FreeRTOS: Introduction to FreeRTOS development

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The FreeRTOS operating system is designed to be compact and task specific. As such, instead of the popular kernel/driver/filesystem/userspace division used by general purpose operating systems, FreeRTOS is just one binary that contains an application and all the relevant kernel code that is necessary to support it. This includes startup code, code that accesses SoC subsystems (drivers), as well as standard kernel functionality such as multitasking, which allows the single-application-per-image model to spawn multiple tasks. In addition to this standard functionality, the CL-SOM-iMX7 FreeRTOS BSP is augmented with the Open-AMP software stack, which allows the M4 and A7 cores to communicate.

Project architecture

FreeRTOS consists of multiple modular parts:

Unit Description Location
kernel FreeRTOS kernel rtos/FreeRTOS/Source
CMSIS headers and libraries Cortex-M specific support platform/CMSIS
SOC specific definitions This BSP only has MCIMX7D definitions platform/devices
Drivers All the drivers in the FreeRTOS BSP are stateless platform/drivers
Middleware libraries Open-AMP stack, including RPmsg support middleware/multicore/open-amp
Additional utilities Contains some printing functions platform/utilities
App code Application code and project files examples/cl-som-imx7

For development, the focal point is the apps projects, which pick and choose the various pieces of FreeRTOS and combine them with the app code into a single project. A good starting point is to pick an example project and start adding functionality.

FreeRTOS API reference

The CL-SOM-iMX7 FreeRTOS BSP is based on the NXP FreeRTOS BSP. The NXP BSP has some useful documents in the BSP archive, one of which provides a comprehensive documentation of FreeRTOS API. The NXP BSP can be downloaded here in the Board Support Packages section.

Multicore development and coexisting with Linux

Since Cortex-A7 and Cortex-M4 share both memory and peripherals, it is necessary to isolate or synchronize between these cores to prevent them from interfering with one another. The RDC (Resource Domain Controller) subsystem of iMX7 provides ways to implement this isolation/synchronization on the hardware level, and RDC support is part of the FreeRTOS API. Nevertheless, the general purpose OS that runs on the Cortex-A7 may not be prepared to handle the possibility that a bus or subsystem may be locked from it, or concurrently used by somebody else, and so disabling functionality on the Cortex-A7 OS may be necessary. In the case of Linux, a special device tree can be used to tell Linux not to touch iMX7 subsystems that we expect to be used by FreeRTOS. Such a device tree is provided in the CL-SOM-iMX7 FreeRTOS BSP.

Building FreeRTOS

Setup development tools

For FreeRTOS development it is possible to use either the GNU ARM Embedded toolchain, or the ARM DS5 IDE suite. The FreeRTOS BSP for CL-SOM-iMX7 has build scripts/project files for both tools.

GNU ARM Embedded toolchain

  • Create a directory for downloading.
mkdir -p ~/toolchain
cd ~/toolchain
tar xvf gcc-arm-none-eabi-6-2017-q1-update-linux.tar.bz2
  • Let the host system know where the toolchain is located
export ARMGCC_DIR=~/toolchain


  • Create a directory for downloading.
mkdir -p ~/toolchain
  • Download ARM DS5 IDE to the toolchain directory.
  • Extract the downloaded archive:
cd ~/toolchain
tar xvf DS500-BN-00019-r5p0-27rel1.tgz
  • Run the installation script and follow the instructions

Getting FreeRTOS BSP sources

  • Create a directory for development.
mkdir -p ~/development
cd ~/development
  • Extract the BSP source archive inside the BSP package:
cd cl-som-imx7-freertos/freertos
tar xvf freertos.tar.bz2

Building FreeRTOS

In this example we will use the hello_world demo project as reference.

Invoking build

GNU ARM Embedded toolchain
  • Go to the armgcc folder inside the hello_world project directory
cd ~/development/cl-som-imx7-freertos/freertos/freertos_source/examples/cl_som_imx7_m4/demo_apps/hello_world/armgcc
  • Run the build script for either of build configurations:
    • For Debug configuration:
    • For Release configuration:
    • For Both (debug and release) configurations:
  • Open DS5 IDE
  • Go to File->Import
  • Select General->Existing Projects into Workspace
  • Click Browse and go to ~/development/cl-som-imx7-freertos/freertos/freertos_source/examples/cl_som_imx7_m4/demo_apps/hello_world/armgcc and click OK
  • Click Finish

Now you can right click on the project and select the build option.

Image location

Depending on the tool used, and the selected build target, the built binary will reside in ~/development/cl-som-imx7-freertos/freertos/freertos_source/examples/cl_som_imx7_m4/demo_apps/hello_world/{armgcc,ds5}/{debug,release}/hello_world.bin.

See also