跨时钟域违例(Clock crossing violation)

简介

SpinalHDL将检查您的设计中的每个寄存器是否仅（通过组合逻辑路径）与相同或同步时钟域的寄存器连接。

示例

下面的代码：

class TopLevel extends Component {
  val clkA = ClockDomain.external("clkA")
  val clkB = ClockDomain.external("clkB")

  val regA = clkA(Reg(UInt(8 bits)))   // PlayDev.scala:834
  val regB = clkB(Reg(UInt(8 bits)))   // PlayDev.scala:835

  val tmp = regA + regA                // PlayDev.scala:838
  regB := tmp
}

会报错：

CLOCK CROSSING VIOLATION from (toplevel/regA :  UInt[8 bits]) to (toplevel/regB :  UInt[8 bits]).
- Register declaration at
  ***
  Source file location of the toplevel/regA definition via the stack trace
  ***
- through
      >>> (toplevel/regA :  UInt[8 bits]) at ***(PlayDev.scala:834) >>>
      >>> (toplevel/tmp :  UInt[8 bits]) at ***(PlayDev.scala:838) >>>
      >>> (toplevel/regB :  UInt[8 bits]) at ***(PlayDev.scala:835) >>>

有多种可能的修复方法，如下所示：

• crossClockDomain 标签

• setSynchronousWith 方法

• BufferCC 类型

crossClockDomain标签

标签 crossClockDomain 可用于向 SpinalHDL 编译器传达“没关系，不要对这个特定的跨时钟域操作感到恐慌”的信息。

class TopLevel extends Component {
  val clkA = ClockDomain.external("clkA")
  val clkB = ClockDomain.external("clkB")

  val regA = clkA(Reg(UInt(8 bits)))
  val regB = clkB(Reg(UInt(8 bits))).addTag(crossClockDomain)


  val tmp = regA + regA
  regB := tmp
}

setSynchronousWith

您还可以使用 ClockDomain 对象的 setSynchronousWith 方法指定两个时钟域同步。

class TopLevel extends Component {
  val clkA = ClockDomain.external("clkA")
  val clkB = ClockDomain.external("clkB")
  clkB.setSynchronousWith(clkA)

  val regA = clkA(Reg(UInt(8 bits)))
  val regB = clkB(Reg(UInt(8 bits)))


  val tmp = regA + regA
  regB := tmp
}

BufferCC

当交换单比特信号（如 Bool 类型）或格雷码时，您可以使用 BufferCC 安全地跨不同的 ClockDomain 时钟域。

警告

不要将 BufferCC 用于多比特信号，因为如果时钟异步，那么接收端会存在读取损坏的风险。有关更多详细信息，请参阅 时钟域 页面。

class AsyncFifo extends Component {
   val popToPushGray = Bits(ptrWidth bits)
   val pushToPopGray = Bits(ptrWidth bits)

   val pushCC = new ClockingArea(pushClock) {
     val pushPtr     = Counter(depth << 1)
     val pushPtrGray = RegNext(toGray(pushPtr.valueNext)) init(0)
     val popPtrGray  = BufferCC(popToPushGray, B(0, ptrWidth bits))
     val full        = isFull(pushPtrGray, popPtrGray)
     ...
   }

   val popCC = new ClockingArea(popClock) {
     val popPtr      = Counter(depth << 1)
     val popPtrGray  = RegNext(toGray(popPtr.valueNext)) init(0)
     val pushPtrGray = BufferCC(pushToPopGray, B(0, ptrWidth bits))
     val empty       = isEmpty(popPtrGray, pushPtrGray)
     ...
   }
}