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未赋值的寄存器(Unassigned register)

简介

SpinalHDL将检查所有影响设计的寄存器是否已在某处被赋值。

示例

下面的代码：

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

会报错：

UNASSIGNED REGISTER (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***

一个可能的修复方法是：

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

只有初始化(init)的寄存器

在某些情况下，由于设计参数化，生成一个没有赋值而只有 init 语句的寄存器可能是有意义的。

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

  if(something)
    a := somethingElse
  result := a
}

会报错：

UNASSIGNED REGISTER (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***

要修复这个问题，如果寄存器有一个 init 语句但没有赋值，你可以让SpinalHDL将该未赋值的寄存器转换为组合逻辑：

class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = Reg(UInt(8 bits)).init(42).allowUnsetRegToAvoidLatch

  if(something)
    a := somethingElse
  result := a
}