From 4541224c007a1818ffdcd69a7dc4a8e5392bb44d Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sat, 26 Mar 2022 09:50:26 -0700
Subject: Add basic clock-domain-crossing FIFO.

---
 hdl/fifo.sv | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 hdl/fifo.sv

diff --git a/hdl/fifo.sv b/hdl/fifo.sv
new file mode 100644
index 0000000..4158067
--- /dev/null
+++ b/hdl/fifo.sv
@@ -0,0 +1,68 @@
+module fifo
+    #(  type    data_t  = bit
+
+    ,           DEPTH_BITS  = 10
+    )
+    (   input   bit     clock_in
+    ,   input   bit     clock_out
+    ,   input   bit     reset
+
+    ,   output  bit     in_ready
+    ,   input   bit     in_valid
+    ,   input   data_t  in_data
+
+    ,   input   bit     out_ready
+    ,   output  bit     out_valid
+    ,   output  data_t  out_data
+    );
+
+    localparam  DEPTH   = 1<<DEPTH_BITS;
+
+    typedef bit [DEPTH_BITS-1:0]    addr_t;
+
+    data_t  data    [DEPTH-1:0];
+
+    addr_t  oldest, youngest;
+    bit     oldest_wrap, youngest_wrap;
+
+    addr_t  oldest_grey, youngest_grey;
+
+    always @(posedge clock_in, posedge reset) begin
+        if (reset) begin
+            in_ready = 0;
+            youngest = 0;
+            youngest_wrap = 0;
+            youngest_grey = 0;
+        end else begin
+            if (in_ready && in_valid) begin
+                data[youngest] = in_data;
+                if (++youngest == 0)
+                    ++youngest_wrap;
+            end
+
+            youngest_grey = youngest ^ (youngest >> 1);
+
+            in_ready = oldest_grey != youngest_grey || youngest_wrap == oldest_wrap;
+        end
+    end
+
+    always @(posedge clock_out, posedge reset) begin
+        if (reset) begin
+            out_valid = 0;
+            oldest = 0;
+            oldest_wrap = 0;
+            oldest_grey = 0;
+        end else begin
+            if (out_ready && out_valid) begin
+                if (++oldest == 0)
+                    ++oldest_wrap;
+            end
+
+            oldest_grey = oldest ^ (oldest >> 1);
+
+            out_valid = oldest_grey != youngest_grey || youngest_wrap != oldest_wrap;
+            out_data = data[oldest];
+        end
+    end
+
+endmodule
-- 
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