From 6aff3f26db8d8477ed7cab43b9dc03473627b172 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Mon, 28 Mar 2022 10:36:35 -0700
Subject: Integrate wrap bits into grey code for FIFO.

---
 hdl/fifo.sv | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

(limited to 'hdl')

diff --git a/hdl/fifo.sv b/hdl/fifo.sv
index a067914..374f051 100644
--- a/hdl/fifo.sv
+++ b/hdl/fifo.sv
@@ -19,13 +19,14 @@ module fifo
 
     typedef bit [WIDTH_BITS-1:0]    data_t;
     typedef bit [DEPTH_BITS-1:0]    addr_t;
+    typedef bit [DEPTH_BITS:0]      grey_t;
 
     data_t  data    [DEPTH-1:0];
 
     addr_t  oldest, youngest;
     bit     oldest_wrap, youngest_wrap;
 
-    addr_t  oldest_grey, youngest_grey;
+    grey_t  oldest_grey, oldest_plus_one_wrap_grey, youngest_grey;
 
     always @(posedge clock_in, posedge reset) begin
         if (reset) begin
@@ -40,9 +41,9 @@ module fifo
                     ++youngest_wrap;
             end
 
-            youngest_grey = youngest ^ (youngest >> 1);
+            youngest_grey = {youngest_wrap, youngest} ^ ({youngest_grey, youngest} >> 1);
 
-            in_ready = oldest_grey != youngest_grey || youngest_wrap == oldest_wrap;
+            in_ready = oldest_plus_one_wrap_grey != youngest_grey;
         end
     end
 
@@ -52,15 +53,17 @@ module fifo
             oldest = 0;
             oldest_wrap = 0;
             oldest_grey = 0;
+            oldest_plus_one_wrap_grey = {!oldest_wrap, oldest} ^ ({!oldest_wrap, oldest} >> 1);
         end else begin
             if (out_ready && out_valid) begin
                 if (++oldest == 0)
                     ++oldest_wrap;
             end
 
-            oldest_grey = oldest ^ (oldest >> 1);
+            oldest_grey = {oldest_grey, oldest} ^ ({oldest_grey, oldest} >> 1);
+            oldest_plus_one_wrap_grey = {!oldest_wrap, oldest} ^ ({!oldest_wrap, oldest} >> 1);
 
-            out_valid = oldest_grey != youngest_grey || youngest_wrap != oldest_wrap;
+            out_valid = oldest_grey != youngest_grey;
             out_data = data[oldest];
         end
     end
-- 
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