diff --git a/Makefile b/Makefile
index 52477a1..aacc8c7 100644
--- a/Makefile
+++ b/Makefile
@@ -26,7 +26,7 @@ BINARY = firmware
 
 # which development board is used
 # supported are: SYSTEM_BOARD, MAPLE_MINI, BLUE_PILL
-BOARD ?= SYSTEM_BOARD
+BOARD = SYSTEM_BOARD
 
 # source files
 CSRC = $(wildcard *.c)
@@ -45,32 +45,24 @@ LIB_OBJ = $(patsubst %.c,%.o,$(LIB_CSRC))
 # populates LIB_CSRC based on the library files used
 -include $(DEPENDENCIES)
 
-# cross-compiler tools
+# executables for linking, compiling, debugging, ...
 PREFIX ?= arm-none-eabi
-# compile using gcc (requires gcc-arm-none-eabi)
-#CC := $(PREFIX)-gcc
-# compile using clang (requires clang gcc-multilib)
-CC := clang -target $(PREFIX)
-# link using gcc (requires gcc-arm-none-eabi)
-#LD := $(PREFIX)-gcc
-# link (requires binutils-arm-none-eabi)
+CC := $(PREFIX)-gcc
 LD := $(PREFIX)-ld
-# create archive (requires binutils-arm-none-eabi)
 AR := $(PREFIX)-ar
-# compile assembly (requires binutils-arm-none-eabi)
 AS := $(PREFIX)-as
 OBJCOPY := $(PREFIX)-objcopy
 OBJDUMP := $(PREFIX)-objdump
-# debugger (requires)
 GDB := $(PREFIX)-gdb
 
 # opencm3 libraries
 OPENCM3_DIR := libopencm3
-INCLUDE_DIR = $(OPENCM3_DIR)/include
-LIB_DIR = $(OPENCM3_DIR)/lib
-SCRIPT_DIR = $(OPENCM3_DIR)/scripts
+OPENCM3_INC = $(OPENCM3_DIR)/include
+OPENCM3_LIB = $(OPENCM3_DIR)/lib
+# library for the STM32F1 (provided by opencm3)
+STM32F1_LIB = opencm3_stm32f1
 
-# linker script
+# linker script for STN32F1 boards
 ifeq ($(BOARD),SYSTEM_BOARD)
 LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/f1/stm32f103x8.ld
 else ifeq ($(BOARD),BLUE_PILL)
@@ -79,44 +71,54 @@ else ifeq ($(BOARD),MAPLE_MINI)
 LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/f1/stm32f103xb.ld
 endif
 
-# verify if libopencm3 has been downloaded
+# verify if opencm3 has been downloaded
 OPENCM3_DIR_EXISTS = $(shell [ -f $(LDSCRIPT) ] && echo 1 || echo 0 )
 ifeq ($(OPENCM3_DIR_EXISTS), 0)
 $(info run "git submodule init" and "git submodule update" before runnig make)
 $(error libopencm3 repository is not initialized)
 endif
 
-# device flags
+# device micro-controller and board
 DEFS += -DSTM32F1 -D$(BOARD)
 
 # C flags
-CFLAGS += -Os -g -std=c99 -nostdlib -nostdinc
-CFLAGS += -fno-common -ffunction-sections -fdata-sections -ffreestanding
-CFLAGS += -Wpedantic -Wall -Werror -Wextra -Wshadow -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes -Wstrict-overflow=5
-CFLAGS += -I/usr/include/newlib/ -I/usr/lib/gcc/arm-none-eabi/*/include/ -I. -I$(INCLUDE_DIR) $(patsubst %,-I%,$(LIB))
+# optimize for size
+CFLAGS += -Os
+# add debug symbols (remove for smaller release)
+CFLAGS += -g
+# use C99 (supported by most an sufficient)
+CFLAGS += -std=c99
+# have strict warning (for better code)
+CFLAGS += -Wpedantic -Wall -Werror -Wundef -Wextra -Wshadow -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes -Wstrict-overflow=5
+# add options for better code optimization
+CFLAGS += -fno-common -ffunction-sections -fdata-sections
+# don't use system main definition (the starting point)
+CFLAGS += -ffreestanding
+# include own libraries 
+CFLAGS += -I . $(patsubst %,-I%,$(LIB))
+# include opencm3 libraries
+CFLAGS += -I $(OPENCM3_INC)
+# add defines for micro-controller and board
 CFLAGS += $(DEFS)
 
 # linker flags
-LDFLAGS += -static -nostartfiles -nostdlib
-# opencm3 libraries
-LDFLAGS += --library-path $(LIB_DIR)
-LIBNAME = opencm3_stm32f1
-LDLIBS += --library $(LIBNAME)
-# use newlib (providing libc, libm, libnosys, libg)
-# TODO retrieve path automatically
-LDFLAGS += --library-path /usr/lib/arm-none-eabi/newlib/armv7-m/
-# use libgcc (provides __aeabi_*)
-# TODO retrieve path automatically (arm-none-eabi-gcc does it on its own)
-LDFLAGS += --library-path /usr/lib/gcc/arm-none-eabi/*/armv7-m/
-# own libraries
-LDFLAGS += --dynamic-linker . $(patsubst %,--dynamic-linker %,$(LIB))
-# libc and other system libraries
-LDLIBS += --library c --library gcc --library nosys --library g --library m
-# linker script for micro-controller
-LDFLAGS += -T $(LDSCRIPT)
+LDFLAGS += -static -nostartfiles -nostdlib -nostdinc
+LDFLAGS += -L$(OPENCM3_LIB)
+LDFLAGS += -L. $(patsubst %,-L%,$(LIB))
+# add c,m,nosys system libraries
+LDFLAGS += -L/usr/arm-none-eabi/lib/armv7-m/
+# add gcc system library
+LDFLAGS += -L/usr/lib/gcc/arm-none-eabi/6.1.1/armv7-m/
+# linker definitions for micro-controller
+LDFLAGS += -T$(LDSCRIPT)
+# only keep used sections
+LDFLAGS += --gc-sections
 
-# device specific flags
-ARCH_FLAGS = -mthumb -mcpu=cortex-m3
+# used libraries (must be provided after objects)
+LDLIBS += -l$(STM32F1_LIB) -lc -lgcc -lnosys -lm
+
+# target micro-controller information (ARM Cortex-M3 supports thumb and thumnb2, but does not include a floating point unit)
+ARCH_FLAGS = -mthumb -mcpu=cortex-m3 -msoft-float
 
 # SWD adapter used
 # supported are : st-link v2 (STLINKV2), black magic probe (BMP)
@@ -154,12 +156,13 @@ list: $(BINARY).list
 %.map %.list: %.elf
 	$(Q)$(OBJDUMP) -S $(<) > $(@)
 
-%.elf: $(LDSCRIPT) $(LIB_DIR)/lib$(LIBNAME).a $(OBJ) $(LIB_OBJ)
-	$(info compiling $(@))
+%.elf: $(LDSCRIPT) $(OPENCM3_LIB)/lib$(STM32F1_LIB).a $(OBJ) $(LIB_OBJ)
+	$(info linking $(@))
 	$(Q)$(LD) $(LDFLAGS) $(OBJ) $(LIB_OBJ) $(LDLIBS) -o $(@)
 	$(Q)size $(@)
 
 %.o: %.c $(CHDR) $(LIB_CHDR)
+	$(info compiling $(@))
 	$(Q)$(CC) $(CFLAGS) $(ARCH_FLAGS) -o $(@) -c $(<)
 
 # generate dependencies
@@ -178,8 +181,8 @@ doc: Doxyfile README.md $(CSRC) $(CHDR) $(LIB_CSRC) $(LIB_CHDR)
 clean:
 	$(Q)$(RM) $(BINARY).elf $(BINARY).bin $(BINARY).hex $(BINARY).map $(OBJ) $(LIB_OBJ) $(LIB)/*.o $(DEPENDENCIES)
 
-# make libopencm3 if not done
-$(LIB_DIR)/lib$(LIBNAME).a:
+# make libopencm3 if library for STM32F1 is not yet existing
+$(OPENCM3_LIB)/lib$(STM32F1_LIB).a:
 	$(info compiling libopencm3 library)
 	$(Q)$(MAKE) -C $(OPENCM3_DIR)