fifo randomizer for emi

1 files changed, 66 insertions, 0 deletions

diff --git a/usrp2/extramfifo/refill_randomizer.v b/usrp2/extramfifo/refill_randomizer.v
new file mode 100644
index 000000000..0b30f4049
--- /dev/null
+++ b/usrp2/extramfifo/refill_randomizer.v
@@ -0,0 +1,66 @@
+//
+// EMI mitigation.
+// Process FULL flag from FIFO so that de-assertion
+// (FIFO now not FULL) is delayed by a pseudo random
+// value, but assertion is passed straight through.
+// 
+
+
+module refill_randomizer
+  #(parameter BITS=7)
+    (
+     input clk,
+     input rst,
+     input full_in,
+     output full_out
+     );
+   
+   wire 	    feedback;
+   reg 	    full_last;
+   wire     full_deasserts;
+   reg [6:0] shift_reg;
+   reg [6:0] count;
+   reg 	     delayed_fall;
+   
+
+   always @(posedge clk)
+     full_last <= full_in;
+   
+   assign    full_deasserts = full_last & ~full_in;
+
+   // 7 bit LFSR
+   always @(posedge clk)
+     if (rst)
+       shift_reg <= 7'b1;
+     else
+       if (full_deasserts)
+	 shift_reg <= {shift_reg[5:0],feedback};
+
+   assign    feedback = ^(shift_reg & 7'h41);
+
+   always @(posedge clk)
+     if (rst)
+       begin
+	  count <= 1;
+	  delayed_fall  <= 1;
+       end
+     else if (full_deasserts)
+       begin
+	  count <= shift_reg;
+	  delayed_fall <= 1;
+       end
+     else if (count == 1)
+       begin
+	  count <= 1;
+	  delayed_fall <= 0;
+       end
+     else
+       begin
+	  count <= count - 1;
+	  delayed_fall <= 1;
+       end
+   
+   // Full_out goes instantly high if full_in does. However its fall is delayed.
+   assign    full_out = (full_in == 1) || (full_last == 1) || delayed_fall;
+
+endmodule
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