Add tests for Xilinx UG901 examples

tests/xilinx_ug901/sfir_shifter.v

@@ -0,0 +1,19 @@
+//sfir_shifter.v
+(* dont_touch = "yes" *)
+module sfir_shifter #(parameter dsize = 16, nbtap = 4)
+                     (input clk,input [dsize-1:0] datain, output [dsize-1:0] dataout);
+
+   (* srl_style = "srl_register" *) reg [dsize-1:0] tmp [0:2*nbtap-1];
+   integer i;
+
+   always @(posedge clk)
+     begin
+        tmp[0] <= datain;
+        for (i=0; i<=2*nbtap-2; i=i+1)
+          tmp[i+1] <= tmp[i];
+     end
+
+   assign dataout = tmp[2*nbtap-1];
+
+endmodule
+// sfir_shifter