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No driver on

Introduction

SpinalHDL will check that all combinatorial signals which have impacts on the design are assigned by something.

Example

The following code :

class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = UInt(8 bits)
  result := a
}

will throw :

NO DRIVER ON (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***

A fix could be :

class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = UInt(8 bits)
  a := 42
  result := a
}