diff --git a/lib/SDRAM/ddr_sdr_v1_5/src/ddr_phy_virtex4.vhd b/lib/SDRAM/ddr_sdr_v1_5/src/ddr_phy_virtex4.vhd
index ef9f524..1c58a02 100644
--- a/lib/SDRAM/ddr_sdr_v1_5/src/ddr_phy_virtex4.vhd
+++ b/lib/SDRAM/ddr_sdr_v1_5/src/ddr_phy_virtex4.vhd
@@ -95,7 +95,11 @@ architecture tech of sdram_phy is
 	type u_tag_array_t is array (natural range 0 to 4) of user_tag_t;
 	signal u_tag_pipe : u_tag_array_t;
 	
-begin
+	attribute KEEP : string;
+	attribute KEEP of clk0		: signal is "TRUE";
+
+begin
+
 	clk0_out	<= clk0_s;
 	clk0		<= clk0_s after 10 ps;		-- fix delta delay
 	rst0_out	<= rst0;