I’m assuming you already have a QNX image up and running on your Raspberry Pi 5. I’m also assuming that you know what I2C is and how it works on your usual real-time microcontroller, like an arduino or an STM32 with an RTOS installed.

Interfacing with devices on UNIX-like operating systems

UNIX convention means everything is a file. If you’re used to microcontroller code with no operating system, you would directly read and write to pins on the board or maybe use a library to hide the busy work with communication on the pins.

With an operating system you would prefer to avoid direct pin access and use the file system as an abstraction.

What does it mean to be a resource manager

Resource managers on QNX are similar to drivers. In a microkernel architecture they receive and send messages as their interface. This all gets abstracted away into devctl calls, so the standard POSIX methods are still working.

Making sure I2C is enabled

UNIX-like operating systems by convention expose devices with a file descriptor, found in /dev/. You need to have the I2C driver (called a kernel module on the other nix) enabled and loaded. By default the Raspberry Pi auto-loads needed overlays for any device descriptor accessed. Read the /boot/overlays/readme file for more info about what an overlay is. Loading overlays is a Raspberry Pi 5 concept, implemented by the board designers. The i2c-dwc-rpi5 command just starts the driver.

On the default parameters you wouldn’t need to run or configure anything. If you run ls /dev/i2c* and see I2C devices present, you should be set. On my machine I see there’s already file descriptors at /dev/i2c1 and /dev/i2c6. i2c1 corresponds to the top pins on my board

First interactions with an I2C userspace interface

First thing you’ll note is that there’s no i2c-tools available for QNX. Cross-compiling it would need a patch but I haven’t gotten around to doing that just yet. Any interactions with I2C will need you to run the program as root.

QNX has a very extensive GPIO library ¹ and one of the solutions there is a very convenient API ² to talk on the I2C file descriptor. It’s still possible to create the structures on your own and pass pointers around with devctl, but this library is only one header + one source file and it handles what is essentially standard I2C protocols. The repository also contains a library for SPI, we’ll be using it for communicating with the LoRa module.

Suppose you’ve got an ADS1115 connected on the usual Pi I2C pins, then you could perform I2C operations via the /dev/i2c1 file descriptor.

Documentation and links

• https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino.resmgr/topic/overview_I2C_example.html This explains how I2C communication could work as a resource manager.

• https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino.custom_bsp/topic/i2c/i2c_framework_Sample_calls.html

• https://gitlab.com/qnx/projects/hardware-component-samples/-/tree/main/common/rpi_i2c?ref_type=heads :: Actual code on how to interact with the I2C driver on the Pi 5.