Chisel modules are very similar to Verilog modules in defining a hierarchical structure in the generated circuit.

The hierarchical module namespace is accessible in downstream tools to aid in debugging and physical layout. A user-defined module is defined as a class which:

  • inherits from Module,
  • contains at least one interface wrapped in a Module‚Äôs IO() method (traditionally stored in a port field named io), and
  • wires together subcircuits in its constructor.

As an example, consider defining your own two-input multiplexer as a module:

import chisel3._
class Mux2IO extends Bundle {
  val sel = Input(UInt(1.W))
  val in0 = Input(UInt(1.W))
  val in1 = Input(UInt(1.W))
  val out = Output(UInt(1.W))

class Mux2 extends Module {
  val io = IO(new Mux2IO)
  io.out := (io.sel & io.in1) | (~io.sel & io.in0)

The wiring interface to a module is a collection of ports in the form of a Bundle. The interface to the module is defined through a field named io. For Mux2, io is defined as a bundle with four fields, one for each multiplexer port.

The := assignment operator, used here in the body of the definition, is a special operator in Chisel that wires the input of left-hand side to the output of the right-hand side.

Module Hierarchy

We can now construct circuit hierarchies, where we build larger modules out of smaller sub-modules. For example, we can build a 4-input multiplexer module in terms of the Mux2 module by wiring together three 2-input multiplexers:

class Mux4IO extends Bundle {
  val in0 = Input(UInt(1.W))
  val in1 = Input(UInt(1.W))
  val in2 = Input(UInt(1.W))
  val in3 = Input(UInt(1.W))
  val sel = Input(UInt(2.W))
  val out = Output(UInt(1.W))
class Mux4 extends Module {
  val io = IO(new Mux4IO)

  val m0 = Module(new Mux2) := io.sel(0) := io.in0 := io.in1

  val m1 = Module(new Mux2) := io.sel(0) := io.in2 := io.in3

  val m3 = Module(new Mux2) := io.sel(1) := :=

  io.out :=

We again define the module interface as io and wire up the inputs and outputs. In this case, we create three Mux2 children modules, using the Module constructor function and the Scala new keyword to create a new object. We then wire them up to one another and to the ports of the Mux4 interface.

Note: Chisel Modules have an implicit clock (called clock) and an implicit reset (called reset). To create modules without implicit clock and reset, Chisel provides RawModule.

Historical Note: Prior to Chisel 3.5, Modules were restricted to only having a single user-defined port named io. There was also a type called MultiIOModule that provided implicit clock and reset while allowing the user to define as many ports as they want. This is now the functionality of Module.


A RawModule is a module that does not provide an implicit clock and reset. This can be useful when interfacing a Chisel module with a design that expects a specific naming convention for clock or reset.

Then we can use it in place of Module usage :

import chisel3.{RawModule, withClockAndReset}

class Foo extends Module {
  val io = IO(new Bundle{
    val a = Input(Bool())
    val b = Output(Bool())
  io.b := !io.a

class FooWrapper extends RawModule {
  val a_i  = IO(Input(Bool()))
  val b_o  = IO(Output(Bool()))
  val clk  = IO(Input(Clock()))
  val rstn = IO(Input(Bool()))

  val foo = withClockAndReset(clk, !rstn){ Module(new Foo) } := a_i
  b_o :=

In the example above, the RawModule is used to change the reset polarity of module SlaveSpi. Indeed, the reset is active high by default in Chisel modules, then using withClockAndReset(clock, !rstn) we can use an active low reset in the entire design.

The clock is just wired as is, but if needed, RawModule can be used in conjunction with BlackBox to connect a differential clock input for example.