Hierarchy violation

Introduction

SpinalHDL will check that signals are never accessed outside of the current component’s scope.

The following signals can be read inside a component:

All directionless signals defined in the current component
All in/out/inout signals of the current component
All in/out/inout signals of child components

In addition, the following signals can be assigned to inside of a component:

All directionless signals defined in the current component
All out/inout signals of the current component
All in/inout signals of child components

If a HIERARCHY VIOLATION error appears, it means that one of the above rules was violated.

Example

The following code:

class TopLevel extends Component {
  val io = new Bundle {
    // This is an 'in' signal of the current component 'Toplevel'
    val a = in UInt(8 bits)
  }
  val tmp = U"x42"
  io.a := tmp  // ERROR: attempting to assign to an input of current component
}

will throw:

HIERARCHY VIOLATION : (toplevel/io_a : in UInt[8 bits]) is driven by (toplevel/tmp :  UInt[8 bits]), but isn't accessible in the toplevel component.
  ***
  Source file location of the `io.a := tmp` via the stack trace
  ***

A fix could be :

class TopLevel extends Component {
  val io = new Bundle {
    val a = out UInt(8 bits) // changed from in to out
  }
  val tmp = U"x42"
  io.a := tmp  // now we are assigning to an output
}