object graph representing the domain-specific data model from different points in the hierarchy.