Provide API to set concrete type of implicit reset (#1361)

Introduces mutually-exclusive traits RequireAsyncReset and
RequireSyncReset to set the type of the implicit reset in
MultiIOModules. The Scala-type remains Reset, but the Chisel
elaboration-time checks apply.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

2 files changed, 36 insertions, 2 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/RawModule.scala b/chiselFrontend/src/main/scala/chisel3/RawModule.scala
index ae3b6fe7..407ed931 100644
--- a/chiselFrontend/src/main/scala/chisel3/RawModule.scala
+++ b/chiselFrontend/src/main/scala/chisel3/RawModule.scala
@@ -134,6 +134,14 @@ abstract class RawModule(implicit moduleCompileOptions: CompileOptions)
   }
 }
 
+trait RequireAsyncReset extends MultiIOModule {
+  override private[chisel3] def mkReset: AsyncReset = AsyncReset()
+}
+
+trait RequireSyncReset extends MultiIOModule {
+  override private[chisel3] def mkReset: Bool = Bool()
+}
+
 /** Abstract base class for Modules, which behave much like Verilog modules.
   * These may contain both logic and state which are written in the Module
   * body (constructor).
@@ -145,10 +153,12 @@ abstract class MultiIOModule(implicit moduleCompileOptions: CompileOptions)
     extends RawModule {
   // Implicit clock and reset pins
   val clock: Clock = IO(Input(Clock()))
-  val reset: Reset = {
+  val reset: Reset = IO(Input(mkReset))
+
+  private[chisel3] def mkReset: Reset = {
     // Top module and compatibility mode use Bool for reset
     val inferReset = _parent.isDefined && moduleCompileOptions.inferModuleReset
-    IO(Input(if (inferReset) Reset() else Bool()))
+    if (inferReset) Reset() else Bool()
   }
 
   // Setup ClockAndReset
diff --git a/src/test/scala/chiselTests/ResetSpec.scala b/src/test/scala/chiselTests/ResetSpec.scala
index 2a17d52f..d08be8fa 100644
--- a/src/test/scala/chiselTests/ResetSpec.scala
+++ b/src/test/scala/chiselTests/ResetSpec.scala
@@ -68,4 +68,28 @@ class ResetSpec extends ChiselFlatSpec {
     })
     async should include ("always @(posedge clk or posedge rst)")
   }
+
+  behavior of "Users"
+
+  they should "be able to force implicit reset to be synchronous" in {
+    val fir = generateFirrtl(new MultiIOModule with RequireSyncReset {
+      reset shouldBe a [Bool]
+    })
+    fir should include ("input reset : UInt<1>")
+  }
+
+  they should "be able to force implicit reset to be asynchronous" in {
+    val fir = generateFirrtl(new MultiIOModule with RequireAsyncReset {
+      reset shouldBe an [AsyncReset]
+    })
+    fir should include ("input reset : AsyncReset")
+  }
+
+  "Chisel" should "error if sync and async modules are nested" in {
+    a [ChiselException] shouldBe thrownBy {
+      elaborate(new MultiIOModule with RequireAsyncReset {
+        val mod = Module(new MultiIOModule with RequireSyncReset)
+      })
+    }
+  }
 }