Added more variation for f_or_s_i.

sim/tb.v

@@ -59,8 +59,21 @@ module tb;
 
     initial begin
         #10000;
-        forever #(100 * ($random % (20 + 1)))
-            f_or_s_i = $random;
+        forever begin
+            f_or_s_i <= ~f_or_s_i;
+
+            if ($random % 10 < 4)
+                repeat (($random % 2) + 1)
+                    @(posedge clock);
+            else
+                repeat (($random % 200) + 1)
+                    @(posedge clock);
+
+            f_or_s_i <= ~f_or_s_i;
+
+            repeat (($random % 2000) + 1)
+                @(posedge clock);
+        end
     end
 
     initial