Bare metal blinky Mbed OS example

This example shows how to achieve memory optimizations in Mbed OS. Starting with a blinky application, the example illustrates how to enable the bare metal profile and further memory optimizations.

You can build this project with all supported Mbed OS build tools. However, this example project specifically refers to the command-line interface tool Arm Mbed CLI.

Install Mbed CLI.
From the command-line, import the example: mbed import mbed-os-example-blinky-baremetal
Change the current directory to where the project was imported.

Application functionality

The main() function toggles the state of a digital output connected to an LED on the board.

Building and running

Connect a USB cable between the USB port on the target and the host computer.
Run the following command to build the example project, program the microcontroller flash memory, and open a serial terminal:
```
$ mbed compile -m <TARGET> -t <TOOLCHAIN> --flash --sterm
```

Your PC may take a few minutes to compile your code.

The binary is located at ./BUILD/<TARGET>/<TOOLCHAIN>/mbed-os-example-blinky-baremetal.bin.

Alternatively, you can manually copy the binary to the target, which gets mounted on the host computer through USB.

Depending on the target, you can build the example project with the GCC_ARM, ARM or IAR toolchain. After installing Arm Mbed CLI, run the command below to determine which toolchain supports your target:

$ mbed compile -S

Expected output

The LED on your target turns on and off every 500 milliseconds, and the serial terminal shows an output similar to:

--- Terminal on /dev/tty.usbmodem21102 - 9600,8,N,1 ---
This is the bare metal blinky example running on Mbed OS 99.99.99.

Configuring the application

The bare metal profile

The bare metal profile is a configuration of Mbed OS that excludes the RTOS, as well as other features. We designed it specifically for ultraconstrained devices because it gives you more control over the system. For more details, please see the bare metal documentation

To build with the bare metal profile, the application configuration file must contain:

{
    "requires": ["bare-metal"]
}

Futher optimizations

Some of the configurations shown below are already set by default in targets/targets.json and platform/mbed_lib.json.

Linking with smaller C libraries

Both the ARM and GCC_ARM toolchains support optimized versions of their C standard libraries, microlib and newlib-nano. We recommend using them with the bare metal profile.

To build with the smaller C libraries, modify the application configuration file:

{
    "target_overrides": {
        "*": {
            "target.c_lib": "small"
        }
    }
}

The build system reverts to the standard C library if support for the small C library is not enabled for your target. You can find more information here.

Using Mbed minimal printf library

Mbed OS offers a smaller printf() alternative. The minimal printf library implements a subset of the v/s/f/printf function family, and you can disable floating points to further reduce code size.

To build with the minimal printf library and disable floating points printing, you need to modify the application configuration file:

{
    "target_overrides": {
        "*": {
            "target.printf_lib": "minimal-printf",
            "platform.minimal-printf-enable-floating-point": false
        }
    }
}

Further optimizations are possible. For more details, please see the minimal printf README.

Using a minimal console

If your application only needs unbuffered I/O operations, you can save additional memory by using a configuration of the platform library, which removes file handling functionality from the system I/O retarget code.

To build with the minimal console functionality, modify the application configuration file:

{
    "target_overrides": {
        "*": {
            "platform.stdio-minimal-console-only": true
        }
    }
}

Memory comparison

The below table shows the result for the blinky bare metal application compiled with the release profile on K64F for the GCC_ARM toolchain.

The baseline configuration used is the blinky bare metal application built with the standard C library.

Mbed OS release: mbed-os-6.0.0-alpha-2

Standard C lib	Small C lib	Minimal printf	Minimal console	RAM	Flash
X				0	0
	X			-2,592	-28,581
	X	X		-2,592	-29,918
	X	X	X	-2,592	-30,810

Troubleshooting

If you have problems, you can review the documentation for suggestions on what could be wrong and how to fix it.

noutram/Task-102-SOS