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Component and hierarchy


Like in VHDL and Verilog, you can define components that can be used to build a design hierarchy. However, in SpinalHDL, you don’t need to bind their ports at instantiation:

class AdderCell() extends Component {
  // Declaring external ports in a Bundle called `io` is recommended
  val io = new Bundle {
    val a, b, cin = in Bool()
    val sum, cout = out Bool()
  // Do some logic
  io.sum := io.a ^ io.b ^ io.cin
  io.cout := (io.a & io.b) | (io.a & io.cin) | (io.b & io.cin)

class Adder(width: Int) extends Component {
  // Create 2 AdderCell instances
  val cell0 = new AdderCell()
  val cell1 = new AdderCell() :=   // Connect cout of cell0 to cin of cell1

  // Another example which creates an array of ArrayCell instances
  val cellArray = Array.fill(width)(new AdderCell())
  cellArray(1).io.cin := cellArray(0).io.cout   // Connect cout of cell(0) to cin of cell(1)


val io = new Bundle { ... }
Declaring external ports in a Bundle called io is recommended. If you name your bundle io, SpinalHDL will check that all of its elements are defined as inputs or outputs.


If it is better to your taste, you can use the Module syntax instead of Component (they are the same thing)

Input / output definition

The syntax to define inputs and outputs is as follows:




in Bool()/out Bool()

Create an input Bool/output Bool


in/out Bits/UInt/SInt[(x bits)]

Create an input/output of the corresponding type



For all other data types, you may have to add some brackets around it. Sorry, this is a Scala limitation.



This syntax is provided by the spinal.lib library (If you annotate your object with the slave syntax, then import spinal.lib.slave instead). T should extend IMasterSlave – Some documentation is available here. You may not actually need the brackets, so master T is fine as well.


There are some rules to follow with component interconnection:

  • Components can only read output and input signals of child components.

  • Components can read their own output port values (unlike in VHDL).


If for some reason you need to read signals from far away in the hierarchy (such as for debugging or temporal patches), you can do it by using the value returned by some.where.else.theSignal.pull()

Pruned signals

SpinalHDL will generate all the named signals and their depedencies, while all the useless anonymous / zero width ones are removed from the RTL generation.

You can collect the list of all the removed ans useless signals via the printPruned and the printPrunedIo functions on the generated SpinalReport object:

class TopLevel extends Component {
  val io = new Bundle {
    val a,b = in UInt(8 bits)
    val result = out UInt(8 bits)

  io.result := io.a + io.b

  val unusedSignal = UInt(8 bits)
  val unusedSignal2 = UInt(8 bits)

  unusedSignal2 := unusedSignal

object Main {
  def main(args: Array[String]) {
    SpinalVhdl(new TopLevel).printPruned()
    //This will report :
    //  [Warning] Unused wire detected : toplevel/unusedSignal : UInt[8 bits]
    //  [Warning] Unused wire detected : toplevel/unusedSignal2 : UInt[8 bits]

Parametrized Hardware (“Generic” in VHDL, “Parameter” in Verilog)

If you want to parameterize your component, you can give parameters to the constructor of the component as follows:

class MyAdder(width: BitCount) extends Component {
  val io = new Bundle {
    val a, b   = in UInt(width)
    val result = out UInt(width)
  io.result := io.a + io.b

object Main {
  def main(args: Array[String]) {
    SpinalVhdl(new MyAdder(32 bits))

If you have several parameters, it is a good practice to give a specific configuration class as follows:

case class MySocConfig(axiFrequency  : HertzNumber,
                       onChipRamSize : BigInt,
                       cpu           : RiscCoreConfig,
                       iCache        : InstructionCacheConfig)

class MySoc(config: MySocConfig) extends Component {

You can add functions inside the config, along with requirements on the config attributes:

case class MyBusConfig(addressWidth: Int, dataWidth: Int) {
  def bytePerWord = dataWidth / 8
  def addressType = UInt(addressWidth bits)
  def dataType = Bits(dataWidth bits)

  require(dataWidth == 32 || dataWidth == 64, "Data width must be 32 or 64")

Synthesized component names

Within a module, each component has a name, called a “partial name”. The “full” name is built by joining every component’s parent name with “_”, for example: io_clockDomain_reset. You can use setName to replace this convention with a custom name. This is especially useful when interfacing with external components. The other methods are called getName, setPartialName, and getPartialName respectively.

When synthesized, each module gets the name of the Scala class defining it. You can override this as well with setDefinitionName.