Adding and configuring mbed targets

mbed uses JSON as a description language for its build targets. The JSON description of mbed targets can be found in targets/targets.json. To better understand how a target is defined, we’ll use this example (taken from targets.json):

    "TEENSY3_1": {
        "inherits": ["Target"],
        "core": "Cortex-M4",
        "extra_labels": ["Freescale", "K20XX", "K20DX256"],
        "OUTPUT_EXT": "hex",
        "is_disk_virtual": true,
        "supported_toolchains": ["GCC_ARM", "ARM"],
        "post_binary_hook": {
            "function": "TEENSY3_1Code.binary_hook",
            "toolchains": ["ARM_STD", "ARM_MICRO", "GCC_ARM"]
        "device_name": "MK20DX256xxx7",
        "detect_code": ["0230"]

The definition of the target called TEENSY3_1 is a JSON object. The properties in the object are either “standard” (understood by the mbed build system) or specific to the target.

Standard properties

This section lists all the properties that are known to the mbed build system. Unless specified otherwise, all properties are optional.


The description of an mbed target can “inherit” from one or more descriptions of other targets. When a target A inherits from another target B (A is the child of B and B is the parent of A), it automatically “borrows” all the definitions of properties from B and can modify them as needed (if you’re familiar with Python, this is very similar to class inheritance). In our example above, TEENSY3_1 inherits from Target (most mbed targets inherit from Target). This is how Target is defined:

"Target": {
    "core": null,
    "default_toolchain": "ARM",
    "supported_toolchains": null,
    "extra_labels": [],
    "is_disk_virtual": false,
    "macros": [],
    "detect_code": [],
    "public": false

Since TEENSY3_1 inherits from Target:

  • core is a property defined both in TEENSY3_1 and Target. Since TEENSY3_1 redefines it, the value of core for TEENSY3_1 is Cortex-M4.
  • default_toolchain is not defined in TEENSY3_1, but since it is defined in Target, TEENSY3_1 borrows it, so the value of default_toolchain for TEENSY3_1 is ARM.

A target can add properties that don’t exist in its parent(s). For example, OUTPUT_EXT is defined in TEENSY3_1, but doesn’t exist in Target.

It’s possible to inherit from more than one target. For example:

"ImaginaryTarget": {
    "inherits": ["Target", "TEENSY3_1"]

In this case, ImaginaryTarget inherits the properties of both Target and TEENSY3_1, so:

  • The value of ImaginaryTarget.default_toolchain is ARM (from Target).
  • The value of ImaginaryTarget.OUTPUT_EXT is hex (from TEENSY3_1).
  • The value of ImaginaryTarget.core is null (from Target, since that’s the first parent of ImaginaryTarget that defines core).

Avoid using multiple inheritance for your targets if possible, since it can get pretty tricky to figure out how a property is inherited. If you have to use multiple inheritance, keep in mind that the mbed target description mechanism uses the old (pre 2.3) Python mechanism for finding the method resolution order:

  • Look for the property in the current target.
  • If not found, look for the property in the first target’s parent, then in the parent of the parent and so on.
  • If not found, look for the property in the rest of the target’s parents, relative to the current inheritance level.

For more details about the Python method resolution order, check this link.


The name of the target’s ARM core.

Possible values: "Cortex-M0", "Cortex-M0+", "Cortex-M1", "Cortex-M3", "Cortex-M4", "Cortex-M4F", "Cortex-M7", "Cortex-M7F", "Cortex-A9"


Some mbed targets may be defined solely for the purpose of serving as an inheritance base for other targets (as opposed to being used to build mbed code). When such a target is defined, its description must have the public property set to false, to prevent the mbed build system from considering it as a build target. An example is the Target target shown above.

If public is not defined for a target, it defaults to true.

Note: unlike other target properties, the value of public is not inherited from a parent to its children.

macros, macros_add, macros_remove

The macros in this list will be defined when compiling mbed code. The macros can be defined with or without a value. For example, the declaration "macros": ["NO_VALUE", "VALUE=10"] will add these definitions to the compiler’s command line: -DNO_VALUE -DVALUE=10.

When target inheritance is used, it’s possible to alter the values of macros in inherited targets without re-defining macros completely:

  • An inherited target can use macros_add to add its own macros.
  • An inherited target can use macros_remove to remove macros defined by its parents.

For example:

    "TargetA": {
        "macros": ["PARENT_MACRO1", "PARENT_MACRO2"]
    "TargetB": {
        "inherits": ["TargetA"],
        "macros_add": ["CHILD_MACRO1"],
        "macros_remove": ["PARENT_MACRO2"]

In this configuration, the value of TargetB.macros is ["PARENT_MACRO1", "CHILD_MACRO1"].

extra_labels, extra_labels_add, extra_labels_remove

The list of labels defines how the build system looks for sources, libraries, include directories and any other files that are needed at compile time. extra_labels makes the build system aware of additional directories that must be scanned for such files.

If target inheritance is used, it’s possible to alter the values of extra_labels using extra_labels_add and extra_labels_remove. This is similar to the macros_add and macros_remove mechanism described in the previous paragraph.

features, features_add, features_remove

The list of features defines what hardware a device has.

mbed, libraries, or application source code can then select different implementations of drivers based on hardware availability; selectively compile drivers for existing hardware only; or run only the tests that apply to a particular platform.

If target inheritance is used, it’s possible to alter the values of features using features_add and features_remove. This is similar to the macros_add and macros_remove mechanism described in the previous two paragraphs.


This is the list of toolchains that can be used to compile code for the target. The known toolchains are ARM, uARM, GCC_ARM, GCC_CR, IAR.


The name of the toolchain that will be used by default to compile this target (if another toolchain is not specified). Possible values are ARM or uARM.


Some mbed targets require specific actions for generating a binary image that can be flashed to the target. If that’s the case, these actions can be specified using the post_binary_hook property and custom Python code. For the TEENSY3_1 target above, the definition of post_binary_hook looks like this:

"post_binary_hook": {
    "function": "TEENSY3_1Code.binary_hook",
    "toolchains": ["ARM_STD", "ARM_MICRO", "GCC_ARM"]

In this example, after the initial binary image for the target is generated, the build system will call the function binary_hook in the TEENSY3_1Code class.

Note: The definition of the TEENSY3_1Code class must exist in the file.

Since toolchains is also specified, binary_hook will only be called if the toolchain used for compiling the code is either ARM_STD, ARM_MICRO or GCC_ARM. Note that specifying toolchains is optional: if it’s not specified, the hook will be called no matter what toolchain is used.

As for the binary_hook code, this is how it looks in

class TEENSY3_1Code:
    def binary_hook(t_self, resources, elf, binf):
        from intelhex import IntelHex
        binh = IntelHex()
        binh.loadbin(binf, offset = 0)

        with open(binf.replace(".bin", ".hex"), "w") as f:
            binh.tofile(f, format='hex')

In this case, it converts the output file (binf) from binary format to Intel HEX format.

The hook code can look quite different for different targets. Take a look at the other classes in for more examples of hook code.


Passes necessary data for exporting the mbed code to various third party tools and IDEs.

This is possible because the device name corresponds to a field in publicly hosted CMSIS packs. These packs hold target properties. This is the PDSC that contains the TEENSY_31 device (MK20DX256xxx7). The device information begins on line 156. The dname (device name) field on line 156 directly corresponds to that in the Uvision5 IDE target selection window. Beginning on line 15 of tools/export/uvision/uvision.tmpl, target information from these packs is used to generate valid Uvision5 projects. If the device name is not found, we use a generic ARM CPU target in Uvision5.

tools/export/iar/iar_definitions.json uses this device name to store information necessary to set the target in an IAR project.