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Add narrowData option to AsyncQueue.

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This option reduces the number of wires that cross the clock boundary.
This can be a useful feature if the clock boundary coincides with
a voltage boundary, in which case the number of level shifters is reduced.
However, this introduces a path that crosses from sink->source->sink domain,
so the option is disabled by default.
This commit is contained in:
Ben Keller 2017-04-21 16:31:17 -07:00
parent c72b15f2a0
commit 0aa8f7d61d
1 changed files with 16 additions and 9 deletions
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25
src/main/scala/util/AsyncQueue.scala
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@ -35,7 +35,7 @@ class AsyncValidSync(sync: Int, desc: String) extends Module {
io.out := UIntSyncChain(io.in.asUInt, sync, desc)(0)
}
class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = true) extends Module {
class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = true, narrowData: Boolean = false) extends Module {
val bits = log2Ceil(depth)
val io = new Bundle {
// These come from the source domain
@ -43,7 +43,8 @@ class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean =
// 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 = Vec(if(narrowData) 1 else depth, gen).asOutput
val index = narrowData.option(UInt(INPUT, width = bits))
// Signals used to self-stabilize a safe AsyncQueue
val sink_reset_n = Bool(INPUT)
val ridx_valid = Bool(INPUT)
@ -65,7 +66,7 @@ class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean =
val widx_reg = AsyncResetReg(widx, "widx_gray")
io.widx := widx_reg
io.mem := mem
if(narrowData) io.mem(0) := mem(io.index.get) else io.mem := mem
io.widx_valid := Bool(true)
if (safe) {
@ -92,7 +93,7 @@ class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean =
}
}
class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = true) extends Module {
class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = true, narrowData: Boolean = false) extends Module {
val bits = log2Ceil(depth)
val io = new Bundle {
// These come from the sink domain
@ -100,7 +101,8 @@ class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = t
// 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 mem = Vec(if(narrowData) 1 else depth, gen).asInput
val index = narrowData.option(UInt(OUTPUT, width = bits))
// Signals used to self-stabilize a safe AsyncQueue
val source_reset_n = Bool(INPUT)
val ridx_valid = Bool(OUTPUT)
@ -117,11 +119,12 @@ class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = t
// 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))
if(narrowData) io.index.get := index
// This register does not NEED to be reset, as its contents will not
// be considered unless the asynchronously reset deq valid register is set.
// It is possible that bits latches when the source domain is reset / has power cut
// This is safe, because isolation gates brought mem low before the zeroed widx reached us
io.deq.bits := RegEnable(io.mem(index), valid)
io.deq.bits := RegEnable(io.mem(if(narrowData) UInt(0) else index), valid)
val valid_reg = AsyncResetReg(valid.asUInt, "valid_reg")(0)
io.deq.valid := valid_reg && source_ready
@ -154,13 +157,16 @@ class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = t
}
}
class AsyncQueue[T <: Data](gen: T, depth: Int = 8, sync: Int = 3, safe: Boolean = true) extends Crossing[T] {
// If narrowData is true then the read mux is moved to the source side of the crossing.
// This reduces the number of level shifters in the case where the clock crossing is also a voltage crossing,
// at the expense of a combinational path from the sink to the source and back to the sink.
class AsyncQueue[T <: Data](gen: T, depth: Int = 8, sync: Int = 3, safe: Boolean = true, narrowData: Boolean = false) extends Crossing[T] {
require (sync >= 2)
require (depth > 0 && isPow2(depth))
val io = new CrossingIO(gen)
val source = Module(new AsyncQueueSource(gen, depth, sync, safe))
val sink = Module(new AsyncQueueSink (gen, depth, sync, safe))
val source = Module(new AsyncQueueSource(gen, depth, sync, safe, narrowData))
val sink = Module(new AsyncQueueSink (gen, depth, sync, safe, narrowData))
source.clock := io.enq_clock
source.reset := io.enq_reset
@ -174,6 +180,7 @@ class AsyncQueue[T <: Data](gen: T, depth: Int = 8, sync: Int = 3, safe: Boolean
io.deq <> sink.io.deq
sink.io.mem := source.io.mem
if(narrowData) source.io.index.get <> sink.io.index.get
sink.io.widx := source.io.widx
source.io.ridx := sink.io.ridx
sink.io.widx_valid := source.io.widx_valid