@LiamDonovan Actually I am thinking that to write to the I2C device from a time-critical thread you may be able to simply the call to write() in I2c_Codec::writeRegister with rt_fprintf().

Alternatively, the proper "Xenomai" way of communicating between rt and non-rt threads is rt_pipe. You would then need a "consumer" non-rt thread which reads from the pipe and writes to the I2C pseudo-file . Xenomai pipes allow bi-directional communication so you could use the same thread to read from I2C and send back.

The catch in these cases is always that you have a finite amount of memory pre-allocated, but that would be the same catch for any other rt implementation. So it is important that the user a) does not write too much data at once (and if they do they should increase the buffer size beforehand) and, b) the user checks for the return value of writeRegister().

I used pipes in the latest Midi https://github.com/BelaPlatform/Bela/blob/dev-midi/core/Midi.cpp but only in one-direction.

thought rt_fprintfis probably less deterministic than using the queue (as the buffering is left to Xenomai), though it probably does not make a huge difference either way for most I2C applications.

ben_wizards I'll get on it once I figure out how rt_pipes work. At the moment it occasionally causes mode switches so it would probably not be very useful in its current state.

    Ok I've got this working. You can get it by checking out the dev-i2c branch from the repo, then take a look at the 06-sensors/i2c_accelerometer example. A disclaimer: it is not yet well tested or documented.

    The important thing is to use i2c.request and i2c.write to read and write from the audio thread (i.e in render). In setup or on an auxiliary task you should instead use i2c.requestNow and i2c.writeNow.

    Suggestions / comments / testing are all welcome.

      LiamDonovan Great! I only have one problem-the I2C libraries for the common OLEDs are all either written for Arduinos or are written in Python. Any idea how I can get around that?

        Can you link to one of the Arduino ones you are looking into using?

        I'm using the adafruit library right now on an small Arduino project I'm doing for a client.

        https://github.com/adafruit/Adafruit_SSD1306

        It's very easy to use (with the Arduino) and the display is awesome-one or two of them would be PERFECT for a Bela based Eurorack module.

        2 months later

        ben_wizards I did have a go at porting the arduino library to Bela but I ended up needing to include the entire arduino codebase within the project just to get it to compile, and it was getting very messy so I abandoned the attempt. When I have a little more time I plan to try again, this time using the datasheet for the SSD1306 and Bela's I2C API. It should just be a case of following the instructions on the data sheet, but the instructions for this particular module are infuriatingly complicated and obscure.

            4 days later

            ben_wizards the one I could find seems to be a python library, did you find any C++ library?
            Going Python is a possible solution, but you would need some sort of inter-process communication (e.g.: sockets or pipes) to communicate between the Bela C++ program and the Python script.
            If you do not need to upgrade the display TOO often (e.g.: less than 5 Hz, I would say) , then you could also consider spawning a python process from the C++ code with a command line parameter to set your visualization, e.g.:
            system("python myscript clearDisplay"), but this is very inefficient as it needs to reload python and import all the libraries for each new command.

              2 months later

              Is there any update on this matter? I bought a display that uses the exact library as above, and am unable to figure out how to get it to work. It seems that it is much harder than I originally expected, especially since their website says "The libraries are written in C++ for Arduino but could easily be ported to any microcontroller by rewriting the low-level pin access functions."

              This is the display I am using:
              https://www.adafruit.com/product/2719

              And the 2 required libraries to use it:
              https://github.com/adafruit/Adafruit-GFX-Library
              https://github.com/adafruit/Adafruit_SSD1306

              Any help would be much appreciated.

              I tried to comment out all the things that were needed in order for these libraries to compile. So now they compile but obviously do not work.
              Anyhow looking at these two commits you will see what would need to be re-implemented (which I have not done and I do not plan to do in the near future):
              https://github.com/giuliomoro/Adafruit-GFX-Library/commit/452dc305f3ded58da66fdd1033d338c6d6e185c1
              https://github.com/giuliomoro/Adafruit_SSD1306/commit/fb63b38e06bed5af5b1f9195b70d4e29f9e07206

              Before going down this rabbit hole, though, I would encourage you to double check if there are any port of the libraries for the BeagleBone. Both projects have several hundred forks, I would be surprised if nothing is there already.

              Great, thanks. I'll give it a look.

              3 months later

              did anyone get this working?

              I'm planning on giving this a go, via the python library... (then communicate with the audio process via osc)
              the following seems a good guide for the BBB

              https://learn.adafruit.com/ssd1306-oled-displays-with-raspberry-pi-and-beaglebone-black/overview
              my questions is... is the bela (or audio cape) likely to 'get in the way', for either SPI or I2C, and which is the preferred approach?
              I guess, looking at the above instructions the question is, are the following valid

              for i2c
              using /dev/i2c-1
              (there is talk in the instruction of ensuring to device overlays are already using)

              for spi
              using /dev/spidev1.0
              enabling via
              optargs=capemgr.enable_partno=BB-SPIDEV0

              thoughts?

              if I can get it working, i'll report back with details 🙂

              a couple of notes, for me as much as anything:
              - the above python method, is built on Adafruit_BBIO, this is probably not going to be in xenomai dist, but it can be built from https://github.com/adafruit/adafruit-beaglebone-io-python
              - someone appears to already have Adafruit_BBIO running, see https://forum.bela.io/d/273-reading-digital-pins-from-pd-in-python

              ok, so got errors whist using apt-get
              E: Archives directory /var/cache/apt/archives/partial is missing. - Acquire (2: No such file or directory)

              resolved by creating,
              now having issues with the python image libraries.....
              ive found these are supposed to be replaced by Pillow,
              so trying to do

              pip install Pillow

              but this fails, due to headers for zlib not being available,
              so trying to install zlib dev with

              apt-get install zlib1g-dev

              but thats not found... any ideas, how to install zlib dev?

              EDIT:
              ok, compiled and installed zlib from source (GitHub), now have the same issue with libjpeg8-dev
              (unfortunately you cannot disable jpeg support in pillow in current release!)

              ok, I'm confused... what is going on with the distribution sources for bela (0.2.0a)

              I can see that an armhf packages is available on debian wheezy, and it appears this is in the source list
              deb http://ftp.us.debian.org/debian/ wheezy main contrib non-free

              but its just saying no candidate for things like libzlib1g-dev and libjpeg8-dev...
              and ive done both apt-get update and upgrade

              unfortunately i cannot find the source for libjpeg8 so i cant build it , like i did for zlib....

              really frustrating, that i cant just pick up the sources, or the packages...

                for me apt-get install worked fine with zlib1g-dev, while libjpeg8-dev seems to have been replaced by libjpeg62-turbo-dev, which installs just fine, at which point
                pip install Pillow works fine.
                Check the following:

                a) before any apt-get operation, is the date on the board correct? (it should get it from the network, but just double check. Opening the IDE page in the browser after a reboot will set the current date (which will be reset at the next boot)
                b) run apt-get update before trying to apt-get install
                c) I am using the image v0.2.0b, December 2016, I think you are not (as denoted by the missing /var/cache/apt/archives/partial, see here). I think apt-get update should fix your issue, otherwise try upgrading to this release: https://github.com/BelaPlatform/bela-image/releases/tag/v0.2.0b_stable

                Or perhaps the apt-get upgrade bit screwed up things? We had to add jessie repositories to this image (despite it being Wheezy) to get some of the newest stuff, which is why - I imagine - some things may get screwed if you upgrade.
                Again, the steps above work fine on 0.2.0b