NuttX support for Gemstone is currently under development. Not all peripherals on the board are usable at this time.
NuttX is an open-source operating system designed for real-time applications. Compliant with POSIX and ANSI standards, NuttX is optimized for embedded systems and microcontrollers. The primary reason for choosing NuttX for the R5F cores on the Gemstone board is its deterministic behavior and reliable management of real-time tasks. Additionally, NuttX is a platform supported by popular autopilot software like Ardupilot and PX4, making it critical for drone and autonomous vehicle projects. Since there is no official NuttX support for the AM67A SoC yet, a custom configuration named beagley-ai has been created. This configuration is tailored to the hardware features of our board and the capabilities of the R5F cores.

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Toolchain Installation

To compile NuttX, we need a cross-compiler compatible with ARM Cortex-R5F cores. The gcc-arm-none-eabi toolchain is the standard solution for this requirement. On Ubuntu/Debian systems, you can install the toolchain using the package manager: 
sudo apt update
sudo apt install gcc-arm-none-eabi
Alternatively, you can download the latest version from ARM’s official website and perform a manual installation. After installation, verify the toolchain is working correctly with the command arm-none-eabi-gcc --version.

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Downloading the Source Code

The NuttX ecosystem consists of two main repositories: the core operating system and the application framework. To download the customized versions for the Gemstone board, follow these steps: 
# Clone the main NuttX repository
git clone https://github.com/t3gemstone/nuttx.git

# Clone the NuttX applications as the 'apps' folder
git clone https://github.com/t3gemstone/nuttx-apps.git apps

# Navigate to the main working directory
cd nuttx
This setup creates the standard directory structure expected by NuttX. The apps folder contains the applications and services that will run on the operating system.

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Applying the Configuration

NuttX uses pre-prepared configurations for different hardware platforms. Let’s start by applying the beagley-ai configuration for our Gemstone board: 
./tools/configure.sh -l beagley-ai:nsh
This command configures the basic settings tailored to the AM67A SoC and enables NuttShell (nsh), a simple command-line interface. NuttShell allows you to perform basic file operations and system management tasks.

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Customizing the Configuration

Sometimes, you may need to modify the default configuration. NuttX uses a menuconfig tool similar to the Linux kernel: 
make menuconfig
Through this graphical interface, you can enable drivers, adjust memory settings, or add new features. To save your changes permanently, use the savedefconfig command: 
make savedefconfig
This command saves only the configurations that differ from the defaults, preventing unnecessary bloating of the configuration file.

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Debug Configuration

By default, NuttX is configured in release mode, producing a small-sized binary optimized for performance but lacking debug information. For step-by-step debugging during development, you need to switch to debug mode. Open the menuconfig tool and navigate to: Build Setup → Debug Options → Enable Debug Features. Also, enable the Generate Debug Symbols option. These settings ensure the compiler embeds debug information into the binary, allowing the debugger to perform source-level debugging.

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Compilation Process

Once the configuration is complete, you can compile NuttX: 
make -j$(nproc)
This command performs parallel compilation using all available CPU cores. The compilation results in an ELF file named nuttx, which contains the operating system image to be loaded onto the R5F core.

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CCS IDE Integration

Texas Instruments Code Composer Studio (CCS) is a development environment specifically optimized for the AM67A SoC family. To integrate your NuttX project into CCS, open the IDE and use File → Import → C/C++ → Existing Code as Makefile Project. Select the nuttx folder as the project directory and specify Cross GCC as the toolchain. This configuration allows CCS to understand the NuttX source code and provide features like syntax highlighting and code navigation.

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Debug Preparation with SBL NULL

Under normal operating conditions, the tiboot3.bin file located in the SD card’s boot partition launches the U-Boot bootloader. However, to enable debug access to the R5F cores, we need to replace this file with the tiboot3.bin produced by the SBL NULL project. SBL NULL is a special version of the System Boot Loader. This version initializes the SoC at a basic level and leaves the R5F cores in WFI (Wait For Interrupt) mode. This state allows the cores to wait for the debugger to connect without executing any code. Insert the SD card into your computer, back up the tiboot3.bin file in the boot partition, and replace it with the file produced by SBL NULL. Reinsert the card into the Gemstone board and power it on.

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Debugging Process

To start debugging in CCS IDE, first open your Target Configuration file and connect to the AM67A SoC. From the Run → Debug menu, select the MCU R5F core. Once the debugger is connected, use Run → Load → Load Program to select the nuttx ELF file. This process loads the NuttX operating system into the R5F core’s memory. After the program is loaded, you can set breakpoints, monitor variables, and step through the code. This allows you to examine critical components like NuttX’s task scheduler and interrupt handlers in detail.

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Cleanup Operations

During development, you may occasionally need a fresh start. NuttX provides two levels of cleanup commands: 
# Cleans only object files and binaries
make clean

# Cleans all build files and configurations
make distclean
The make clean command removes files generated during compilation but preserves the configuration. make distclean resets the project to its initial state, requiring you to reconfigure it. Once this process is complete, your Gemstone board’s R5F cores will be running a fully functional NuttX operating system, providing a solid foundation for your real-time applications.