We are pleased to announce that we have just released the full mbed SDK under a permissive open source license!

About the mbed SDK

The mbed Software Development Kit (SDK) is a C/C++ microcontroller software platform already relied upon by tens of thousands of developers to write code for ARM microcontrollers. It has always been free for commercial and noncommercial use, but this latest release under an open source license will extend its applicability further to be relied on in more commercial, open source and educational projects.

We've designed the mbed SDK to provide enough hardware abstraction to be intuitive and concise, but powerful enough to build complex projects. It includes all the base level startup code, C runtime and library pre-integrated and tested for the target microcontrollers, and high-level MCU peripheral APIs that allow you to drive the peripherals of the microcontrollers without going near a datasheet. It is built on the low-level ARM CMSIS APIs, which is great for allowing you to code down to the metal if needed. In addition to RTOS, USB and Networking libraries, the mbed Cookbook now has hundreds of reusable peripheral and module libraries that have been built on top of the SDK by the hard work of the mbed developer community.

Open Source License

The SDK is now licensed under the permissive Apache 2.0 open source licence.

We wanted to make sure the license we chose made it possible to use the SDK in both commercial and personal projects with confidence, including no obligations to open source your own code if you didn't want to. Whilst we encourage sharing of code and experience to be reusable by others, we certainly don't want to enforce it, and a permissive license provides that freedom for our users to keep the options open.

The most widely used permissive licenses are MIT, BSD and Apache 2.0. We've been a fan of MIT for a while because the license and intent is so simple to understand, but the Apache 2.0 license, as used by Android, is really the result of bring the MIT/BSD-style license up to date. It meant a bit more work on our part to ensure we could release under this license, but the result is much better for everyone choosing to use the SDK, especially in a commercial environment where choosing to use open source code may depend on the better guarantees Apache 2.0 brings.

We'll also be making it easy for and encouraging other contributed code to be published under the same Apache 2.0 license, so there is a growing level of consistency in the licensing of other code published on mbed.org too.

Why now?

Up to this point, we've always provided the SDK as a pre-built library. One of our main reasons for doing this was to establish a really stable API that developers could rely on and build up trust in, and ensure we got the feedback when they couldn't do something. We also wanted to avoid people dipping in to and coming to rely on the library internals meaning we couldn't transparently change the implementation details in the future without breaking compatibility.

And the main reason we wanted to be able to change the implementation details under the hood without impacting API compatibility was to do what we've just spent the last year working hard on; the SDK now gives you transparent portability for code based on the SDK across:

• Multiple microcontrollers: Our official mbed Microcontrollers and other supported hardware targets
• Multiple toolchains: Our Free Online Compiler and other Professional and Open Source Toolchains

This is really powerful. All the effort we've put in to making and testing the SDK to be transparently portable also means libraries built on the mbed SDK, such as all component and module libraries already in the mbed Cookbook, can be written once and used totally unchanged across targets and toolchains! This is great for enabling reuse and continued improvement in library quality and functionality.

With the APIs now established with such a critical mass and SDK trusted by so many, and all this portability framework now in place, the build-only library restriction has achieved the goals we set out for. There is still a lot more to do on the SDK to get it where we are aiming, but benefits of it being open as we work on it now outweigh the costs, so the time is right to donate the source to the community as well. Thanks for your patience!

What it means for the community

We have been lucky enough to attract and build a very friendly and active community, and members have a huge breadth and depth of knowledge and expertise in mbed and lots of related technologies. There are already thousands of published open source programs and libraries based on the SDK, so this means the source for the whole program stack is now available for all these programs.

We think the most important aspects of open sourcing of the mbed SDK and giving the community access to all the mbed library sources, and its multi-toolchain build system, will be:

• Developers working on commercial products will not have to worry of any lock-in and they will be able to modify trade-offs unsuitable for their embedded system
• Open source projects based on the mbed platform will be able to provide a completely open software stack
• Those looking to learn or contribute will now be able to delve in to the depth of the lowest level implementations

Whilst the SDK is already very established and stable, it is still very much a work in progress for where we've set our sights. We hope that working on it more publicly will also help make sure we end up in the right place for all our developers we support.

In these first few years of mbed it has been amazing to witness what everyone has been able to achieve with the mbed platform, very often going beyond our original use cases. We hope this milestone is the next step in making the platform even more powerful, flexible and applicable to lots of projects that could benefit from our work and all the efforts of the amazing mbed developer community!

To read more and get access to the mbed SDK, see:

• mbed SDK

For more information on the mbed project, see:

• Explore mbed

Have fun and keep innovating!