Move Deprecated LFSR16 to Compatibility

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

1 files changed, 35 insertions, 1 deletions

diff --git a/src/main/scala/chisel3/compatibility.scala b/src/main/scala/chisel3/compatibility.scala
index 9584fad6..0c4c18a9 100644
--- a/src/main/scala/chisel3/compatibility.scala
+++ b/src/main/scala/chisel3/compatibility.scala
@@ -4,6 +4,8 @@
   *  while moving to the more standard package naming convention `chisel3` (lowercase c).
   */
 import chisel3._    // required for implicit conversions.
+import chisel3.experimental.chiselName
+import chisel3.util.random.FibonacciLFSR
 
 package object Chisel {     // scalastyle:ignore package.object.name number.of.types number.of.methods
   import chisel3.internal.firrtl.Width
@@ -561,7 +563,39 @@ package object Chisel {     // scalastyle:ignore package.object.name number.of.t
     }
   }
 
-  val LFSR16 = chisel3.util.LFSR16
+  /** LFSR16 generates a 16-bit linear feedback shift register, returning the register contents.
+    * This is useful for generating a pseudo-random sequence.
+    *
+    * The example below, taken from the unit tests, creates two 4-sided dice using `LFSR16` primitives:
+    * @example {{{
+    *   val bins = Reg(Vec(8, UInt(32.W)))
+    *
+    *   // Create two 4 sided dice and roll them each cycle.
+    *   // Use tap points on each LFSR so values are more independent
+    *   val die0 = Cat(Seq.tabulate(2) { i => LFSR16()(i) })
+    *   val die1 = Cat(Seq.tabulate(2) { i => LFSR16()(i + 2) })
+    *
+    *   val rollValue = die0 +& die1  // Note +& is critical because sum will need an extra bit.
+    *
+    *   bins(rollValue) := bins(rollValue) + 1.U
+    *
+    * }}}
+    */
+  // scalastyle:off magic.number
+  object LFSR16 {
+    /** Generates a 16-bit linear feedback shift register, returning the register contents.
+      * @param increment optional control to gate when the LFSR updates.
+      */
+    @chiselName
+    def apply(increment: Bool = true.B): UInt =
+      VecInit( FibonacciLFSR
+                .maxPeriod(16, increment, seed = Some(BigInt(1) << 15))
+                .asBools
+                .reverse )
+        .asUInt
+
+  }
+  // scalastyle:on magic.number
 
   val ListLookup = chisel3.util.ListLookup
   val Lookup = chisel3.util.Lookup