Add shift register with reset (#439)

* [stevo]: add reset initialization to shift register

* [stevo]: better comment

* [stevo]: add tests, fix bug

2 files changed, 48 insertions, 0 deletions

diff --git a/src/main/scala/chisel3/util/Reg.scala b/src/main/scala/chisel3/util/Reg.scala
index 00005e3a..27785dfb 100644
--- a/src/main/scala/chisel3/util/Reg.scala
+++ b/src/main/scala/chisel3/util/Reg.scala
@@ -61,4 +61,20 @@ object ShiftRegister
       in
     }
   }
+
+  /** Returns the n-cycle delayed version of the input signal with reset initialization.
+    *
+    * @param in input to delay
+    * @param n number of cycles to delay
+    * @param resetData reset value for each register in the shift
+    * @param en enable the shift
+    */
+  def apply[T <: Data](in: T, n: Int, resetData: T, en: Bool): T = {
+    // The order of tests reflects the expected use cases.
+    if (n != 0) {
+      RegEnable(apply(in, n-1, resetData, en), resetData, en)
+    } else {
+      in
+    }
+  }
 }
diff --git a/src/test/scala/chiselTests/Reg.scala b/src/test/scala/chiselTests/Reg.scala
index 43e64fe7..ef66c30a 100644
--- a/src/test/scala/chiselTests/Reg.scala
+++ b/src/test/scala/chiselTests/Reg.scala
@@ -4,7 +4,9 @@ package chiselTests
 
 import firrtl.ir.Input
 import org.scalatest._
+import org.scalatest.prop._
 import chisel3._
+import chisel3.util._
 import chisel3.core.DataMirror
 import chisel3.testers.BasicTester
 
@@ -46,3 +48,33 @@ class RegSpec extends ChiselFlatSpec {
     elaborate{ new RegForcedWidthTester }
   }
 }
+
+class ShiftTester(n: Int) extends BasicTester {
+  val (cntVal, done) = Counter(true.B, n)
+  val start = 23.U
+  val sr = ShiftRegister(cntVal + start, n)
+  when(done) {
+    assert(sr === start)
+    stop()
+  }
+}
+
+class ShiftResetTester(n: Int) extends BasicTester {
+  val (cntVal, done) = Counter(true.B, n-1)
+  val start = 23.U
+  val sr = ShiftRegister(cntVal + 23.U, n, 1.U, true.B)
+  when(done) {
+    assert(sr === 1.U)
+    stop()
+  }
+}
+
+class ShiftRegisterSpec extends ChiselPropSpec {
+  property("ShiftRegister should shift") {
+    forAll(smallPosInts) { (shift: Int) => assertTesterPasses{ new ShiftTester(shift) } }
+  }
+
+  property("ShiftRegister should reset all values inside") {
+    forAll(smallPosInts) { (shift: Int) => assertTesterPasses{ new ShiftResetTester(shift) } }
+  }
+}