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The spinal.core components

The core components of the language are described in this document. It is part of the general

The core language components are as follows:

  • *Clock domains*, which allow to define and interoperate multiple clock domains within a design

  • Memory instantiation, which permit the automatic instantiation of RAM and ROM memories.

  • IP instantiation, using either existing VHDL or Verilog component.

  • Assignments

  • When / Switch

  • Component hierarchy

  • Area

  • Functions

  • Utility functions

  • VHDL generator

Clock domains definitions

In Spinal, clock and reset signals can be combined to create a clock domain. Clock domains could be applied to some area of the design and then all synchronous elements instantiated into this area will then implicitly use this clock domain.

Clock domain application work like a stack, which mean, if you are in a given clock domain, you can still apply another clock domain locally.

Clock domain syntax

The syntax to define a clock domain is as follows (using EBNF syntax):

ClockDomain(clock : Bool[,reset : Bool[,enable : Bool]]])

This definition takes three parameters:

  1. The clock signal that defines the domain

  2. An optional resetsignal. If a register which need a reset and his clock domain didn’t provide one, an error message happen

  3. An optional enable signal. The goal of this signal is to disable the clock on the whole clock domain without having to manually implement that on each synchronous element.

An applied example to define a specific clock domain within the design is as follows:

val coreClock = Bool()
val coreReset = Bool()

// Define a new clock domain
val coreClockDomain = ClockDomain(coreClock,coreReset)


// Use this domain in an area of the design
val coreArea = new ClockingArea(coreClockDomain){
  val coreClockedRegister = Reg(UInt(4 bits))

Clock configuration

In addition to the constructor parameters given here , the following elements of each clock domain are configurable via a ClockDomainConfig class :


Valid values






true, false


true, false

class CustomClockExample extends Component {
  val io = new Bundle {
    val clk = in Bool()
    val resetn = in Bool()
    val result = out UInt (4 bits)
  val myClockDomainConfig = ClockDomainConfig(
    clockEdge = RISING,
    resetKind = ASYNC,
    resetActiveLevel = LOW
  val myClockDomain = ClockDomain(io.clk,io.resetn,config = myClockDomainConfig)
  val myArea = new ClockingArea(myClockDomain){
    val myReg = Reg(UInt(4 bits)) init(7)
    myReg := myReg + 1

    io.result := myReg

By default, a ClockDomain is applied to the whole design. The configuration of this one is :

  • clock : rising edge

  • reset: asynchronous, active high

  • no enable signal

External clock

You can define everywhere a clock domain which is driven by the outside. It will then automatically add clock and reset wire from the top level inputs to all synchronous elements.

class ExternalClockExample extends Component {
  val io = new Bundle {
    val result = out UInt (4 bits)
  val myClockDomain = ClockDomain.external("myClockName")
  val myArea = new ClockingArea(myClockDomain){
    val myReg = Reg(UInt(4 bits)) init(7)
    myReg := myReg + 1

    io.result := myReg

Cross Clock Domain

SpinalHDL checks at compile time that there is no unwanted/unspecified cross clock domain signal reads. If you want to read a signal that is emitted by another ClockDomain area, you should add the crossClockDomain tag to the destination signal as depicted in the following example:

val asynchronousSignal = UInt(8 bits)
val buffer0 = Reg(UInt(8 bits)).addTag(crossClockDomain)
val buffer1 = Reg(UInt(8 bits))
buffer0 := asynchronousSignal
buffer1 := buffer0   // Second register stage to be avoid metastability issues
// Or in less lines:
val buffer0 = RegNext(asynchronousSignal).addTag(crossClockDomain)
val buffer1 = RegNext(buffer0)


There are multiple assignment operator :




Standard assignment, equivalent to ‘<=’ in VHDL/Verilog
last assignment win, value updated at next delta cycle


Equivalent to := in VHDL and = in Verilog
value updated instantly


Automatic connection between 2 signals. Direction is inferred by using signal direction (in/out)
Similar behavioural than :=
//Because of hardware concurrency is always read with the value '1' by b and c
val a,b,c = UInt(4 bits)
a := 0
b := a
a := 1  //a := 1 win
c := a

var x = UInt(4 bits)
val y,z = UInt(4 bits)
x := 0
y := x      //y read x with the value 0
x \= x + 1
z := x      //z read x with the value 1

SpinalHDL check that bitcount of left and right assignment side match. There is multiple ways to adapt bitcount of BitVector (Bits, UInt, SInt) :

Resizing ways


x := y.resized

Assign x wit a resized copy of y, resize value is automatically inferred to match x

x := y.resize(newWidth)

Assign x with a resized copy of y, size is manually calculated

There are 2 cases where spinal automaticly resize things :



SpinalHDL action

myUIntOf_8bit := U(3)

U(3) create an UInt of 2 bits, which don’t match with left side

Because U(3) is a “weak” bit inferred signal, SpinalHDL resize it automatically

myUIntOf_8bit := U(2 -> False default -> true)

The right part infer a 3 bit UInt, which doesn’t match with the left part

SpinalHDL reapply the default value to bit that are missing

When / Switch

As VHDL and Verilog, wire and register can be conditionally assigned by using when and switch syntaxes

  //execute when      cond1 is true
  //execute when (not cond1) and cond2
  //execute when (not cond1) and (not cond2)

    //execute when x === value1
    //execute when x === value2
    //execute if none of precedent condition meet

You can also define new signals into a when/switch statement. It’s useful if you want to calculate an intermediate value.

val toto,titi = UInt(4 bits)
val a,b = UInt(4 bits)

  val tmp = a + b
  toto := tmp
  titi := tmp + 1
} otherwise {
  toto := 0
  titi := 0


Like in VHDL and Verilog, you can define components that could be used to build a design hierarchy. But unlike them, you don’t need to bind them at instantiation.

class AdderCell extends Component {
  //Declaring all in/out in an io Bundle is probably a good practice
  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
  val cell0 = new AdderCell
  val cell1 = new AdderCell := //Connect carrys
  val cellArray = Array.fill(width)(new AdderCell)

Syntax to define in/out is the following :




in/out(x : Data)

Set x an input/output


in/out Bool()

Create an input/output Bool


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

Create an input/output of the corresponding type


There is some rules about component interconnection :

  • Components can only read outputs/inputs signals of children components

  • Components can read outputs/inputs ports values

  • If for some reason, you need to read a signals from far away in the hierarchy (debug, temporal patch) you can do it by using the value returned by some.where.else.theSignal.pull().


Sometime, creating a component to define some logic is overkill and to much verbose. For this kind of cases you can use Area :

class UartCtrl extends Component {
  val timer = new Area {
    val counter = Reg(UInt(8 bits))
    val tick = counter === 0
    counter := counter - 1
    when(tick) {
      counter := 100
  val tickCounter = new Area {
    val value = Reg(UInt(3 bits))
    val reset = False
    when(timer.tick) {          // Refer to the tick from timer area
      value := value + 1
    when(reset) {
      value := 0
  val stateMachine = new Area {


The ways you can use Scala functions to generate hardware are radically different than VHDL/Verilog for many reasons:

  • You can instantiate register, combinatorial logic and component inside them.

  • You don’t have to play with process/@always that limit the scope of assignment of signals

  • Everything work by reference, which allow many manipulation.
    For example you can give to a function an bus as argument, then the function can internaly read/write it.
    You can also return a Component, a Bus, are anything else from scala the scala world.

RGB to gray

For example if you want to convert a Red/Green/Blue color into a gray one by using coefficient, you can use functions to apply them :

// Input RGB color
val r,g,b = UInt(8 bits)

// Define a function to multiply a UInt by a scala Float value.
def coef(value : UInt,by : Float) : UInt = (value * U((255*by).toInt,8 bits) >> 8)

//Calculate the gray level
val gray = coef(r,0.3f) +
           coef(g,0.4f) +

Valid Ready Payload bus

For instance if you define a simple Valid Ready Payload bus, you can then define useful function inside it.

class MyBus(payloadWidth:  Int) extends Bundle {
  val valid = Bool()
  val ready = Bool()
  val payload = Bits(payloadWidth bits)

  //connect that to this
  def <<(that: MyBus) : Unit = {
    this.valid := that.valid
    that.ready := this.ready
    this.payload := that.payload

  // Connect this to the FIFO input, return the fifo output
  def queue(size: Int): MyBus = {
    val fifo = new Fifo(payloadWidth, size) << this

VHDL generation

There is a small component and a main that generate the corresponding VHDL.

// spinal.core contain all basics (Bool, UInt, Bundle, Reg, Component, ..)
import spinal.core._

//A simple component definition
class MyTopLevel extends Component {
  //Define some input/output. Bundle like a VHDL record or a verilog struct.
  val io = new Bundle {
    val a = in Bool()
    val b = in Bool()
    val c = out Bool()

  //Define some asynchronous logic
  io.c := io.a & io.b

//This is the main of the project. It create a instance of MyTopLevel and
//call the SpinalHDL library to flush it into a VHDL file.
object MyMain {
  def main(args: Array[String]) {
    SpinalVhdl(new MyTopLevel)

Instantiate VHDL and Verilog IP

In some cases, it could be useful to instantiate a VHDL or a Verilog component into a SpinalHDL design. To do that, you need to define BlackBox which is like a Component, but its internal implementation should be provided by a separate VHDL/Verilog file to the simulator/synthesis tool.

class Ram_1w_1r(_wordWidth: Int, _wordCount: Int) extends BlackBox {
  val generic = new Generic {
    val wordCount = _wordCount
    val wordWidth = _wordWidth

  val io = new Bundle {
    val clk = in Bool()

    val wr = new Bundle {
      val en = in Bool()
      val addr = in UInt (log2Up(_wordCount) bits)
      val data = in Bits (_wordWidth bits)
    val rd = new Bundle {
      val en = in Bool()
      val addr = in UInt (log2Up(_wordCount) bits)
      val data = out Bits (_wordWidth bits)



The SpinalHDL core contain some utils :




log2Up(x : BigInt)

Return the number of bit needed to represent x states


isPow2(x : BigInt)

Return true if x is a power of two


Much more tool and utils are present in spinal.lib