Asynchronous Reset (#1011)

Fixes #219

* Adds AsyncResetType (similar to ClockType)
* Registers with reset signal of type AsyncResetType are async reset
  registers
* Registers with async reset can only be reset to literal values
* Add initialization logic for async reset registers

1 files changed, 6 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/ir/IR.scala b/src/main/scala/firrtl/ir/IR.scala
index 4f647c0c..4c00bdd1 100644
--- a/src/main/scala/firrtl/ir/IR.scala
+++ b/src/main/scala/firrtl/ir/IR.scala
@@ -591,6 +591,12 @@ case object ClockType extends GroundType {
   def mapWidth(f: Width => Width): Type = this
   def foreachWidth(f: Width => Unit): Unit = Unit
 }
+case object AsyncResetType extends GroundType {
+  val width = IntWidth(1)
+  def serialize: String = "AsyncReset"
+  def mapWidth(f: Width => Width): Type = this
+  def foreachWidth(f: Width => Unit): Unit = Unit
+}
 case class AnalogType(width: Width) extends GroundType {
   def serialize: String = "Analog" + width.serialize
   def mapWidth(f: Width => Width): Type = AnalogType(f(width))