mbed-cmake supports a number of upload methods to help get code onto your device. Some uploaders also support debugging, and for these CMake will help you by automatically running the needed commands to start a debugging session.

In order to enable uploading, you'll need to configure which upload methods your project supports. To do this, you'll need to create an UploadMethodConfig.cmake file and include it in your top-level CMakeLists.txt. You can start by using the template UploadMethodConfig.cmake in the example project (which is for Nucleo F429ZI). You'll then edit it to control the upload methods that are available for your target, and set the parameters that they need to operate. The different parameters and their values are explained in this document.

This page lists each upload method, its parameters (variables that will have a fixed value) and its options (variables that are different for different people and should be set on the command line when you run cmake). Since this config is set by standard CMake code, it's very easy to add your own additional logic (including command line options) to adjust these variables!

Once you configure some upload methods, you can then run cmake with the -DUPLOAD_METHOD=<method> argument to enable uploading your code to the target.

To upload an executable, just run make flash-xxx, where xxx is replaced by the name of the executable target. Don't forget that you can also use make help to see the list of all available targets. Also, for Ninja users, just replace make with ninja in these commands.

To debug, first run make gdbserver in one terminal to start up the debug probe server. Then, you can connect to the GDB server in your debug tool of choice, or run make debug-xxx in another terminal to start command-line GDB on the given program.

All current upload methods (see below for details):

Name	CMake Argument	Supports Uploading	Supports Debugging	Upload Speed	Supported On
No upload method	-DUPLOAD_METHOD=NONE	❌	❌	N/A	N/A
MBed Tools	-DUPLOAD_METHOD=MBED	✔️	❌	Fast	All Mbed Boards, no WSL host OS support
MBed File Copy	-DUPLOAD_METHOD=MBEDCOPY	✔️	❌	Fast	All Mbed Boards, recommended for WSL hosts
J-Link	-DUPLOAD_METHOD=JLINK	✔️	✔️	Fast	Mbed boards with J-Link On-Board. Custom boards with a J-Link probe.
pyOCD	-DUPLOAD_METHOD=PYOCD	✔️	✔️	Medium	Almost all Mbed boards. Custom boards with an ST-Link or DAPLink probe.
OpenOCD	-DUPLOAD_METHOD=OPENOCD	✔️	✔️	(Varies)	A number of different debug probes, but requires configuration.
NXP Bootloader	-DUPLOAD_METHOD=NXPPROG	✔️	❌	Slow	LPC1768 and LPC1114 on custom boards.
STM32Cube	-DUPLOAD_METHOD=STM32CUBE	✔️	✔️	Fast	All STMicroelectronics Mbed boards, custom boards with ST_Link probes.

UPLOAD_METHOD_DEFAULT

Type: String

This sets the default upload method that CMake will use if one is not explicitly set.

GDB_PORT

Type: Integer

This controls the port that GDB servers will be run on when debugging. A value higher than 1024 is recommended to allow debugging without root on Linux/Mac.

This upload method interfaces with standard MBed boards which present themselves as USB drives. The Mbed python tools are used to automatically locate and flash boards connected to the system. So, you will need the mbed-os-tools Python package installed.

MBED_UPLOAD_ENABLED

Type: Bool

Whether the MBed upload method can be activated.

MBED_RESET_BAUDRATE

Type: Integer

Default: 9600

On some boards, Mbed Tools has to connect to the board's serial port in order to reset them. This configuration requires Mbed Tools to know the board rate the board is operating at (though you can also likely get away with setting a slower baud rate here than what's in use).

MBED_TARGET_UID

Type: String

UID of the Mbed board to be programmed. This is only needed if multiple of the same target are connected to your machine -- if it is not set, any one target will be flashed. You can get a the list of UIDs from python -m pyocd list.

This upload method works through the simplest method possible: it just copies the .bin file from your build dir onto the Mbed device's virtual drive.

Notes for Windows: Some STMicro devices don't seem to play well with CMake's built-in file copying routine, and will freeze when code is copied to them using it. To work around this, make sure a "cp" command is available on your path, e.g. from here, and the buildscript will use it instead of the

MBEDCOPY_UPLOAD_ENABLED

Type: Bool

Whether the Mbed file copy upload method can be activated.

MBED_TARGET_UID

Type: String

Path to copy the file to. Set this to the mount point / drive letter of the Mbed device. Note: On Windows, you need a trailing forward slash after the drive letter, or things won't work right, e.g. -DMBED_PATH=D:/.

This upload method connects to your processor via a J-Link JTAG box and the J-Link command line tools. It supports both flashing and debugging. CMake will automatically locate the J-Link tools in their standard install locations on Windows, Mac, and Linux.

JLINK_UPLOAD_ENABLED

Type: Bool

Whether the J-Link upload method can be activated.

JLINK_CPU_NAME

Type: String

The name that your processor is known by to J-link. These are listed here.

JLINK_CLOCK_SPEED

Type: Integer or String

Clock speed of the JTAG or SWD connection. Accepts either a speed in kHz or "adaptive" to automatically determine speed using the RTCK pin.

JLINK_UPLOAD_INTERFACE

Type: String

Interface to use with J-Link. Should be "JTAG" or "SWD". Defaults to JTAG if not set.

This upload method utilizes Mbed's own pyOCD application to flash and debug your processor. pyOCD mainly supports the CMSIS-DAP, DAPLink, and STLink debug probes integrated into Mbed dev boards, but can also use standalone DAP-based programmers and has experimental support for the J-Link probe. Unlike all other debugging programs, pyOCD has the ability to recognize and display the threads that are currently running in Mbed RTOS. This makes it the most convenient debugging solution for many MBed applications.

Installation of pyOCD is usually as simple as python3 -m pip install pyocd, though on some platforms there are additional binary components that need to be installed for certain debug probes.

NOTE: Some older Mbed boards will need to have their firmware updated to work with pyOCD.

PYOCD_UPLOAD_ENABLED

Type: Bool

Whether the pyOCD upload method can be activated.

PYOCD_TARGET_NAME

Type: String

Name of your processor as passed to the -t option of pyOCD. This is usually the full or partial model number.

PYOCD_CLOCK_SPEED

Type: Integer or String

Clock speed of the JTAG or SWD connection. Default is in Hz, but can use k and M suffixes for MHz and GHz

PYOCD_PROBE_UID

Type: String

Probe UID to pass to pyOCD commands. This selects which upload tool is used when multiple are connected to your computer. You can get the UIDs from python -m pyocd list.

This upload method utilizes the OpenOCD application to flash and debug your processor. OpenOCD is highly configurable and supports a huge array of targets, from processors to flash memories to FPGAs. However, this flexibility comes at a cost: it can be a bit of a pain to configure. Using OpenOCD with your target requires you to select the appropriate configuration scripts from its vast array of included scripts, or to write your own if there is no appropriate option. Once this is set up, OpenOCD can then be used as a GDB server and flash programmer.

OpenOCD can be installed through most distro package managers, and Windows binaries can be downloaded from here. Once OpenOCD is installed to a standard location (extract the Windows version to Program Files), it should be automatically be detected by CMake.

OPENOCD_UPLOAD_ENABLED

Type: Bool

Whether the OpenOCD upload method can be activated.

OPENOCD_CHIP_CONFIG_COMMANDS

Type: List of Strings

This config option specifies all OpenOCD commands needed to configure the program for your target processor. At minimum, this should include loading an interface config file and a target config file. Since these options may need to access scripts in the OpenOCD install dir, CMake provides the variable OpenOCD_SCRIPT_DIR which will resolve to the scripts directory of OpenOCD on the current machine.

Uploader that uses the bootloader on certain NXP LPC1XXX and LPC2XXX processors to flash code. USCRPL's fork of nxpprog.py is used to do the upload so programming from Windows 10 using FTDI adapters should work fine.

Using this uploader requires some special considerations in your hardware design: Unless you want to manually reset the CPU each time you upload code, the processor's ISP and reset pins must be connected to the serial port. Specifically, a low on the processor reset pin should be triggered by a high on DTR, and a low on the processor ISP pin should be triggered by a high on RTS.

To use this uploader, you will need to have the serial package installed into the Python installation that CMake is using.

NXPPROG_UPLOAD_ENABLED

Type: Bool

Whether the NXP Bootloader upload method can be activated.

NXPPROG_BAUD

Type: Integer

Baudrate to talk to the bootloader over.

NXPPROG_OSCFREQ

Type: Integer

The bootloader needs to be sent the chip's internal oscillator frequency in order to operate. See your chip's datasheet section on the bootloader for details.

NXPPROG_COM_PORT

Type: Integer

Serial port that the NXP processor is accessible on.

This uploader uses STMicroelectronics' official upload and debugging tools for its ST-LINK programmers. The upload tool can be obtained from the standalone STM32CubeProg package, but unfortunately the GDB server is only included in the STM32Cube IDE, which includes both programs (and also weighs in pretty large at 2.2GB).

In my testing, STM32Cube is at least 5 times faster than PyOCD at uploading code to the chip, so if you have a large program it might be worth taking the time to set up. Also, its debugger seems to be considerably faster at things like setting breakpoints and single-stepping through code.

To use STM32CubeProg only, find the STM32_Programmer_CLI executable on your system and pass the -DSTM32CubeProg_PATH=<path to STM32_Programmer_CLI> argument to CMake to point to it. To use the programmer and the debugger, you must first install STM32Cube IDE. If you installed to the default install location, CMake should find it automatically. If not, set the STM32CUBE_IDE_PATH variable to point to the IDE install dir, which CMake will use to find the other tools. Note that on Macs this needs to point to the IDE dir inside the app package, e.g. -DSTM32CUBE_IDE_PATH=/Applications/STM32CubeIDE.app/Contents/Eclipse

STM32CUBE_UPLOAD_ENABLED

Type: Bool

Whether the STM32Cube upload method can be activated.

STM32CUBE_CONNECT_COMMAND

Type: List

"Connect" (-c) command to pass to the programmer to connect to your target device. port=SWD should be all that's needed for most Mbed boards, but some also seem to need reset=HWrst.

STM32CUBE_GDBSERVER_ARGS

Type: List

Arguments to pass to the ST-Link gdbserver. --swd should be all that's needed in most situations.

STM32CUBE_PROBE_SN

Type: String

Serial number of the ST-Link probe to connect to. If blank, will connect to any probe. You can get the list of serial numbers plugged into your machine with STM32_Programmer_CLI -l.

These configurable options don't cover every single option that each uploader provides. To customize the commands used further, you can create your own uploader CMake module for your needs. First, copy one of the cmake scripts under mbed-cmake/upload_methods to your own project (make sure that the location you add it to is on CMAKE_MODULE_PATH). Then, give it a new name, and change all variables using the old name to use the new name (e.g. UPLOAD_JLINK_FOUND -> UPLOAD_MYMETHOD_FOUND). Next, make the changes you need to the options and commands used, and add any needed configuration settings to your buildscript (including UPLOAD_MYMETHOD_ENABLED). Finally, you can activate the new upload method by passing the name to CMake via -DUPLOAD_METHOD=MYMETHOD.