Clock crossing violation

Introduction

SpinalHDL will check that every register of your design only depends (through combinational logic paths) on registers which use the same or a synchronous clock domain.

Example

The following code:

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
}

will throw:

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) >>>

There are multiple possible fixes, listed below:

crossClockDomain tags

setSyncronousWith method

BufferCC type

crossClockDomain tag

The crossClockDomain tag can be used to communicate “It’s alright, don’t panic about this specific clock crossing” to the SpinalHDL compiler.

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
}

setSyncronousWith

You can also specify that two clock domains are synchronous together by using the setSynchronousWith method of one of the ClockDomain objects.

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

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


  val tmp = regA + regA
  regB := tmp
}

BufferCC

When exchanging single-bit signals (such as Bool types), or Gray-coded values, you can use BufferCC to safely cross different ClockDomain regions.

Warning

Do not use BufferCC with multi-bit signals, as there is a risk of corrupted reads on the receiving side if the clocks are asynchronous. See the Clock Domains page for more details.

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 bit))
     val empty       = isEmpty(popPtrGray, pushPtrGray)
     ...
   }
}