junctions: add an AsyncQueue

src/main/scala/junctions/asyncqueue.scala

@@ -0,0 +1,88 @@
+// See LICENSE for license details.
+
+package junctions
+import Chisel._
+
+object GrayCounter {
+  def apply(bits: Int, increment: Bool = Bool(true)): UInt = {
+    val binary = RegInit(UInt(0, width = bits))
+    val incremented = binary + increment.asUInt()
+    binary := incremented
+    incremented ^ (incremented >> UInt(1))
+  }
+}
+
+object AsyncGrayCounter {
+  def apply(in: UInt, sync: Int): UInt = {
+    val syncv = RegInit(Vec.fill(sync){UInt(0, width = in.getWidth)})
+    syncv.last := in
+    (syncv.init zip syncv.tail).foreach { case (sink, source) => sink := source }
+    syncv(0)
+  }
+}
+
+class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, clockIn: Clock, resetIn: Bool)
+    extends Module(_clock = clockIn, _reset = resetIn) {
+  val bits = log2Ceil(depth)
+  val io = new Bundle {
+    // These come from the source domain
+    val enq  = Decoupled(gen).flip()
+    // These cross to the sink clock domain
+    val ridx = UInt(INPUT,  width = bits+1)
+    val widx = UInt(OUTPUT, width = bits+1)
+    val mem  = Vec(depth, gen).asOutput
+  }
+
+  val mem = Reg(Vec(depth, gen))
+  val widx = GrayCounter(bits+1, io.enq.fire())
+  val ridx = AsyncGrayCounter(io.ridx, sync)
+  val ready = widx =/= (ridx ^ UInt(depth | depth >> 1))
+
+  val index = if (depth == 1) UInt(0) else io.widx(bits-1, 0) ^ (io.widx(bits, bits) << (bits-1))
+  when (io.enq.fire() && !reset) { mem(index) := io.enq.bits }
+  io.enq.ready := RegNext(ready, Bool(false))
+  io.widx := RegNext(widx, UInt(0))
+  io.mem := mem
+}
+
+class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, clockIn: Clock, resetIn: Bool)
+    extends Module(_clock = clockIn, _reset = resetIn) {
+  val bits = log2Ceil(depth)
+  val io = new Bundle {
+    // These come from the sink domain
+    val deq  = Decoupled(gen)
+    // These cross to the source clock domain
+    val ridx = UInt(OUTPUT, width = bits+1)
+    val widx = UInt(INPUT,  width = bits+1)
+    val mem  = Vec(depth, gen).asInput
+  }
+
+  val ridx = GrayCounter(bits+1, io.deq.fire())
+  val widx = AsyncGrayCounter(io.widx, sync)
+  val valid = ridx =/= widx
+
+  // The mux is safe because timing analysis ensures ridx has reached the register
+  // On an ASIC, changes to the unread location cannot affect the selected value
+  // On an FPGA, only one input changes at a time => mem updates don't cause glitches
+  // The register only latches when the selected valued is not being written
+  val index = if (depth == 1) UInt(0) else ridx(bits-1, 0) ^ (ridx(bits, bits) << (bits-1))
+  io.deq.bits  := RegEnable(io.mem(index), valid && !reset)
+  io.deq.valid := RegNext(valid, Bool(false))
+  io.ridx := RegNext(ridx, UInt(0))
+}
+
+class AsyncQueue[T <: Data](gen: T, depth: Int = 8, sync: Int = 3) extends Crossing[T] {
+  require (sync >= 2)
+  require (depth > 0 && isPow2(depth))
+
+  val io = new CrossingIO(gen)
+  val source = Module(new AsyncQueueSource(gen, depth, sync, io.enq_clock, io.enq_reset))
+  val sink   = Module(new AsyncQueueSink  (gen, depth, sync, io.deq_clock, io.deq_reset))
+
+  source.io.enq <> io.enq
+  io.deq <> sink.io.deq
+
+  sink.io.mem := source.io.mem
+  sink.io.widx := source.io.widx
+  source.io.ridx := sink.io.ridx
+}