Add support for asynchronous reset (#1011)

Adds new AsyncReset and "abstract" Reset types. Reset is inferred
in FIRRTL to be either AsyncReset or Bool. The "reset type" of a
register is set by the type of its reset signal:

  val asyncReset: AsyncReset = IO(Input(AsyncReset()))
  val syncReset: Bool = IO(Input(Bool()))
  val abstractReset: Reset = IO(Input(Reset()))

  val asyncReg = withReset(asyncReset) { RegInit(0.U) }
  val syncReg = withReset(syncReset) { RegInit(0.U) }
  val inferredReg = withReset(abstractReset) { RegInit(0.U) }

AsyncReset can be cast to and from Bool. Whereas synchronous reset is
equivalent to a mux in front of a flip-flop and thus can be driven by
logic, asynchronous reset requires that the reset value is a constant.
This is checked in FIRRTL.

Inference of the concrete type of a Reset occurs based on the type the
Reset's drivers. This inference is very simple, it is simple forward propagation
of the type, but it allows for writing blocks and modules that are agnostic
to the reset type. In particular, the implicit `reset` value in MultiIOModule
and thus Module is now concretely an instance of Reset and thus will be
inferred in FIRRTL.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/scala/examples/SimpleVendingMachine.scala b/src/test/scala/examples/SimpleVendingMachine.scala
index 4bb6027a..2021ece8 100644
--- a/src/test/scala/examples/SimpleVendingMachine.scala
+++ b/src/test/scala/examples/SimpleVendingMachine.scala
@@ -52,7 +52,7 @@ class VerilogVendingMachine extends BlackBox {
   // Because this is a blackbox, we must explicity add clock and reset
   val io = IO(new SimpleVendingMachineIO {
     val clock = Input(Clock())
-    val reset = Input(Bool())
+    val reset = Input(Reset())
   })
 }