diff --git a/tdes/sim/vhdl/makefile b/tdes/sim/vhdl/makefile
index 1d54344..05b7eb8 100644
--- a/tdes/sim/vhdl/makefile
+++ b/tdes/sim/vhdl/makefile
@@ -19,12 +19,15 @@
 # ======================================================================
 
 
+SRC_FILES = ../../rtl/vhdl/des_pkg.vhd ../../rtl/vhdl/des.vhd ../../rtl/vhdl/tdes.vhd
+
+
 all : sim wave
 
 sim : tb_tdes.ghw
 
-tb_tdes.ghw : ../rtl/*.vhd tb_tdes.vhd
-	ghdl -a ../rtl/des_pkg.vhd ../rtl/des.vhd ../rtl/tdes.vhd tb_tdes.vhd
+tb_tdes.ghw : $(SRC_FILES) tb_tdes.vhd
+	ghdl -a $(SRC_FILES) tb_tdes.vhd
 	ghdl -e tb_tdes
 	ghdl -r tb_tdes --wave=tb_tdes.ghw --assert-level=error --stop-time=45us