Register defined as component input

Introduction

In SpinalHDL, you are not allowed to define a component that has a register as an input. The reasoning behind this is to prevent surprises when the user tries to drive the inputs of child components with the registered signal. If a registered input is desired, you will need to declare the unregistered input in the io bundle, and register the signal in the body of the component.

Example

The following code :

class TopLevel extends Component {
  val io = new Bundle {
    val a = in(Reg(UInt(8 bits)))
  }
}

will throw:

REGISTER DEFINED AS COMPONENT INPUT : (toplevel/io_a : in UInt[8 bits]) is defined as a registered input of the toplevel component, but isn't allowed.
  ***
  Source file location of the toplevel/io_a definition via the stack trace
  ***

A fix could be :

class TopLevel extends Component {
  val io = new Bundle {
    val a = in UInt(8 bits)
  }
}

If a registered a is required, it can be done like so:

class TopLevel extends Component {
  val io = new Bundle {
    val a = in UInt(8 bits)
  }
  val a = RegNext(io.a)
}