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The dapboot project is an open-source USB Device Firmware Upgrade (DFU) bootloader for STM32 devices.

Currently, the only targets officially supported are the STM32F103x series.

Build instructions

The default target is a generic STM32F103 dev board with an LED on PC13, commonly referred to as a "bluepill" board.

To build other targets, you can override the TARGET variable when invoking make.

make clean


Target Name Description Link
BLUEPILL Cheap dev board
MAPLEMINI LeafLabs Maple Mini board and clone derivatives
STLINK STLink/v2 hardware clones
OLIMEXSTM32H103 Olimex STM32-H103
BTTSKRMINIE3V2 BigTreeTech SKR MINI E3 V2.0 (3D printer motherboard)
BTTSKRMINIE3V2_USBMOD BTT SKR MINI E3 V2.0 with USB pullup removed

For the above targets there are some potential variants that can be added to the target name based on what the target supports:

Target Variant Description
Standard bootloader, using first 8kB of flash
_HIGH High memory bootloader for 64kB chips (experimental)
_HIGH_128 High memory bootloader for 128kB chips (experimental)
_HIGH_256 High memory bootloader for 256kB chips (experimental)

The high memory bootloader is a variation that doesn't require the application to be at an offset, the bootloader resides in the top 6.5kB of ROM and hides its reset and stack vectors inside unused entries of the application vector table. As an example, to compile for a Bluepill board with 128kB flash, use:

make clean

Flash instructions

The make flash target will use openocd to upload the bootloader to an attached board. By default, the Makefile assumes you're using a CMSIS-DAP based probe, but you can override this by overriding OOCD_INTERFACE variable. For example:

make OOCD_INTERFACE=interface/stlink-v2.cfg flash

Overriding defaults

Local makefile settings can be set by creating a, which is automatically included.

Here is an example that changes the default target to the STLink/v2 and uses an unmodified STLink/v2 to flash it.

OOCD_INTERFACE ?= interface/stlink-v2.cfg

You can also use the env variable DEFS to override default configuration for a target, like:

# Disable LED on BluePill

# Allow access to all Flash on MapleMini and change the app base address
DEFS="-DFLASH_SIZE_OVERRIDE=0x20000 -DAPP_BASE_ADDRESS=0x08004000" make TARGET=MAPLEMINI LDSCRIPT="/some/folder/stm32f103x8-16kb-boot.ld"

Using the bootloader

Building for the bootloader

The standard bootloader occupies the lower 8KiB of flash, so your application must offset its flash contents by 8KiB. This can be done by modifying your linker script or flags as appropriate.

The high memory bootloaders do not use the lower part of the flash, so you only need to make sure your application leaves 6.5kB of flash free.

Switching to the bootloader

The bootloader can be built to look for arbitrary patterns, but the default looks for a magic value stored in the RTC backup registers. Writing the magic value and then resetting will run the bootloader instead of the main application.

In version v1.11 and earlier, the bootloader for STM32F103 targets looks for 0x544F in RTC backup register 2 and 0x4F42 in RTC backup register 1 (together they spell "BOOT" in ASCII). From v1.20 on, the bootloader will only use RTC backup register 1 and check for 0x4F42 on targets with 16-bit backup registers and 0x544F4F42 on targets with 32-bit backup registers.

The backup register and bootloader command word can be customized with the REG_BOOT and CMD_BOOT defines respectively.

You can also use a button to stay in bootloader while booting. It's configured using HAVE_BUTTON define. If your button has a debounce capacitor, you can use BUTTON_SAMPLE_DELAY_CYCLES define to specify how many cycles to wait before sampling the I/O pin, by default it is approximately 20ms in a 72Mhz MCU.

On the bluepill boards, the default is using the BOOT1 input (available on jumper in the board) to enter the bootloader.


This bootloader implements the draft WebUSB specification, which allows web pages to access the bootloader (after presenting the user with a device picker dialog).

For a demo implementing dfu-util features in the browser, see

To customize the WebUSB landing page, you can use the LANDING_PAGE_URL define. To set it from the command line, you can use the DEFS environment variable:


Note that the URL scheme shoul not be part of the LANDING_PAGE_URL string. As of this writing, it is hardcoded to HTTPS.

Manifestation behavior

There are two different manifestation behaviors that can be selected using the DFU_WILL_DETACH define. By default, DFU_WILL_DETACH is set to 1 for backwards compatibility with existing behavior.

When DFU_WILL_DETACH is enabled, the bootloader autonomously reboot into the new firmware after it has been successfully downloaded and passes the basic validation check. This can be helpful on platforms such as Windows where it is not possible to generate a USB bus reset to signal that the bootloader should switch to the application.

When DFU_WILL_DETACH is disabled, the bootloader will return to the idle state after validating the firmware. The DFU host application can then perform other operations or reboot the target into the new firmware by issuing a DFU detach request or generating a USB bus reset. (This can be done with dfu-util by passing the -R flag).


The default USB VID/PID pair (1209/DB42) is allocated through the open-source USB PID program.

To use a custom VID/PID pair, you need to set the macros USB_VID and USB_PID. One way to do this is by setting the DEFS environment variable when compiling:

 DEFS="-DUSB_VID=0x1209 -DUSB_PID=0xCAFE" make

USB Vendor, Product, and Interface strings

To customize the USB vendor, product, and interface strings that DAPBoot reports to the USB host, you may set the macros USB_VENDOR_STRING, USB_PRODUCT_STRING, and USB_INTERFACE_STRING, respectively. You can do this by setting the DEFS environment variable or including these macros in your board's config.h


All contents of the dapboot project are licensed under terms that are compatible with the terms of the GNU Lesser General Public License version 3.

Non-libopencm3 related portions of the dapboot project are licensed under the less restrictive ISC license, except where otherwise specified in the headers of specific files.

See the LICENSE file for full details.