diff --git a/make/ghdl.mk b/make/ghdl.mk
index caf9b4e..bad195a 100644
--- a/make/ghdl.mk
+++ b/make/ghdl.mk
@@ -10,7 +10,7 @@ WORK_LIB ?= work
 LANG_STD ?= 93c
 IEEE_STD ?= standard
 RUN_TIME ?= 1000ns
-WAVE_FORMAT ?= FST
+WAVE_FORMAT ?= fst
 
 # Build path
 BUILD_DIR ?= build/$(TARGET)
@@ -42,14 +42,14 @@ GHDL := ghdl
 GHDL_GCC := ghdl-gcc
 
 # Wave format selection
-SIM_FILE-VCD := $(BUILD_DIR)/$(TARGET).vcd
-GTK_OPTS-VCD := --vcd=$(SIM_FILE-VCD)
+SIM_FILE-vcd := $(BUILD_DIR)/$(TARGET).vcd
+GTK_OPTS-vcd := --vcd=$(SIM_FILE-VCD)
 
-SIM_FILE-FST := $(BUILD_DIR)/$(TARGET).fst
-GTK_OPTS-FST := --fst=$(SIM_FILE-FST)
+SIM_FILE-fst := $(BUILD_DIR)/$(TARGET).fst
+GTK_OPTS-fst := --fst=$(SIM_FILE-FST)
 
-SIM_FILE-GHW := $(BUILD_DIR)/$(TARGET).ghw
-GTK_OPTS-GHW := --wave=$(SIM_FILE-GHW)
+SIM_FILE-ghw := $(BUILD_DIR)/$(TARGET).ghw
+GTK_OPTS-ghw := --wave=$(SIM_FILE-GHW)
 
 SIM_FILE := $(SIM_FILE-$(WAVE_FORMAT))
 GTK_OPTS := $(GTK_OPTS-$(WAVE_FORMAT))