rocket-chip/src/main/scala/util/SynchronizingReg.scala

// See LICENSE.SiFive for license details.

package freechips.rocketchip.util

import Chisel._

/** These wrap behavioral 
  *  shift registers  into specific
  *  modules to allow for 
  *  backend flows to replace or constrain
  *  them properly when used for CDC synchronization,
  *  rather than buffering.
  *  
  *  The 3 different types vary in their reset behavior:
  *  AsyncResetShiftReg -- This is identical to the AsyncResetSynchronizerShiftReg, 
  *      it is just named differently
  *      to distinguish its use case. This is a ShiftRegister meant for timing, 
  *      not for synchronization.
  *  AsyncResetSynchronizerShiftReg -- asynchronously reset to 0
  *  SynchronizerShiftReg -- no reset, pipeline only.
  *  
  */

abstract class AbstractPipelineReg(w: Int = 1) extends Module {

  val io = new Bundle {
    val d = UInt(INPUT, width = w)
    val q = UInt(OUTPUT, width = w)
  }

}

object AbstractPipelineReg {

  def apply [T <: Chisel.Data](gen: => AbstractPipelineReg, in: T, name: Option[String] = None): T = {
    val chain = Module(gen)
    name.foreach{ chain.suggestName(_) }
    chain.io.d := in.asUInt

    chain.io.q.asTypeOf(in)
  }
}

class AsyncResetShiftReg(w: Int = 1, depth: Int = 1, init: Int = 0, name: String = "pipe") extends AbstractPipelineReg(w) {
  require(depth > 0, "Depth must be greater than 0.")

  override def desiredName = s"AsyncResetShiftReg_w${w}_d${depth}_i${init}"

  val chain = List.tabulate(depth) { i =>
    Module (new AsyncResetRegVec(w, init)).suggestName(s"${name}_${i}")
  }

  chain.last.io.d := io.d
  chain.last.io.en := Bool(true)

  (chain.init zip chain.tail).foreach { case (sink, source) =>
    sink.io.d := source.io.q
    sink.io.en := Bool(true)
  }
  io.q := chain.head.io.q

}

object AsyncResetShiftReg {

  def apply [T <: Chisel.Data](in: T, depth: Int = 1, init: Int = 0, name: Option[String] = None ): T =
    AbstractPipelineReg(gen = {new AsyncResetShiftReg(in.getWidth, depth, init)}, in, name)
}

//Note that it is important to ovveride "name" in order to ensure that the Chisel dedup does
// not try to merge instances of this with instances of the superclass.
class AsyncResetSynchronizerShiftReg(w: Int = 1, sync: Int = 3) extends AsyncResetShiftReg(w, depth = sync, name = "sync") {
  require(sync > 0, "Sync must be greater than 0.")
  override def desiredName = s"AsyncResetSynchronizerShiftReg_w${w}_d${sync}"
}

object AsyncResetSynchronizerShiftReg {

  def apply [T <: Chisel.Data](in: T, sync: Int = 3, name: Option[String] = None): T =
    AbstractPipelineReg(gen = {new AsyncResetSynchronizerShiftReg(in.getWidth, sync)}, in, name)
}

class SynchronizerShiftReg(w: Int = 1, sync: Int = 3) extends AbstractPipelineReg(w) {
  require(sync > 0, "Sync must be greater than 0.")

  override def desiredName = s"SynchronizerShiftReg_w${w}_d${sync}"

  val syncv = List.tabulate(sync) { i =>
    val r = Reg(UInt(width = w))
    r.suggestName(s"sync_${i}")
  }

  syncv.last := io.d

  (syncv.init zip syncv.tail).foreach { case (sink, source) =>
    sink := source
  }
  io.q := syncv.head

}

object SynchronizerShiftReg {

  def apply [T <: Chisel.Data](in: T, sync: Int = 3, name: Option[String] = None): T =
    AbstractPipelineReg(gen = { new SynchronizerShiftReg(in.getWidth, sync)}, in, name)
}