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The ChiselEnum type can be used to reduce the chance of error when encoding mux selectors, opcodes, and functional unit operations. In contrast with Chisel.util.Enum, ChiselEnum are subclasses of Data, which means that they can be used to define fields in Bundles, including in IOs.

Functionality and Examples

// Imports used in the following examples
import circt.stage.ChiselStage
import chisel3._
import chisel3.util._

Below we see ChiselEnum being used as mux select for a RISC-V core. While wrapping the object in a package is not required, it is highly recommended as it allows for the type to be used in multiple files more easily.

// package CPUTypes {
object AluMux1Sel extends ChiselEnum {
val selectRS1, selectPC = Value
// We can see the mapping by printing each Value
// AluMux1Sel(0=selectRS1)
// AluMux1Sel(1=selectPC)

Here we see a mux using the AluMux1Sel to select between different inputs.

import AluMux1Sel._

class AluMux1Bundle extends Bundle {
val aluMux1Sel = Input(AluMux1Sel())
val rs1Out = Input(Bits(32.W))
val pcOut = Input(Bits(32.W))
val aluMux1Out = Output(Bits(32.W))

class AluMux1File extends Module {
val io = IO(new AluMux1Bundle)

// Default value for aluMux1Out
io.aluMux1Out := 0.U

switch (io.aluMux1Sel) {
is (selectRS1) {
io.aluMux1Out := io.rs1Out
is (selectPC) {
io.aluMux1Out := io.pcOut
// Generated by CIRCT firtool-1.78.1
module AluMux1File( //
input clock, //
reset, //
io_aluMux1Sel, //
input [31:0] io_rs1Out, //
io_pcOut, //
output [31:0] io_aluMux1Out //

assign io_aluMux1Out = io_aluMux1Sel ? io_pcOut : io_rs1Out; //, :68:26, :70:21

ChiselEnum also allows for the user to directly set the Values by passing an UInt to Value(...) as shown below. Note that the magnitude of each Value must be strictly greater than the one before it.

object Opcode extends ChiselEnum {
val load = Value(0x03.U) // i "load" -> 000_0011
val imm = Value(0x13.U) // i "imm" -> 001_0011
val auipc = Value(0x17.U) // u "auipc" -> 001_0111
val store = Value(0x23.U) // s "store" -> 010_0011
val reg = Value(0x33.U) // r "reg" -> 011_0011
val lui = Value(0x37.U) // u "lui" -> 011_0111
val br = Value(0x63.U) // b "br" -> 110_0011
val jalr = Value(0x67.U) // i "jalr" -> 110_0111
val jal = Value(0x6F.U) // j "jal" -> 110_1111

The user can 'jump' to a value and continue incrementing by passing a start point then using a regular Value definition.

object BranchFunct3 extends ChiselEnum {
val beq, bne = Value
val blt = Value(4.U)
val bge, bltu, bgeu = Value
// We can see the mapping by printing each Value
// BranchFunct3(0=beq)
// BranchFunct3(1=bne)
// BranchFunct3(4=blt)
// BranchFunct3(5=bge)
// BranchFunct3(6=bltu)
// BranchFunct3(7=bgeu)


You can cast an enum to a UInt using .asUInt:

class ToUInt extends RawModule {
val in = IO(Input(Opcode()))
val out = IO(Output(UInt(in.getWidth.W)))
out := in.asUInt

You can cast from a UInt to an enum by passing the UInt to the apply method of the ChiselEnum object:

class FromUInt extends Module {
val in = IO(Input(UInt(7.W)))
val out = IO(Output(Opcode()))
out := Opcode(in)

However, if you cast from a UInt to an Enum type when there are undefined states in the Enum values that the UInt could hit, you will see a warning like the following:

[warn] 136:16: [W001] Casting non-literal UInt to repl.MdocSession$MdocApp$Opcode. You can use repl.MdocSession$MdocApp$ to cast without this warning.
[warn] There were 1 warning(s) during hardware elaboration.

(Note that the name of the Enum is ugly as an artifact of our documentation generation flow, it will be cleaner in normal use).

You can avoid this warning by using the .safe factory method which returns the cast Enum in addition to a Bool indicating if the Enum is in a valid state:

class SafeFromUInt extends Module {
val in = IO(Input(UInt(7.W)))
val out = IO(Output(Opcode()))
val (value, valid) =
assert(valid, "Enum state must be valid, got %d!", in)
out := value

Now there will be no warning:

You can also suppress the warning by using suppressEnumCastWarning. This is primarily used for casting from [[UInt]] to a Bundle type that contains an Enum, where the [[UInt]] is known to be valid for the Bundle type.

class MyBundle extends Bundle {
val addr = UInt(8.W)
val op = Opcode()

class SuppressedFromUInt extends Module {
val in = IO(Input(UInt(15.W)))
val out = IO(Output(new MyBundle()))
suppressEnumCastWarning {
out := in.asTypeOf(new MyBundle)


The Type of the enums values is <ChiselEnum Object>.Type which can be useful for passing the values as parameters to a function (or any other time a type annotation is needed). Calling .litValue on an enum value will return the integer value of that object as a BigInt.

def expectedSel(sel: AluMux1Sel.Type): Boolean = sel match {
case AluMux1Sel.selectRS1 => (sel.litValue == 0)
case AluMux1Sel.selectPC => (sel.litValue == 1)
case _ => false

The enum value type also defines some convenience methods for working with ChiselEnum values. For example, continuing with the RISC-V opcode example, one could easily create hardware signal that is only asserted on LOAD/STORE operations (when the enum value is equal to Opcode.load or using the .isOneOf method:

class LoadStoreExample extends Module {
val io = IO(new Bundle {
val opcode = Input(Opcode())
val load_or_store = Output(Bool())
io.load_or_store := io.opcode.isOneOf(Opcode.load,

Some additional useful methods defined on the ChiselEnum object are:

  • .all: returns the enum values within the enumeration
  • .getWidth: returns the width of the hardware type


As of Chisel v3.4.3 (1 July 2020), the width of the values is always inferred. To work around this, you can add an extra Value that forces the width that is desired. This is shown in the example below, where we add a field ukn to force the width to be 3 bits wide:

object StoreFunct3 extends ChiselEnum {
val sb, sh, sw = Value
val ukn = Value(7.U)
// We can see the mapping by printing each Value
// StoreFunct3(0=sb)
// StoreFunct3(1=sh)
// StoreFunct3(2=sw)
// StoreFunct3(7=ukn)

Signed values are not supported so if you want the value signed, you must cast the UInt with .asSInt.

Additional Resources

The ChiselEnum type is much more powerful than stated above. It allows for Sequence, Vec, and Bundle assignments, as well as a .next operation to allow for stepping through sequential states and an .isValid for checking that a hardware value is a valid Value. The source code for the ChiselEnum can be found here in the class EnumFactory. Examples of the ChiselEnum operations can be found here.