diff --git a/Makefile b/Makefile
index f2620df..e5d724e 100644
--- a/Makefile
+++ b/Makefile
@@ -21,28 +21,24 @@ Q := @
 NULL := 1> /dev/null 2> /dev/null
 endif
 
-# the final binary name (without extension)
-BINARY = firmware
+# main names (without extension, for input source file and output binary)
+APPLICATION = application
+BOOTLOADER = bootloader
+FIRMWARE = $(APPLICATION) $(BOOTLOADER)
 
 # which development board is used
 # supported are: SYSTEM_BOARD, MAPLE_MINI, BLUE_PILL, CORE_BOARD
-BOARD = CORE_BOARD
+BOARD = BLUE_PILL
 
-# source files
-CSRC = $(wildcard *.c)
-CHDR = $(wildcard *.h)
+# source files (this will be populated using includes based DEPENDENCIES)
+CSRC = global.c
+# headers corresponding to source files
+CHDR = $(patsubst %.c,%.h,$(CSRC))
+# objects compiled from source files
 OBJ = $(patsubst %.c,%.o,$(CSRC))
-
-# my library collection
-LIB = lib
-# the library files to use (this will be populated using includes based DEPENDENCIES)
-LIB_CSRC =
-LIB_CHDR = $(patsubst %.c,%.h,$(LIB_CSRC))
-LIB_OBJ = $(patsubst %.c,%.o,$(LIB_CSRC))
-
-# figure out based on the includes which library files are used in the main CSRC files
-DEPENDENCIES = $(patsubst %.c,%.inc,$(CSRC))
-# populates LIB_CSRC based on the library files used
+# figure out based on the main sources files which library files are used
+DEPENDENCIES = $(patsubst %,%.inc,$(FIRMWARE))
+# populates CSRC based on the library files used
 -include $(DEPENDENCIES)
 
 # executables for linking, compiling, debugging, ...
@@ -56,8 +52,9 @@ AR := $(ELLCC)bin/ecc-ar
 AS := $(ELLCC)bin/ecc-as
 OBJCOPY := $(ELLCC)bin/ecc-objcopy
 OBJDUMP := $(ELLCC)bin/ecc-objdump
-# ecc-gdb is buggy and crashes on "kill"
-GDB := gdb
+GDB := $(ELLCC)bin/ecc-gdb
+# ecc-gdb (0.13.3) is buggy (crash on kill, can't load elf)
+GDB := arm-none-eabi-gdb
 
 # opencm3 libraries
 OPENCM3_DIR := libopencm3
@@ -66,7 +63,7 @@ OPENCM3_LIB = $(OPENCM3_DIR)/lib
 # library for the STM32F1 (provided by opencm3)
 STM32F1_LIB = opencm3_stm32f1
 
-# linker script for STN32F1 boards
+# linker script for STM32F103x8 boards (MAPLE_MINI has STM32F103xB with just more 128 kB flash instead of 64 kB, thus is compatible)
 ifeq ($(BOARD),SYSTEM_BOARD)
 LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/f1/stm32f103x8.ld
 else ifeq ($(BOARD),BLUE_PILL)
@@ -76,6 +73,7 @@ LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/f1/stm32f103x8.ld
 else ifeq ($(BOARD),MAPLE_MINI)
 LDSCRIPT = $(OPENCM3_DIR)/lib/stm32/f1/stm32f103xb.ld
 endif
+LDSCRIPT = application.ld
 
 # device micro-controller and board
 DEFS += -DSTM32F1 -D$(BOARD)
@@ -100,9 +98,9 @@ CFLAGS += -ffreestanding
 # don't use the standard library
 CFLAGS += -nostdlib -nostdinc
 # include ELLCC libraries
-CFLAGS += -I$(ELLCC)libecc/include/ -I$(ELLCC)libecc/include/arm/
+CFLAGS += -I $(ELLCC)libecc/include/ -I $(ELLCC)libecc/include/arm/
 # include own libraries 
-CFLAGS += -I . $(patsubst %,-I%,$(LIB))
+CFLAGS += -I . -I lib
 # include opencm3 libraries
 CFLAGS += -I $(OPENCM3_INC)
 # add defines for micro-controller and board
@@ -122,7 +120,7 @@ LDFLAGS += --library-path $(ELLCC)libecc/lib/cortex-m3-linux/
 # opencm3 libraries
 LDFLAGS += --library-path $(OPENCM3_LIB)
 # linker script with definitions for micro-controller
-LDFLAGS += --script $(LDSCRIPT)
+#LDFLAGS += --script $(LDSCRIPT)
 # used libraries (gcc provides the ARM ABI, not sure how to replace with compiler-rt)
 LDLIBS += --library $(STM32F1_LIB) --library c --library m --library gcc
 
@@ -131,7 +129,7 @@ ARCH_FLAGS = -mthumb -mcpu=cortex-m3 -msoft-float
 
 # SWD adapter used
 # supported are : st-link v2 (STLINKV2), black magic probe (BMP)
-SWD_ADAPTER ?= BMP
+SWD_ADAPTER ?= STLINKV2
 ifeq ($(SWD_ADAPTER),STLINKV2)
 # OpenOCD configuration
 OOCD ?= openocd
@@ -139,38 +137,31 @@ OOCD_INTERFACE ?= stlink-v2
 OOCD_TARGET ?= stm32f1x
 else ifeq ($(SWD_ADAPTER),BMP)
 # the black magic probe has a SWD controller built in
-BMPPORT ?= /dev/ttyACM0
+BMP_PORT ?= /dev/ttyACM0
 endif
 
-# which USB CDC ACM port is used bu the device, so we can reset it
-ifeq ($(SWD_ADAPTER),STLINKV2)
-ACMPORT = /dev/ttyACM0
-else ifeq ($(SWD_ADAPTER),BMP)
-ACMPORT = /dev/ttyACM2
-endif
-ACMPORT_EXISTS = $(shell [ -e $(ACMPORT) ] && echo 1 || echo 0 )
-
 # compile target rules
-all: elf
+all: elf hex bin
 
-elf: $(BINARY).elf
-bin: $(BINARY).bin
-hex: $(BINARY).hex
-srec: $(BINARY).srec
-list: $(BINARY).list
+elf: $(patsubst %,%.elf,$(FIRMWARE))
+bin: $(patsubst %,%.bin,$(FIRMWARE))
+hex: $(patsubst %,%.hex,$(FIRMWARE))
 
-%.bin %.hex %.srec: %.elf
-	$(Q)$(OBJCOPY) -Osrec $(<) $(@)
+%.hex: %.elf
+	$(Q)$(OBJCOPY) --strip-all --strip-debug --output-target ihex $(<) $(@)
+
+%.bin: %.elf
+	$(Q)$(OBJCOPY) --strip-all --strip-debug --output-target binary $(<) $(@)
 
 %.map %.list: %.elf
 	$(Q)$(OBJDUMP) -S $(<) > $(@)
 
-%.elf: $(OPENCM3_LIB)/lib$(STM32F1_LIB).a $(OBJ) $(LIB_OBJ)
+%.elf: %.o %.ld $(OBJ) $(OPENCM3_LIB)/lib$(STM32F1_LIB).a
 	$(info linking $(@))
-	$(Q)$(LD) $(LDFLAGS) $(OBJ) $(LIB_OBJ) $(LDLIBS) -o $(@)
+	$(Q)$(LD) $(LDFLAGS) --script $(*).ld $(<) $(OBJ) $(LDLIBS) -o $(@)
 	$(Q)size $(@)
 
-%.o: %.c $(CHDR) $(LIB_CHDR)
+%.o: %.c $(CHDR)
 	$(info compiling $(@))
 	$(Q)$(CC) $(CFLAGS) $(ARCH_FLAGS) -o $(@) -c $(<)
 
@@ -181,14 +172,14 @@ list: $(BINARY).list
 
 # figure out which library source files are used for later inclusion
 %.inc: %.d
-	$(Q)grep -o -e " ${LIB}\/[^ ]*\.h" $(<) | sed -e 's|\(.*\)\.h$$|LIB_CSRC +=\1.c\n-include\1.inc|g' -e 's|.*${*}.*||g' > $(@)
+	$(Q)grep -o -e " lib\/[^ ]*\.h" $(<) | sed -e 's|\(.*\)\.h$$|CSRC +=\1.c\n-include\1.inc|g' -e 's|.*${*}.*||g' > $(@)
 
 # doxygen documentation
-doc: Doxyfile README.md $(CSRC) $(CHDR) $(LIB_CSRC) $(LIB_CHDR)
+doc: Doxyfile README.md $(patsubst %,%.c,$(FIRMWARE)) $(CSRC) $(CHDR)
 	$(Q)doxygen $(<)
 
 clean:
-	$(Q)$(RM) $(BINARY).elf $(BINARY).bin $(BINARY).hex $(BINARY).map $(OBJ) $(LIB_OBJ) $(LIB)/*.o $(DEPENDENCIES) $(LIB)/*.inc
+	$(Q)$(RM) $(BOOTLOADER).elf $(BOOTLOADER).bin $(BOOTLOADER).hex $(OBJ) $(DEPENDENCIES)
 
 # make libopencm3 if library for STM32F1 is not yet existing
 $(OPENCM3_LIB)/lib$(STM32F1_LIB).a:
@@ -197,28 +188,45 @@ $(OPENCM3_LIB)/lib$(STM32F1_LIB).a:
 	git submodule update
 	$(Q)$(MAKE) CFLAGS=-fno-short-enums -C $(OPENCM3_DIR)
 
-flash: $(BINARY).hex
+# flash application using DFU
+flash: $(APPLICATION).bin
+	$(Q)dfu-util -d c440:0d00 -D $(<)
+
+# flash bootloader using SWD
+flash_bootloader: $(BOOTLOADER).hex
 	$(info flashing $(<) using SWD)
 ifeq ($(SWD_ADAPTER),STLINKV2)
 	$(Q)$(OOCD) --file interface/$(OOCD_INTERFACE).cfg --file target/$(OOCD_TARGET).cfg --command "init" --command "reset init" --command "flash write_image erase $(<)" --command "reset" --command "shutdown" $(NULL)
 else ifeq ($(SWD_ADAPTER),BMP)
-	$(Q)$(GDB) --eval-command="target extended-remote $(BMPPORT)" --eval-command="set confirm off" --eval-command="kill" --eval-command="monitor swdp_scan" --eval-command="attach 1" --eval-command="load" --eval-command="kill" --eval-command="detach" --eval-command="quit" $(<)
+	$(Q)$(GDB) --eval-command="target extended-remote $(BMP_PORT)" --eval-command="set confirm off" --eval-command="monitor swdp_scan" --eval-command="attach 1" --eval-command="load" --eval-command="detach" --eval-command="quit" $(<)
+endif
+
+# flash application using SWD
+flash_application: $(APPLICATION).hex
+	$(info flashing $(<) using SWD)
+ifeq ($(SWD_ADAPTER),STLINKV2)
+	$(Q)$(OOCD) --file interface/$(OOCD_INTERFACE).cfg --file target/$(OOCD_TARGET).cfg --command "init" --command "reset init" --command "flash write_image erase $(<)" --command "reset" --command "shutdown" $(NULL)
+else ifeq ($(SWD_ADAPTER),BMP)
+	$(Q)$(GDB) --eval-command="target extended-remote $(BMP_PORT)" --eval-command="set confirm off" --eval-command="monitor swdp_scan" --eval-command="attach 1" --eval-command="load" --eval-command="detach" --eval-command="quit" $(<)
 endif
 
 # reset device by setting the data width to 5 bis on the USB CDC ACM port
-reset:
-ifeq ($(ACMPORT_EXISTS), 1)
-	$(Q)stty --file $(ACMPORT) 115200 raw cs5
-	$(Q)sleep 0.5
+ifeq ($(SWD_ADAPTER)$(BMP_PORT),BMP/dev/ttyACM0)
+ACM_PORT := /dev/ttyACM2
+else
+ACM_PORT := /dev/ttyACM0
 endif
+reset:
+	$(Q)stty --file $(ACM_PORT) raw cs5
+	$(Q)sleep 0.5
 
-# debug using GDB
-debug: $(BINARY).elf
+# debug application using GDB
+debug: $(APPLICATION).elf
 ifeq ($(SWD_ADAPTER),STLINKV2)
 	# for GDB to work with openOCD the firmware needs to be reloaded
 	$(Q)$(GDB) --eval-command="target remote | $(OOCD) --file interface/$(OOCD_INTERFACE).cfg --file target/$(OOCD_TARGET).cfg --command \"gdb_port pipe; log_output /dev/null; init\"" --eval-command="monitor reset halt" --eval-command="load" --eval-command="monitor reset init" $(<)
 else ifeq ($(SWD_ADAPTER),BMP)
-	$(Q)$(GDB) --eval-command="target extended-remote $(BMPPORT)" --eval-command="monitor version" --eval-command="monitor swdp_scan" --eval-command="attach 1" $(<)
+	$(Q)$(GDB) --eval-command="target extended-remote $(BMP_PORT)" --eval-command="monitor version" --eval-command="monitor swdp_scan" --eval-command="attach 1" $(<)
 endif
 
 .PHONY: clean elf bin hex srec list flash reset
diff --git a/main.c b/application.c
similarity index 100%
rename from main.c
rename to application.c
diff --git a/application.ld b/application.ld
new file mode 100644
index 0000000..ab7deea
--- /dev/null
+++ b/application.ld
@@ -0,0 +1,16 @@
+/* linker script for application running on STM32F103x8 micro-controller
+ * the STM32F103x8 has 64kB of flash starting at 0x0800 0000, and 20kB of RAM starting at 0x2000 0000
+ * the bootloader will take the first 9 kB of flash, followed by the application
+ */
+
+/* Define memory regions. */
+MEMORY
+{
+	rom (rx) : ORIGIN = 0x08000000 + 9K, LENGTH = 55K
+	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 20K
+}
+PROVIDE(__application_beginning = ORIGIN(rom));
+PROVIDE(__application_end = ORIGIN(rom) + LENGTH(rom));
+
+/* include rest of the definitions for the STM32F1 family */
+INCLUDE libopencm3_stm32f1.ld
diff --git a/bootloader.c b/bootloader.c
new file mode 100644
index 0000000..6a90280
--- /dev/null
+++ b/bootloader.c
@@ -0,0 +1,85 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+/** USB DFU bootloader
+ *  @file bootloader.c
+ *  @author King Kévin <kingkevin@cuvoodoo.info>
+ *  @date 2017
+ */
+/* standard libraries */
+#include <stdint.h> // standard integer types
+#include <stdbool.h> // boolean types
+
+/* STM32 (including CM3) libraries */
+#include <libopencm3/cm3/scb.h> // vector table definition
+#include <libopencm3/stm32/rcc.h> // clock utilities
+#include <libopencm3/stm32/gpio.h> // GPIO utilities
+
+/* own libraries */
+#include "global.h" // board definitions
+#include "usb_dfu.h" // USB DFU utilities
+
+/** bootloader entry point */
+void main(void);
+void main(void)
+{
+	// check of DFU mode is forced
+	bool dfu_force = false; // to remember if DFU mode is forced
+	// check if a soft boot has been used
+	if (0==(RCC_CSR&0xfc000000)) { // no reset flag present -> this was a soft reset using csr_reset_core(), very probably to start the DFU mode
+		dfu_force = true;
+	} else { // check if the force DFU mode input is set
+		// disable SWJ pin to use as GPIO
+#if (GPIO(B)==GPIO(DFU_FORCE_PORT)) && (GPIO(4)==GPIO(DFU_FORCE_PIN))
+		gpio_primary_remap(AFIO_MAPR_SWJ_CFG_FULL_SWJ_NO_JNTRST, 0);
+#elif ((GPIO(B)==GPIO(DFU_FORCE_PORT)) && (GPIO(3)==GPIO(DFU_FORCE_PIN))) || ((GPIO(A)==GPIO(DFU_FORCE_PORT)) && (GPIO(15)==GPIO(DFU_FORCE_PIN)))
+		gpio_primary_remap(AFIO_MAPR_SWJ_CFG_JTAG_OFF_SW_ON, 0);
+#elif ((GPIO(A)==GPIO(DFU_FORCE_PORT)) && (GPIO(14)==GPIO(DFU_FORCE_PIN))) || ((GPIO(A)==GPIO(DFU_FORCE_PORT)) && (GPIO(13)==GPIO(DFU_FORCE_PIN)))
+		gpio_primary_remap(AFIO_MAPR_SWJ_CFG_JTAG_OFF_SW_OFF, 0);
+#endif
+		rcc_periph_clock_enable(RCC_GPIO(DFU_FORCE_PORT)); // enable clock for GPIO domain
+		gpio_set_mode(GPIO(DFU_FORCE_PORT), GPIO_MODE_INPUT, GPIO_CNF_INPUT_PULL_UPDOWN, GPIO(DFU_FORCE_PIN)); // set GPIO to input
+		// pull on the opposite of the expected value
+		if (DFU_FORCE_VALUE) {
+			gpio_clear(GPIO(DFU_FORCE_PORT), GPIO(DFU_FORCE_PIN)); // pull down to be able to detect when tied to high
+		} else {
+			gpio_set(GPIO(DFU_FORCE_PORT), GPIO(DFU_FORCE_PIN)); // pull up to be able to detect when tied to low
+		}
+		if ((!DFU_FORCE_VALUE && 0==gpio_get(GPIO(DFU_FORCE_PORT), GPIO(DFU_FORCE_PIN))) || (DFU_FORCE_VALUE && 0!=gpio_get(GPIO(DFU_FORCE_PORT), GPIO(DFU_FORCE_PIN)))) { // check if output is set to the value to force DFU mode
+			dfu_force = true; // DFU mode forced
+		}
+	}
+
+	// start application if valid
+	/* the application starts with the vector table
+	 * the first entry in the vector table is the initial stack pointer (SP) address
+	 * the stack will be placed in RAM
+	 * on STM32F1xx SRAM begins at 0x2000 0000, and on STM32F103x8 there is 20KB of RAM (0x5000).
+	 * since the stack grown "downwards" it should start at the end of the RAM: 0x2000 5000
+	 * if the SP is not in this range (e.g. flash has been erased) there is no valid application
+	 * the second entry in the vector table is the reset address, corresponding to the application start
+	 */
+	volatile uint32_t* application = &__application_beginning; // get the value of the application address symbol (use a register instead on the stack since the stack pointer will be changed)
+	if (!dfu_force && (((*application)&0xFFFE0000)==0x20000000)) { // application at address seems valid
+		SCB_VTOR = (volatile uint32_t)(application); // set vector table to application vector table (store at the beginning of the application)
+		__asm__ volatile ("MSR msp,%0" : :"r"(*application)); // set stack pointer to address provided in the beginning of the application (loaded into a register first)
+		(*(void(**)())(application + 1))(); // start application (by jumping to the reset function which address is stored as second entry of the vector table)
+	}
+
+	rcc_clock_setup_in_hse_8mhz_out_72mhz(); // start main clock
+	board_setup(); // setup board to control LED
+	led_on(); // indicate bootloader started
+	usb_dfu_setup(); // setup USB DFU for firmware upload
+	usb_dfu_start(); // run DFU mode
+}
diff --git a/bootloader.ld b/bootloader.ld
new file mode 100644
index 0000000..da996bd
--- /dev/null
+++ b/bootloader.ld
@@ -0,0 +1,16 @@
+/* linker script for application running on STM32F103x8 micro-controller
+ * the STM32F103x8 has 64kB of flash starting at 0x0800 0000, and 20kB of RAM starting at 0x2000 0000
+ * the bootloader will take the first 9 kB of flash, followed by the application
+ */
+
+/* Define memory regions. */
+MEMORY
+{
+	rom (rx) : ORIGIN = 0x08000000, LENGTH = 9K
+	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 20K
+}
+PROVIDE(__application_beginning = ORIGIN(rom) + LENGTH(rom));
+PROVIDE(__application_end = __application_beginning + 55K);
+
+/* include rest of the definitions for the STM32F1 family */
+INCLUDE libopencm3_stm32f1.ld
diff --git a/global.h b/global.h
index bf35164..03598e4 100644
--- a/global.h
+++ b/global.h
@@ -252,24 +252,23 @@
 #define SPI_MISO_PIN(x) CAT3(GPIO_SPI,x,_MISO)
 /** get SPI pin for MOSI signal based on SPI identifier */
 #define SPI_MOSI_PIN(x) CAT3(GPIO_SPI,x,_MOSI)
-
 /** @} */
 
 /** @defgroup board_led board LED GPIO
  *  @{
  */
 #if defined(SYSTEM_BOARD) || defined(CORE_BOARD)
-/* on system and core board LED is on pin 11/PA1 */
-#define LED_PORT A /**< GPIO port (port A) */
-#define LED_PIN	1 /**< GPIO pin (pin PA1) */
+	/* on system and core board LED is on pin 11/PA1 */
+	#define LED_PORT A /**< GPIO port (port A) */
+	#define LED_PIN	1 /**< GPIO pin (pin PA1) */
 #elif defined(BLUE_PILL)
-/* on minimum system LED is on pin 2/PC13 */
-#define LED_PORT C /**< GPIO port (port C on blue pill) */
-#define LED_PIN 13 /**< GPIO pin (pin PC13 on system board) */
+	/* on minimum system LED is on pin 2/PC13 */
+	#define LED_PORT C /**< GPIO port (port C on blue pill) */
+	#define LED_PIN 13 /**< GPIO pin (pin PC13 on system board) */
 #elif defined (MAPLE_MINI)
-/* on maple mini LED is on pin 19/PB1 */
-#define LED_PORT B /**< GPIO port (port B on maple mini) */
-#define LED_PIN 1 /**< GPIO pin (pin PB1 on maple mini) */
+	/* on maple mini LED is on pin 19/PB1 */
+	#define LED_PORT B /**< GPIO port (port B on maple mini) */
+	#define LED_PIN 1 /**< GPIO pin (pin PB1 on maple mini) */
 #endif
 /** @} */
 
@@ -277,16 +276,46 @@
  *  @{
  */
 #if defined(MAPLE_MINI)
-/* on maple mini user button is on 32/PB8 */
-#define BUTTON_PORT B /**< GPIO port (port B on maple mini) */
-#define BUTTON_PIN 8 /**< GPIO pin (pin PB8 on maple mini) */
+	/* on maple mini user button is on 32/PB8 */
+	#define BUTTON_PORT B /**< GPIO port (port B on maple mini) */
+	#define BUTTON_PIN 8 /**< GPIO pin (pin PB8 on maple mini) */
 #elif defined(CORE_BOARD)
-/* on core board user button is on PA8 */
-#define BUTTON_PORT A /**< GPIO port (port A) */
-#define BUTTON_PIN 8 /**< GPIO pin (pin PA8) */
+	/* on core board user button is on PA8 */
+	#define BUTTON_PORT A /**< GPIO port (port A) */
+	#define BUTTON_PIN 8 /**< GPIO pin (pin PA8) */
 #endif
 /** @} */
 
+/** @defgroup input to force DFU mode on low, even if application is valid
+ *  @{
+ */
+#if defined(MAPLE_MINI)
+	/* use button */
+	#define DFU_FORCE_PORT BUTTON_PORT /**< button port */
+	#define DFU_FORCE_PIN BUTTON_PIN /**< button pin */
+	#define DFU_FORCE_VALUE true /**< button is pulled low unpressed, high pressed to force DFU mode */
+#elif defined(CORE_BOARD)
+	/* use button */
+	#define DFU_FORCE_PORT BUTTON_PORT /**< button port */
+	#define DFU_FORCE_PIN BUTTON_PIN /**< button pin */
+	#define DFU_FORCE_VALUE false /**< button floating unpressed, connected to ground pressed to force DFU mode */
+#else
+	/* use the JNTRST pin as JPIO (this will disable the SWJ function, but we are not using it) */
+	#define DFU_FORCE_PORT B /**< JNTRST port (needs to be remapped to become PB4) */
+	#define DFU_FORCE_PIN 4 /**< JNTRST pin (needs to be remapped to become PB4) */
+	#define DFU_FORCE_VALUE false /**< must be high to force DFU mode, since it's low after reset */
+#endif
+/** @} */
+
+/** symbol for beginning of the application
+ *  @note this symbol will be provided by the linker script
+ */
+extern uint32_t __application_beginning;
+/** symbol for end of the application
+ *  @note this symbol will be provided by the linker script
+ */
+extern uint32_t __application_end;
+
 extern volatile bool button_flag; /**< flag set when board user button has been pressed/released */
 
 /** get binary representation of a number
diff --git a/usb_dfu.c b/usb_dfu.c
new file mode 100644
index 0000000..03ceef6
--- /dev/null
+++ b/usb_dfu.c
@@ -0,0 +1,299 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+/** library for USB DFU to write on internal flash (code)
+ *  @file usb_dfu.c
+ *  @author King Kévin <kingkevin@cuvoodoo.info>
+ *  @date 2017
+ */
+
+/* standard libraries */
+#include <stdint.h> // standard integer types
+#include <stdlib.h> // general utilities
+
+/* STM32 (including CM3) libraries */
+#include <libopencmsis/core_cm3.h> // Cortex M3 utilities
+#include <libopencm3/cm3/scb.h> // reset utilities
+#include <libopencm3/stm32/rcc.h> // real-time control clock library
+#include <libopencm3/stm32/gpio.h> // general purpose input output library
+#include <libopencm3/usb/usbd.h> // USB library
+#include <libopencm3/usb/dfu.h> // USB DFU library
+
+#include "global.h" // global utilities
+#include "usb_dfu.h" // USB DFU header and definitions
+#include "flash_internal.h" // flash reading/writing utilities
+
+static uint8_t usbd_control_buffer[1024] = {0}; /**< buffer to be used for control requests (fit to flash page size) */
+static usbd_device *usb_device = NULL; /**< structure holding all the info related to the USB device */
+static enum dfu_state usb_dfu_state = STATE_DFU_IDLE; /**< current DFU state */
+static enum dfu_status usb_dfu_status = DFU_STATUS_OK; /**< current DFU status */
+
+static uint8_t download_data[sizeof(usbd_control_buffer)] = {0}; /**< downloaded data to be programmed in flash */
+static uint16_t download_length = 0; /**< length of downloaded data */
+static uint32_t flash_pointer = 0; /**< where the downloaded data should be flashed */
+
+/** USB DFU device descriptor
+ *  @note as defined in USB Device Firmware Upgrade specification section 4.2.1
+ */
+static const struct usb_device_descriptor usb_dfu_device = {
+	.bLength = USB_DT_DEVICE_SIZE, /**< the size of this header in bytes, 18 */
+	.bDescriptorType = USB_DT_DEVICE, /**< a value of 1 indicates that this is a device descriptor */
+	.bcdUSB = 0x0200, /**< this device supports USB 2.0 */
+	.bDeviceClass = 0, /**< unused */
+	.bDeviceSubClass = 0, /**< unused */
+	.bDeviceProtocol = 0, /**< unused */
+	.bMaxPacketSize0 = 64, /**< packet size for endpoint zero in bytes */
+	.idVendor = 0xc440, /**< Vendor ID (CuVo...) */
+	.idProduct = 0x0d00, /**< product ID within the Vendor ID space (...odoo) */
+	.bcdDevice = 0x0100, /**< version number for the device */
+	.iManufacturer = 1, /**< the index of the string in the string table that represents the name of the manufacturer of this device */
+	.iProduct = 2, /**< the index of the string in the string table that represents the name of the product */
+	.iSerialNumber = 3, /**< the index of the string in the string table that represents the serial number of this item in string form */
+	.bNumConfigurations = 1, /**< the number of possible configurations this device has */
+};
+
+/** USB DFU functional descriptor
+ *  @note as defined in USB Device Firmware Upgrade specification section 4.2.4
+ */
+static const struct usb_dfu_descriptor usb_dfu_functional = {
+	.bLength = sizeof(struct usb_dfu_descriptor), /**< provide own size */
+	.bDescriptorType = DFU_FUNCTIONAL, /**< functional descriptor type */
+	.bmAttributes = USB_DFU_CAN_DOWNLOAD | USB_DFU_WILL_DETACH, /**< this DFU can download and will detach after download (we don't support manifest for simplicity, technically we could) */
+	.wDetachTimeout = 200, /**< maximum time in milliseconds to detach (and reboot) */
+	.wTransferSize = sizeof(usbd_control_buffer), /**< set max transfer size */
+	.bcdDFUVersion = 0x0110, /**< DFU specification version 1.1 used */
+};
+
+/** USB DFU interface descriptor
+ *  @note as defined in USB Device Firmware Upgrade specification section 4.2.3
+ */
+static const struct usb_interface_descriptor usb_dfu_interface = {
+	.bLength = USB_DT_INTERFACE_SIZE, /**< size of descriptor in byte */
+	.bDescriptorType = USB_DT_INTERFACE, /**< interface descriptor type */
+	.bInterfaceNumber = 0, /**< this interface is the first (and only) */
+	.bAlternateSetting = 0, /**< no alternative settings */
+	.bNumEndpoints = 0, /**< only the control pipe at endpoint 0 is used */
+	.bInterfaceClass = 0xFE, /**< DFU interface class (not defined in libopencm3 dfu lib) */
+	.bInterfaceSubClass = 1, /**< DFU interface subclass (not defined in libopencm3 dfu lib) */
+	.bInterfaceProtocol = 2,  /**< DFU interface mode protocol (not defined in libopencm3 dfu lib) */
+	.iInterface = 4, /**< the index of the string in the string table that represents interface description */
+	.extra = &usb_dfu_functional, /**< point to functional descriptor */
+	.extralen = sizeof(usb_dfu_functional), /**< size of functional descriptor */
+};
+
+/** USB DFU interface descriptor list */
+static const struct usb_interface usb_dfu_interfaces[] = {{
+	.num_altsetting = 1, /**< this is the only alternative */
+	.altsetting = &usb_dfu_interface, /**< point to only interface descriptor */
+}};
+
+/** USB DFU configuration descriptor
+ *  @note as defined in USB Device Firmware Upgrade specification section 4.2.2
+ */
+static const struct usb_config_descriptor usb_dfu_configuration = {
+	.bLength = USB_DT_CONFIGURATION_SIZE, /**< the length of this header in bytes */
+	.bDescriptorType = USB_DT_CONFIGURATION, /**< a value of 2 indicates that this is a configuration descriptor */
+	.wTotalLength = 0, /**< total size of the configuration descriptor including all sub interfaces (automatically filled in by the USB stack in libopencm3) */
+	.bNumInterfaces = LENGTH(usb_dfu_interfaces), /**< the number of interfaces in this configuration */
+	.bConfigurationValue = 1, /**< the index of this configuration */
+	.iConfiguration = 0, /**< a string index describing this configuration (zero means not provided) */
+	.bmAttributes = 0x80, /**< bus powered (1<<7) */
+	.bMaxPower = 0x32, /**< the maximum amount of current that this device will draw in 2mA units */
+	// end of header
+	.interface = usb_dfu_interfaces, /**< pointer to an array of interfaces */
+};
+
+/** USB string table
+ *  @note starts with index 1
+ */
+static const char *usb_dfu_strings[] = {
+	"CuVoodoo",
+	"STM32F1",
+	"DFU",
+	"CuVoodoo DFU bootloader (DFU mode)",
+};
+
+/** disconnect USB to force re-enumerate */
+static void usb_disconnect(void)
+{
+#if defined(MAPLE_MINI)
+	// disconnect USB D+ using dedicated DISC line/circuit on PB9
+	rcc_periph_clock_enable(RCC_GPIOB);
+	gpio_set_mode(GPIOB, GPIO_MODE_OUTPUT_2_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, GPIO9);
+	gpio_set(GPIOB, GPIO9);
+	for (uint32_t i = 0; i < 0x2000; i++) {
+		__asm__("nop");
+	}
+	gpio_clear(GPIOB, GPIO9);
+#else
+	// pull USB D+ low for a short while
+	rcc_periph_clock_enable(RCC_GPIOA);
+	gpio_set_mode(GPIOA, GPIO_MODE_OUTPUT_2_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, GPIO12);
+	gpio_clear(GPIOA, GPIO12);
+	for (uint32_t i = 0; i < 0x2000; i++) {
+		__asm__("nop");
+	}
+#endif
+}
+
+/** flash downloaded data block
+ *  @param[in] usbd_dev USB device (unused)
+ *  @param[in] req USB request (unused)
+ *  @note this function is called after the corresponding GETSTATUS request
+ */
+static void usb_dfu_flash(usbd_device *usbd_dev, struct usb_setup_data *req)
+{
+	(void)usbd_dev; // variable not used
+	(void)req; // variable not used
+	led_off(); // indicate we are processing
+	if (flash_internal_write(flash_pointer, download_data, download_length)) { // write downloaded data
+		flash_pointer += download_length; // go to next segment
+		usb_dfu_state = STATE_DFU_DNLOAD_IDLE; // go back to idle stat to wait for next segment
+	} else { // warn about writing error
+		usb_dfu_status = DFU_STATUS_ERR_WRITE;
+		usb_dfu_state = STATE_DFU_ERROR;
+	}
+	led_on(); // indicate we finished processing
+}
+
+/** disconnect USB and perform system reset
+ *  @param[in] usbd_dev USB device (unused)
+ *  @param[in] req USB request (unused)
+ *  @note this function is called after the corresponding GETSTATUS request
+ */
+static void usb_dfu_reset(usbd_device *usbd_dev, struct usb_setup_data *req)
+{
+	(void)usbd_dev; // variable not used
+	(void)req; // variable not used
+	usb_disconnect(); // USB detach (disconnect to force re-enumeration)
+	scb_reset_system(); // reset device
+	while (true); // wait for the reset to happen
+}
+
+/** handle incoming USB DFU control request
+ *  @param[in] usbd_dev USB device descriptor
+ *  @param[in] req control request information
+ *  @param[in] buf control request data
+ *  @param[in] len control request data length
+ *  @param[in] complete not used
+ *  @return 0 if succeeded, error else
+ *  @note resets device when configured with 5 bits
+ */
+static int usb_dfu_control_request(usbd_device *usbd_dev, struct usb_setup_data *req, uint8_t **buf, uint16_t *len, void (**complete)(usbd_device *usbd_dev, struct usb_setup_data *req))
+{
+	(void)complete;
+	(void)usbd_dev; // device is not used
+
+	// DFU only requires handling class requests
+	if ((req->bmRequestType & USB_REQ_TYPE_TYPE)!=USB_REQ_TYPE_CLASS) {
+		return 0;
+	}
+
+	led_off(); // indicate we are processing request
+	int to_return = 1; // value to return
+	switch (req->bRequest) {
+		case DFU_DETACH: // USB detach requested
+			*complete = usb_dfu_reset; // reset after reply
+			break;
+		case DFU_DNLOAD: // download firmware on flash
+			if (STATE_DFU_IDLE!=usb_dfu_state && STATE_DFU_DNLOAD_IDLE!=usb_dfu_state) { // wrong start to request download
+				// warn about programming error
+				usb_dfu_status = DFU_STATUS_ERR_PROG;
+				usb_dfu_state = STATE_DFU_ERROR;
+			} else if (STATE_DFU_IDLE==usb_dfu_state && ((NULL==len) || (0 == *len))) { // download request should not start empty
+				// warn about programming error
+				usb_dfu_status = DFU_STATUS_ERR_PROG;
+				usb_dfu_state = STATE_DFU_ERROR;
+			} else if (STATE_DFU_DNLOAD_IDLE==usb_dfu_state && ((NULL==len) || (0 == *len))) { // download completed
+				// go to manifestation phase
+				usb_dfu_state = STATE_DFU_MANIFEST_SYNC;
+			} else { // there is data to be flashed
+				if (*len%2) {
+					// we can only write half words
+					usb_dfu_status = DFU_STATUS_ERR_PROG;
+					usb_dfu_state = STATE_DFU_ERROR;
+				} else if (flash_pointer+*len>=(uint32_t)&__application_end) {
+					// application data is too large
+					usb_dfu_status = DFU_STATUS_ERR_ADDRESS;
+					usb_dfu_state = STATE_DFU_ERROR;
+				} else {
+					// save downloaded data to be flashed
+					for (uint16_t i=0; i<*len && i<sizeof(download_data); i++) {
+						download_data[i] = (*buf)[i];
+					}
+					download_length = *len;
+					usb_dfu_state = STATE_DFU_DNLOAD_SYNC; // go to sync state
+					*complete = usb_dfu_flash; // start flashing the downloaded data
+				}
+			}
+			break;
+		case DFU_UPLOAD: // upload firmware from flash
+			to_return = 0; // upload no supported
+			break;
+		case DFU_GETSTATUS: // get status
+			(*buf)[0] = usb_dfu_status; // set status
+			(*buf)[1] = 100; // set poll timeout (24 bits, in milliseconds) to small value for periodical poll
+			(*buf)[2] = 0; // set poll timeout (24 bits, in milliseconds) to small value for periodical poll
+			(*buf)[3] = 0; // set poll timeout (24 bits, in milliseconds) to small value for periodical poll
+			(*buf)[4] = usb_dfu_state; // set state
+			(*buf)[5] = 0; // string not used
+			*len = 6; // set length of buffer to return
+			if (STATE_DFU_DNLOAD_SYNC==usb_dfu_state) {
+				usb_dfu_state = STATE_DFU_DNBUSY; // switch to busy state
+			} else if (STATE_DFU_MANIFEST_SYNC==usb_dfu_state) {
+				usb_dfu_state = STATE_DFU_MANIFEST; // go to manifest mode
+				led_off(); // indicate the end
+				*complete = usb_dfu_reset; // start reset without waiting for request since we advertised we would detach 
+			}
+			break;
+		case DFU_CLRSTATUS: // clear status
+			if (STATE_DFU_ERROR==usb_dfu_state || DFU_STATUS_OK!=usb_dfu_status) { // only clear in case there is an error
+				usb_dfu_status = DFU_STATUS_OK; // clear error status
+				usb_dfu_state = STATE_APP_IDLE; // put back in idle state
+			}
+			break;
+		case DFU_GETSTATE: // get state
+			(*buf)[0] = usb_dfu_state; // return state
+			*len = 1; // only state needs to be provided
+			break;
+		case DFU_ABORT: // abort current operation
+			usb_dfu_state = STATE_APP_IDLE; // put back in idle state (nothing else to do)
+			flash_pointer = (uint32_t)&__application_beginning; // reset download location
+			break;
+		default:
+			to_return = 0;
+	}
+	led_on(); // indicate we finished processing
+
+	return to_return;
+}
+
+void usb_dfu_setup(void)
+{
+	flash_pointer = (uint32_t)&__application_beginning; // set download destination to beginning of application in flash
+	rcc_periph_reset_pulse(RST_USB); // reset USB peripheral
+	usb_disconnect(); // disconnect to force re-enumeration
+	rcc_periph_clock_enable(RCC_GPIOA); // enable clock for GPIO used for USB
+	rcc_periph_clock_enable(RCC_USB); // enable clock for USB domain
+	usb_device = usbd_init(&st_usbfs_v1_usb_driver, &usb_dfu_device, &usb_dfu_configuration, usb_dfu_strings, LENGTH(usb_dfu_strings), usbd_control_buffer, sizeof(usbd_control_buffer)); // configure USB device
+	usbd_register_control_callback(usb_device, USB_REQ_TYPE_CLASS | USB_REQ_TYPE_INTERFACE, USB_REQ_TYPE_TYPE | USB_REQ_TYPE_RECIPIENT, usb_dfu_control_request); // set control request handling DFU operations
+}
+
+void usb_dfu_start(void)
+{
+	// infinitely poll device to handle requests
+	while (true) {
+		usbd_poll(usb_device);
+	}
+}
diff --git a/usb_dfu.h b/usb_dfu.h
new file mode 100644
index 0000000..30ca731
--- /dev/null
+++ b/usb_dfu.h
@@ -0,0 +1,26 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+/** library for USB DFU to write on internal flash (API)
+ *  @file usb_dfu.h
+ *  @author King Kévin <kingkevin@cuvoodoo.info>
+ *  @date 2017
+ */
+#pragma once
+
+/** setup USB DFU peripheral */
+void usb_dfu_setup(void);
+/** start USB DFU handling */
+void usb_dfu_start(void);
+