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Starting a New Project

The main thing you need to know when creating your own new project in GCC4MBED is how to create and configure your project's Makefile. A good start to familiarize yourself with the process is to look at the existing sample makefiles such as HelloWorld or USBMouse. If we look at one of these makefiles, we will see something like:

PROJECT         := HelloWorld
DEVICES         := K64F \
                   LPC1768 \
                   NRF51_DK
GCC4MBED_DIR    := ../..
NO_FLOAT_SCANF  := 1
NO_FLOAT_PRINTF := 1

include $(GCC4MBED_DIR)/build/gcc4mbed.mk

Let's take a look at this sample to discuss what each line is doing:

  • The PROJECT variable is set to create the name of the project. The final output binaries (.elf, .bin, .hex) for this project will have a base name of HelloWorld. You should change this to give your project its own cool name.
  • DEVICES is set to contain a list of target devices for which you want to build your project. The above example will produce binaries for K64F, LPC1768, and NRF51_DK targets. In many cases you will have a single device in mind for your project. For example if you were building your sample to just run on the mbed-LPC1768 then you would set DEVICES to that single target, DEVICES := LPC1768.
  • The GCC4MBED_DIR variable indicates where the root of the GCC4MBED project is in relation to this Makefile. In this case, the Makefile is found in samples/HelloWorld so the root of the GCC4MBED project is two directory levels above. This is represented by the ../.. relative path. I will have a bit more to say about this variable later.
  • NO_FLOAT_SCANF and NO_FLOAT_PRINTF are both set to 1, indicating that this HelloWorld sample doesn't require floating point support for either of the Standard C Library's scanf() or printf() family of routines. When using newlib-nano as the Standard C Library, the linker will pull in smaller versions of these routines and thus produce smaller binaries.
  • The last line includes the main gcc4mbed.mk makefile from the GCC4MBED project. This takes the variables that you have set above and uses them to build your project and its dependencies (mbed-os for example). All the real work for getting your project built happens in gcc4mbed.mk and the makefiles it includes.

There are other variables that you can set in your Makefile to further customize the way that GCC4MBED builds your project. You can find them fully documented here.

Location, Location, Location

Where should your put your project's source code? The GCC4MBED_DIR makefile variable gives you a lot of flexibility for where your code can be located relative to the location of your GCC4MBED install.

Within GCC4MBED

The existing samples are already stored within the GCC4MBED repository and you could place your project right alongside those samples. If you did place the source code for your project in GCC4MBED's sample/ folder as a sibling of the HelloWorld and other samples then you would use the same GCC4MBED_DIR setting as you currently see in the existing samples:

GCC4MBED_DIR    := ../..

GCC4MBED Repository as git Submodule

I often use git's submodule functionality to add GCC4MBED as a folder directly within my project. My LedEyes port is an example of such GCC4MBED usage. GCC4MBED is located in the root of my project and the source code, along with makefile are located in the firmware folder. This means that my source code folder and GCC4MBED folder are actually siblings in the root of my project's git repository. If we look at the makefile we will see the following GCC4MBED_DIR setting:

GCC4MBED_DIR    := ../gcc4mbed

Note: This is the file structure I typically use when I am making use of GCC4MBED and can be seen in a few of my public GitHub repositories.

GCC4MBED Repository as Sibling of Project Repository

Sometimes I will just initialize a new git repository for a project's source in the same folder as I have my GCC4MBED repository located. For example if I have all of my git repositories in a folder named /git on my machine then I have a folder named /git/gcc4mbed for GCC4MBED and another folder named something like /git/NewProject for my cool new project. If /git/NewProject/Makefile was the location of the project's makefile then GCC4MBED_DIR would be set to:

GCC4MBED_DIR    := ../gcc4mbed