Frequent Errors

This page will talk about errors which could happen when people are using SpinalHDL.

Exception in thread “main” java.lang.NullPointerException

Console symptoms :

Exception in thread "main" java.lang.NullPointerException

Code Example :

val a = b + 1         // b can't be read at that time, because b isn't instantiated yet
val b = UInt(4 bits)

Issue explanation :

SpinalHDL is not a language, it is an Scala library, which mean, it obey to the same rules than the Scala general purpose programming language. When you run your SpinalHDL hardware description to generate the corresponding VHDL/Verilog RTL, your SpinalHDL hardware description will be executed as a Scala programm, and b will be a null reference until the programm execution come to that line, and it’s why you can’t use it before.

Hierarchy violation

The SpinalHDL compiler check that all your assignments are legal from an hierarchy perspective. Multiple cases are elaborated in following chapters

Signal X can’t be assigned by Y

Console symptoms :

Hierarchy violation : Signal X can't be assigned by Y

Code Example :

class ComponentX extends Component {
  ...
  val X = Bool()
  ...
}

class ComponentY extends Component {
  ...
  val componentX = new ComponentX
  val Y = Bool()
  componentX.X := Y // This assignment is not legal
  ...
}

class ComponentX extends Component {
  val io = new Bundle {
    val X = Bool() // Forgot to specify an in/out direction
  }
  ...
}

class ComponentY extends Component {
  ...
  val componentX = new ComponentX
  val Y = Bool()
  componentX.io.X := Y // This assignment will be detected as not legal
  ...
}

Issue explanation :

You can only assign input signals of subcomponents, else there is an hierarchy violation. If this issue happened, you probably forgot to specify the X signal’s direction.

Input signal X can’t be assigned by Y

Console symptoms :

Hierarchy violation : Input signal X can't be assigned by Y

Code Example :

class ComponentXY extends Component {
  val io = new Bundle {
    val X = in Bool()
  }
  ...
  val Y = Bool()
  io.X := Y // This assignment is not legal
  ...
}

Issue explanation :

You can only assign an input signals from the parent component, else there is an hierarchy violation. If this issue happened, you probably mixed signals direction declaration.

Output signal X can’t be assigned by Y

Console symptoms :

Hierarchy violation : Output signal X can't be assigned by Y

Code Example :

class ComponentX extends Component {
  val io = new Bundle {
    val X = out Bool()
  }
  ...
}

class ComponentY extends Component {
  ...
  val componentX = new ComponentX
  val Y = Bool()
  componentX.X := Y // This assignment is not legal
  ...
}

Issue explanation :

You can only assign output signals of a component from the inside of it, else there is an hierarchy violation. If this issue happened, you probably mixed signals direction declaration.