diff --git a/src/Makefile b/src/Makefile
index 8a62758..0b6759e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3,7 +3,7 @@ MAKEFLAGS += --no-print-dir
 Q := @
 endif
 
-OPT_FLAGS ?= -O2
+OPT_FLAGS = -O0 -g
 
 CFLAGS += -Wall -Wextra -Werror -Wno-char-subscripts\
 	$(OPT_FLAGS) -std=gnu99 -g3 -MD \
@@ -22,7 +22,6 @@ CROSS_COMPILE ?= arm-none-eabi-
 CC = $(CROSS_COMPILE)gcc
 OBJCOPY = $(CROSS_COMPILE)objcopy
 
-OPT_FLAGS = -Os
 CFLAGS += -mcpu=cortex-m3 -mthumb \
 	-DSTM32F1 -DDISCOVERY_STLINK -I../libopencm3/include \
 	-I .