diff --git a/src/main/scala/util/RationalCrossing.scala b/src/main/scala/util/RationalCrossing.scala
index 8a0f5448..26a7673d 100644
--- a/src/main/scala/util/RationalCrossing.scala
+++ b/src/main/scala/util/RationalCrossing.scala
@@ -7,6 +7,32 @@
 package util
 import Chisel._
 
+// A rational crossing must put registers on the slow side.
+// This trait covers the options of how/where to put the registers.
+// BEWARE: the source+sink must agree on the direction!
+sealed trait RationalDirection {
+  def flip: RationalDirection
+}
+
+// If it's unclear which side will be slow (or it is variable),
+// place registers on both sides of the crossing, by splitting
+// a Queue into flow and pipe parts on either side. This is safe
+// for all possible clock ratios, but has the downside that the
+// path from the slow domain must close timing in the fast domain.
+case object Symmetric extends RationalDirection {
+  def flip = Symmetric
+}
+
+// If the source is fast, place the registers at the sink.
+case object FastToSlow extends RationalDirection {
+  def flip = SlowToFast
+}
+
+// If the source is slow, place the registers at the source.
+case object SlowToFast extends RationalDirection {
+  def flip = FastToSlow
+}
+
 final class RationalIO[T <: Data](gen: T) extends Bundle
 {
   val bits   = gen.chiselCloneType
@@ -23,15 +49,19 @@ object RationalIO
   def apply[T <: Data](gen: T) = new RationalIO(gen)
 }
 
-class RationalCrossingSource[T <: Data](gen: T) extends Module
+class RationalCrossingSource[T <: Data](gen: T, direction: RationalDirection = Symmetric) extends Module
 {
   val io = new Bundle {
     val enq = DecoupledIO(gen).flip
     val deq = RationalIO(gen)
   }
 
-  val enq = Queue(io.enq, 1, flow=true)
   val deq = io.deq
+  val enq = direction match {
+    case Symmetric  => Queue(io.enq, 1, flow=true)
+    case FastToSlow => io.enq
+    case SlowToFast => Queue(io.enq, 2)
+  }
 
   val count = RegInit(UInt(0, width = 2))
   val equal = count === deq.sink
@@ -42,9 +72,16 @@ class RationalCrossingSource[T <: Data](gen: T) extends Module
   enq.ready  := Mux(equal, deq.ready, count(1) =/= deq.sink(0))
 
   when (enq.fire()) { count := Cat(count(0), !count(1)) }
+
+  // Ensure the clocking is setup correctly
+  direction match {
+    case Symmetric  => () // always safe
+    case FastToSlow => assert (equal || count(1) === deq.sink(0))
+    case SlowToFast => assert (equal || count(1) =/= deq.sink(0))
+  }
 }
 
-class RationalCrossingSink[T <: Data](gen: T) extends Module
+class RationalCrossingSink[T <: Data](gen: T, direction: RationalDirection = Symmetric) extends Module
 {
   val io = new Bundle {
     val enq = RationalIO(gen).flip
@@ -53,7 +90,11 @@ class RationalCrossingSink[T <: Data](gen: T) extends Module
 
   val enq = io.enq
   val deq = Wire(io.deq)
-  io.deq <> Queue(deq, 1, pipe=true)
+  direction match {
+    case Symmetric  => io.deq <> Queue(deq, 1, pipe=true)
+    case FastToSlow => io.deq <> Queue(deq, 2)
+    case SlowToFast => io.deq <> deq
+  }
 
   val count = RegInit(UInt(0, width = 2))
   val equal = count === enq.source
@@ -64,14 +105,21 @@ class RationalCrossingSink[T <: Data](gen: T) extends Module
   deq.valid := Mux(equal, enq.valid, count(1) =/= enq.source(0))
 
   when (deq.fire()) { count := Cat(count(0), !count(1)) }
+
+  // Ensure the clocking is setup correctly
+  direction match {
+    case Symmetric  => () // always safe
+    case FastToSlow => assert (equal || count(1) =/= enq.source(0))
+    case SlowToFast => assert (equal || count(1) === enq.source(0))
+  }
 }
 
-class RationalCrossing[T <: Data](gen: T) extends Module
+class RationalCrossing[T <: Data](gen: T, direction: RationalDirection = Symmetric) extends Module
 {
   val io = new CrossingIO(gen)
 
-  val source = Module(new RationalCrossingSource(gen))
-  val sink   = Module(new RationalCrossingSink(gen))
+  val source = Module(new RationalCrossingSource(gen, direction))
+  val sink   = Module(new RationalCrossingSink(gen, direction))
 
   source.clock := io.enq_clock
   source.reset := io.enq_reset
@@ -84,8 +132,8 @@ class RationalCrossing[T <: Data](gen: T) extends Module
 
 object ToRational
 {
-  def apply[T <: Data](x: DecoupledIO[T]): RationalIO[T] = {
-    val source = Module(new RationalCrossingSource(x.bits))
+  def apply[T <: Data](x: DecoupledIO[T], direction: RationalDirection = Symmetric): RationalIO[T] = {
+    val source = Module(new RationalCrossingSource(x.bits, direction))
     source.io.enq <> x
     source.io.deq
   }
@@ -93,8 +141,8 @@ object ToRational
 
 object FromRational
 {
-  def apply[T <: Data](x: RationalIO[T]): DecoupledIO[T] = {
-    val sink = Module(new RationalCrossingSink(x.bits))
+  def apply[T <: Data](x: RationalIO[T], direction: RationalDirection = Symmetric): DecoupledIO[T] = {
+    val sink = Module(new RationalCrossingSink(x.bits, direction))
     sink.io.enq <> x
     sink.io.deq
   }