Ports are used as interfaces to hardware components. A port is simply any Data object that has directions assigned to its members.

Chisel provides port constructors to allow a direction to be added (input or output) to an object at construction time. Primitive port constructors wrap the type of the port in Input or Output.

An example port declaration is as follows:

class Decoupled extends Bundle {
  val ready = Output(Bool())
  val data  = Input(UInt(32.W))
  val valid = Input(Bool())

After defining Decoupled, it becomes a new type that can be used as needed for module interfaces or for named collections of wires.

By folding directions into the object declarations, Chisel is able to provide powerful wiring constructs described later.

Inspecting Module ports

(Chisel 3.2+)

Chisel 3.2+ introduces an API DataMirror.modulePorts which can be used to inspect the IOs of any Chisel module, including MultiIOModules, RawModules, and BlackBoxes.

Here is an example of how to use this API:

import chisel3.experimental.DataMirror
import chisel3.stage.{ChiselGeneratorAnnotation, ChiselStage}

class Adder extends MultiIOModule {
  val a = IO(Input(UInt(8.W)))
  val b = IO(Input(UInt(8.W)))
  val c = IO(Output(UInt(8.W)))
  c := a +& b

class Test extends MultiIOModule {
  val adder = Module(new Adder)
  // for debug only
  adder.a := DontCare
  adder.b := DontCare

  // Inspect ports of adder
  // See the result below.
   DataMirror.modulePorts(adder).foreach { case (name, port) => {
    println(s"Found port $name: $port")

(new ChiselStage).execute(Array.empty, Seq(ChiselGeneratorAnnotation(() => new Test)))
// Elaborating design...
// Found port clock: Clock(IO clock in Adder)
// Found port reset: Reset(IO reset in Adder)
// Found port a: UInt<8>(IO a in Adder)
// Found port b: UInt<8>(IO b in Adder)
// Found port c: UInt<8>(IO c in Adder)
// Done elaborating.
// res0: firrtl.AnnotationSeq = firrtl.AnnotationSeq@6e7520f8