diff --git a/sim/Makefile b/sim/Makefile
index 0814ac2..b541402 100644
--- a/sim/Makefile
+++ b/sim/Makefile
@@ -6,7 +6,7 @@ MAKE := make
 VERILATOR := verilator
 
 VERI_FLAGS += +vcd
-VERI_CFLAGS +=
+VERI_CFLAGS += -DVL_DEBUG
 
 VERI_VFLAGS += -DTRACE_ENABLED