diff --git a/lib/CPUs/MIPS/ghdl/tb_mips_top.make b/lib/CPUs/MIPS/ghdl/tb_mips_top.make
index 2d6d301..0b68143 100644
--- a/lib/CPUs/MIPS/ghdl/tb_mips_top.make
+++ b/lib/CPUs/MIPS/ghdl/tb_mips_top.make
@@ -32,8 +32,8 @@ SRCS += ../src/tb_mips_top.vhd
 # Compile
 TARGET := tb_mips_top
 ENTITY := tb_mips_top
-RUN_TIME := 250us
-WAVE_FORMAT := ghw
+RUN_TIME ?= 250us
+WAVE_FORMAT ?= ghw
 
 include $(VHDL_MAKE_HOME)/ghdl.mk