Porting guide: adding target support to mbed OS 5

There are two typical ways to add target support to mbed OS: Either you’ll be adding a completely new microcontroller and board, or you’ll be adding a new board that contains an already-supported microcontroller.

There is a dependency on CMSIS-CORE and CMSIS-Pack, so make sure the microcontroller already has these available.

Adding a new microcontroller

Add the empty target implementation reference to the working directory using:

mbed import mbed-os-example-new-target
cd mbed-os-example-new-target\mbed-os
git checkout master
git pull
git checkout -b my-new-target
git remote set-url origin https://github.com/USERNAME/mbed-os
cd ..

Target description

Add the target description to mbed-os\targets\targets.json:

    "inherits": ["Target"],
    "core": "Cortex-M3",
    "supported_toolchains": ["ARM", "GCC_ARM", "IAR"],
    "extra_labels": ["VENDOR", "MCU_FAMILY", "MCU_NAME"],
    "macros": [],
    "device_has": ["SERIAL", "STDIO_MESSAGES"]
    "inherits": ["MCU_NAME"],
    "macros_add": []

Bootstrap code

You need a number of files to first successfully compile. You need CMSIS-CORE files for startup and peripheral memory addresses as well as the linker scripts for ARM, IAR and GCC toolchains. These files are usually in the mbed-os\targets\TARGET_VENDOR\TARGET_MCU_FAMILY\TARGET_MCUNAME\device directory.

If the microcontroller has a SYS_TICK, the RTOS will configure this, so only a few macros are needed in mbed-os\targets\TARGET_VENDOR\mbed-rtx.h.

Dynamic vector relocation also requires some extensions to CMSIS-CORE. mbed-os\targets\TARGET_VENDOR\TARGET_MCUNAME\cmsis.h is the entry point to the target for mbed OS software. This will include the vector relocation additions and device-specific headers that include CMSIS-CORE. A relocation routine is needed in mbed-os\targets\TARGET_VENDOR\TARGET_MCUNAME\cmsis_nvic.c and .h

Now verify your target is identified by using mbed compile -m MCU_NAME -t <toolchain>. This compiles.

The next step is to enable the test harness dependencies. To run the test suite, there is a minimum requirement of GPIO, microsecond ticker and serial implementation, all explained below.


The gpio_s is for referencing memory-mapped GPIO registers and passing related pin information or other IO operation data that the HAL needs. These structures are defined in objects.h. The required implementation definition is in mbed-os\hal\gpio_api.h.


The serial_s is for referencing memory-mapped serial registers and passing related pin and peripheral operation information data that the HAL needs. These structures are defined in objects.h. The required implementation definition is in mbed-os\hal\serial_api.h.

Microsecond ticker

The microsecond ticker is a system resource that many APIs and other timing utilities use. It needs a one microsecond resolution and should be implemented using a free-running hardware counter or timer with match register. The required implementation definition is in mbed-os\hal\us_ticker_api.h.

At this point, we should be able to compile a handful of tests:

mbed test -m BOARD_NAME --compile -t <toolchain>

To execute the tests, you’ll need to already support mbed-ls.

All the others

At this point, the HAL structure should be familiar as a programming model. There are many more APIs to implement, which you enable by adding a device_has attribute to the MCU_NAME and then providing the implementation. Other HAL APIs include but are not limited to:

device_has       |   api
ANALOGIN         |   analog_in.h
ANALOGOUT        |   analog_out.h
CAN              |   can_api.h
EMAC             |   emac_api.h
INTERRUPTIN      |   gpio_irq_api.h
I2C I2CSLAVE     |   i2c_api.h
LOWPOWERTIMER    |   lp_ticker_api.h
PORT_IN PORT_OUT |   port_api.h
PWMOUT           |   pwmout_api.h
RTC              |   rtc_api.h
SLEEP            |   sleep_api.h
SPI SPISLAVE     |   spi_api.h
TRNG             |  trng_api.h