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Bundles and Vecs

Bundle and Vec are classes that allow the user to expand the set of Chisel datatypes with aggregates of other types.

Bundles group together several named fields of potentially different types into a coherent unit, much like a struct in C. Users define their own bundles by defining a class as a subclass of Bundle.

import chisel3._
class MyFloat extends Bundle {
val sign = Bool()
val exponent = UInt(8.W)
val significand = UInt(23.W)

class ModuleWithFloatWire extends RawModule {
val x = Wire(new MyFloat)
val xs = x.sign

You can create literal Bundles using the experimental Bundle Literals feature.

Scala convention is to name classes using UpperCamelCase, and we suggest you follow that convention in your Chisel code.

Vecs create an indexable vector of elements, and are constructed as follows:

class ModuleWithVec extends RawModule {
// Vector of 5 23-bit signed integers.
val myVec = Wire(Vec(5, SInt(23.W)))

// Connect to one element of vector.
val reg3 = myVec(3)

(Note that we specify the number followed by the type of the Vec elements. We also specifiy the width of the SInt)

The set of primitive classes (SInt, UInt, and Bool) plus the aggregate classes (Bundles and Vecs) all inherit from a common superclass, Data. Every object that ultimately inherits from Data can be represented as a bit vector in a hardware design.

Bundles and Vecs can be arbitrarily nested to build complex data structures:

class BigBundle extends Bundle {
// Vector of 5 23-bit signed integers.
val myVec = Vec(5, SInt(23.W))
val flag = Bool()
// Previously defined bundle.
val f = new MyFloat

Note that the builtin Chisel primitive and aggregate classes do not require the new when creating an instance, whereas new user datatypes will. A Scala apply constructor can be defined so that a user datatype also does not require new, as described in Function Constructor.

Flipping Bundles

The Flipped() function recursively flips all elements in a Bundle/Record. This is very useful for building bidirectional interfaces that connect to each other (e.g. Decoupled). See below for an example.

class ABBundle extends Bundle {
val a = Input(Bool())
val b = Output(Bool())
class MyFlippedModule extends RawModule {
// Normal instantiation of the bundle
// 'a' is an Input and 'b' is an Output
val normalBundle = IO(new ABBundle)
normalBundle.b := normalBundle.a

// Flipped recursively flips the direction of all Bundle fields
// Now 'a' is an Output and 'b' is an Input
val flippedBundle = IO(Flipped(new ABBundle))
flippedBundle.a := flippedBundle.b

This generates the following Verilog:

// Generated by CIRCT firtool-1.76.0
module MyFlippedModule( //
input normalBundle_a, //
output normalBundle_b, //
flippedBundle_a, //
input flippedBundle_b //

assign normalBundle_b = normalBundle_a; //
assign flippedBundle_a = flippedBundle_b; //


(Chisel 3.2+)

All elements of a Vec must have the same parameterization. If we want to create a Vec where the elements have the same type but different parameterizations, we can use a MixedVec:

import chisel3.util.MixedVec
class ModuleMixedVec extends Module {
val io = IO(new Bundle {
val x = Input(UInt(3.W))
val y = Input(UInt(10.W))
val vec = Output(MixedVec(UInt(3.W), UInt(10.W)))
io.vec(0) := io.x
io.vec(1) := io.y

We can also programmatically create the types in a MixedVec:

class ModuleProgrammaticMixedVec(x: Int, y: Int) extends Module {
val io = IO(new Bundle {
val vec = Input(MixedVec((x to y) map { i => UInt(i.W) }))
// ...
// of the module goes here...

A note on cloneType (For Chisel < 3.5)

NOTE: This section only applies to Chisel before Chisel 3.5. As of Chisel 3.5, Bundles should not override def cloneType, as this is a compiler error when using the chisel3 compiler plugin for inferring cloneType.

Since Chisel is built on top of Scala and the JVM, it needs to know how to construct copies of Bundles for various purposes (creating wires, IOs, etc). If you have a parametrized Bundle and Chisel can't automatically figure out how to clone it, you will need to create a custom cloneType method in your bundle. In the vast majority of cases, this is not required as Chisel can figure out how to clone most Bundles automatically:

class MyCloneTypeBundle(val bitwidth: Int) extends Bundle {
val field = UInt(bitwidth.W)
// ...

The only caveat is if you are passing something of type Data as a "generator" parameter, in which case you should make it a private val, and define a cloneType method with override def cloneType = (new YourBundleHere(...)).asInstanceOf[this.type].

For example, consider the following Bundle. Because its gen variable is not a private val, the user has to explicitly define the cloneType method:

import chisel3.util.{Decoupled, Irrevocable}
class RegisterWriteIOExplicitCloneType[T <: Data](gen: T) extends Bundle {
val request = Flipped(Decoupled(gen))
val response = Irrevocable(Bool())
override def cloneType = new RegisterWriteIOExplicitCloneType(gen).asInstanceOf[this.type]

We can make this this infer cloneType by making gen private since it is a "type parameter":

import chisel3.util.{Decoupled, Irrevocable}
class RegisterWriteIO[T <: Data](private val gen: T) extends Bundle {
val request = Flipped(Decoupled(gen))
val response = Irrevocable(Bool())