hdl-util
diff --git a/‎src/xorshift.sv
Lines changed: 8 additions & 2 deletions b/‎src/xorshift.sv
Lines changed: 8 additions & 2 deletions
diff --git a/‎test/xorshift_tb.sv
Lines changed: 9 additions & 11 deletions b/‎test/xorshift_tb.sv
Lines changed: 9 additions & 11 deletions
@@ -5,9 +5,11 @@ module xorshift #(
     parameter bit [WIDTH-1:0] POLYNOMIAL = WIDTH'(0)
 ) (
     input logic clk,
+    // Synchronous reset
     input logic reset,
+    // An optional seed used to set the initial state upon reset
     input logic [WIDTH-1:0] seed,
-    output logic [WIDTH-1:0] state
+    output logic [WIDTH-1:0] state = WIDTH'(0)
 );
 
 logic [WIDTH-1:0] polynomial;
@@ -16,12 +18,16 @@ logic lfsr_xnor;
 integer i;
 always_comb
 begin
-    lfsr_xnor = 1'b0;
+    lfsr_xnor = 0;
     for (i = 0; i < WIDTH; i++)
     begin
         if (polynomial[i])
             lfsr_xnor = lfsr_xnor ^~ state[i];
     end
+    // Trick to negate the fact that it's initially set to 0. It should be synthesized away.
+    // This works because the maximal length polynomials all have an even number of terms.
+    // Have confirmed it works for a few values, but I may be wrong.
+    lfsr_xnor = lfsr_xnor ^~ 0;
 end
 
 always_ff @(posedge clk)
 
@@ -7,25 +7,23 @@ module xorshift_tb;
         #1;
         clk <= ~clk;
     end
-    localparam int WIDTH = 12;
-    logic reset = 1'b1;
-    logic [WIDTH-1:0] seed = WIDTH'(1), state;
-    xorshift #(.WIDTH(WIDTH)) xorshift(.clk(clk), .reset(reset), .seed(seed), .state(state));
+    localparam int WIDTH = 4;
+    logic reset = 1'b0;
+    logic [WIDTH-1:0] state;
+    xorshift #(.WIDTH(WIDTH)) xorshift(.clk(clk), .reset(reset), .state(state));
 
     logic [WIDTH-1:0] seen [0:2**WIDTH-1];
-    integer i, current = -1;
+    integer i, current = 0;
     always_ff @(posedge clk)
     begin
-        if (current == -1)
-            reset <= 1'b0;
         else if (current == 2**WIDTH - 1)
+        if (current == 3 * 2**WIDTH)
             $finish;
         else
         begin
-            seen[current] <= state;
-            for (i = 0; i < current; i++)
+            seen[current % (2**WIDTH - 1)] <= state;
+            for (i = 0; i < current % (2**WIDTH - 1); i++)
             begin
-                assert(seen[i] != state) else $fatal(1, "Encountered a repeat of %d at %d %d", state, i, current);
+                assert(seen[i] != state) else $fatal(1, "Encountered a repeat of %d at %d %d %d %p", state, i, current % (2**WIDTH - 1), seen);
             end
         end
         current <= current + 1'd1;