diff --git a/hdl/Makefile b/hdl/Makefile
index 76a5431..697f18f 100644
--- a/hdl/Makefile
+++ b/hdl/Makefile
@@ -3,10 +3,13 @@ PIN_DEF = pins.pcf
 DEVICE = hx8k
 PACKAGE = ct256
 
+SOURCE_V = $(wildcard *.v)
+TESTBENCH_V = $(wildcard tb/*.v)
+
 all: $(PROJ).rpt $(PROJ).bin
 
 %.blif: %.v
-	yosys -p 'synth_ice40 -top top -blif $@' $(wildcard *.v)
+	yosys -p 'synth_ice40 -top top -blif $@' $(SOURCE_V)
 	# yosys -p 'synth_ice40 -top top -blif $@' $<
 
 %.asc: $(PIN_DEF) %.blif
@@ -25,7 +28,7 @@ sudo-prog: $(PROJ).bin
 	@echo 'Executing prog as root!!!'
 	sudo iceprog $<
 
-tb.out: $(wildcard *.v) $(wildcard tb/*.v)
+tb.out: $(SOURCE_V) $(TESTBENCH_V)
 	iverilog $^ -o $@
 
 sim: tb.out