How to get started with ARM Cortex-M and RTOS?

The ARM Cortex-M series of microcontrollers are extremely popular in embedded systems due to their low cost, low power consumption, and good performance. Using a Real-Time Operating System (RTOS) allows you to better structure your application software and take advantage of features like multithreading and inter-task communication. Here is a step-by-step guide on how to get started with ARM Cortex-M and RTOS development.

1. Pick an ARM Cortex-M microcontroller

There are many ARM Cortex-M chip variants from various semiconductor vendors like STMicroelectronics, NXP, Microchip, and others. Some popular options are:

• STM32F4 series – Cortex-M4 core, floating point unit, DSP instructions, high performance
• STM32F7 series – Cortex-M7 core, floating point unit, DSP instructions, very high performance
• STM32L4 series – Cortex-M4 core, focus on ultra-low power consumption
• NXP Kinetis K series – Cortex-M4 core, good performance/power ratio
• Microchip SAMD51 – Cortex-M4 core, with integrated USB and good peripherals

Pick a microcontroller that matches your application requirements like performance, power, cost, peripherals, package options, etc. The Cortex-M4 is a good starting point providing a balance of features and cost.

2. Get a development board

Most microcontroller vendors offer low-cost development boards to help you get started quickly. These include the microcontroller chip, peripherals like debug adapters, and breakout pins to easily interface sensors, motors, displays etc. Some options are:

• STM32 Nucleo boards – inexpensive boards with Arduino-compatible connectors
• STM32 Discovery kits – include more features like display, sensors etc.
• NXP LPCXpresso boards – includes embedded debug adapter
• Microchip SAM E70 Xplained board – good for SAM D51 microcontroller

Choose a board that provides the I/O interfaces you need and fits your budget. The development board will allow you to easily program, debug and test the microcontroller.

3. Install Tools

You’ll need toolchain software installed on your computer to build and debug applications for the ARM Cortex-M microcontroller. Common options are:

• IDE: Visual Studio Code, Eclipse, IAR Embedded Workbench, Keil uVision – Easy to use IDEs with debugging/project capabilities
• Compiler: GNU ARM Embedded Toolchain – ARM-optimized version of the open-source GCC compiler
• Debugger: OpenOCD, ST-LINK utility – Open source on-chip debuggers to flash and debug code
• Build system: CMake, Makefile – For building and managing code compilation

IDEs like VS Code or Eclipse allow importing existing projects and provide an integrated editing and debugging environment. The compiler transforms your C/C++ code into ARM instructions. Debuggers communicate with the chip for programming and debugging via a wired interface. Build systems automate the compile/link process.

4. Start with a “Blinky” program

The “blinky” program is a simple application that blinks an LED on your development board. This is the “Hello World” equivalent for embedded programming. It will ensure your toolchain is setup properly and that you can build, flash and debug code on the microcontroller. Here are the steps:

1. Create a new project in your IDE for your particular microcontroller and board.
2. The project configuration should set up include paths, compiler settings, loader settings etc.
3. Write a main.c file that initializes a GPIO pin connected to an LED as output.
4. Add a loop that blinks the LED on/off with some delay.
5. Build the project to compile the code into an executable.
6. Connect the board to your computer and flash the program onto the microcontroller.
7. Run and debug the code, setting breakpoints to pause and observe program flow.

Once you have LED blinking, you have all the basics in place to start developing more complex embedded applications.

5. Select and install an RTOS

Here are some popular real-time operating system choices for Cortex-M microcontrollers:

• FreeRTOS – Popular open source RTOS that is widely used. Lots of online resources.
• Amazon FreeRTOS – Enhanced FreeRTOS from Amazon Web Services.
• ChibiOS – Robust open source RTOS targeted at embedded applications.
• Micrium μC/OS – Commercial RTOS with excellent optimization and small memory footprint.
• Mbed OS – Built around FreeRTOS to provide easy driver integration.

FreeRTOS is a good starting point as it is free to use and has lots of online tutorials and documentation. Download the appropriate version for your compiler toolchain. The RTOS code can be treated as a library and integrated into your project.

6. Create RTOS sample projects

To start learning the RTOS, it is helpful to create some simple example projects that demonstrate RTOS concepts. Some ideas:

• Blinking LEDs – Use separate FreeRTOS threads to blink multiple LEDs at different rates.
• Task Communication – Use FreeRTOS queues, semaphores, or notifications for threads to communicate.
• Task Synchronization – Use mutexes to protect shared resources.
• Task Scheduling – Adjust thread priorities and observe scheduling behavior.

These simple demos will help you understand how to create/delete tasks, pass data between tasks, utilize RTOS synchronization primitives, and how the RTOS scheduler works. The FreeRTOS website has some good example tutorials.

7. Develop application software

With basic familiarity of ARM Cortex-M and FreeRTOS (or your RTOS of choice), you can now start building real-world applications. The RTOS facilitates:

• Breaking complex software into smaller isolated tasks
• Clean concurrency using threads and task synchronization
• Event-driven architecture via inter-task communication
• Prioritized execution based on task importance
• Efficient resource allocation between tasks

Some best practices when developing with RTOS:

• Keep tasks simple and focused on one job
• Use RTOS features for task synchronization and data exchange
• Minimize shared global data
• Test inter-task communication thoroughly
• Measure task execution times and CPU utilization

With these techniques you can create complex, responsive and robust embedded applications using the ARM Cortex-M and RTOS platforms.

8. Optimization and Debugging

Here are some tips for optimizing performance and debugging issues with ARM Cortex-M and RTOS:

• Profile task execution time and memory usage to identify bottlenecks.
• Tune RTOS task priority and scheduler settings for your application.
• Use processor-specific optimizations in the compiler settings.
• Enable microcontroller features like caches to improve performance.
• Use static analysis tools to find bugs and ensure compliance to standards like MISRA C.
• Instrument code and use debug/tracing features to understand runtime behavior.
• Simulate your application using virtual prototyping tools models.
• Stress test inter-task communication, synchronization, interrupt response.

Performance tuning and debugging takes time and effort. Leverage as many tools as possible, and iterate your designs frequently to catch issues early.

Next Steps

Congratulations on getting started with ARM Cortex-M and RTOS! Here are some next things you can look into:

• Adding a real-time operating system (RTOS) like FreeRTOS or Micrium μC/OS
• Interfacing with sensors, motors, radios, and other I/O devices
• Implementing communication interfaces like USB, Ethernet, or wireless radios
• Building out your software architecture and optimizing performance
• Handing interrupts and writing low-level hardware drivers
• Testing for compliance with safety standards like IEC 61508 or ISO 26262
• Migrating your design to an application-specific microcontroller chip

ARM Cortex-M microcontrollers and RTOS enable all kinds of innovative embedded applications. Keep learning and building your skills – have fun creating!