A few weeks back now, I worked on a project for iDAT. I came in at the end to fix a few bugs on a Android/hardware integration project. The idea was to use an Android phone, along with a IOIO board and a Polar heart rate monitor breakout board to collect data on a person’s movements, heart rate (and a few other things).
This was split into two distinct sections; the first was the hardware itself and secondly the software which would poll data from the IOIO board and do something to visualise it.
For the hardware, I started off first trying to get some data from the HMRI (the shorter name of the breakout board) using an Arduino. This worked quite quickly. The board uses I²C to communicate and so this is only a few pins. Whilst Dan Julio provides some example code for the Arduino, this was a bit old and so I needed to update this. You can find the updated code in my GitHub project, hmri_arduino. It should work fine with Arduino 1.0 and greater.
To interface the HMRI with the IOIO board, the process was much the same as the Arudino, but I needed to provide two 4.7kΩ pull up resistors to get a signal (the Arduino provides these for you). I also needed to desolder the SJ1 contacts and solder the OP0 contacts for an I²C connection, a section from the manual is listed below.
After this, I needed to interface the HMRI, through the IOIO board on the Android device. The IOIO project provides a library to do this — the interface is Arduino inspired — and once you get the correct version, it’s quite easy to get working.
I was using a Samsung Galaxy SII (with a slightly older version of Android) and the older style IOIO board. I matched up the board type with the current release as listed on the IOIO downloads page. This ended up stumbling me for quite a while; the board refuses to connect without precisely the right firmware/library combination.
For the implementation itself, I used a singleton[^antipattern] which is started up with the main activity and is then shutdown on completion. The idea behind this was to create a connection to the hardware and keep one thread which would handleit (the singleton starts a thread for interacting with the hardware, and then provides a thread-safe way in which to interface with it). Sadly, this didn’t quite work with the way Android is designed and so doesn’t quite work how I had expected. Instead, the hardware connection is started and stopped when the main activity run-loop is either started or stopped. Due to the way Android implements it’s run-loop this is all that is possible (coming from predominantly iOS, this such an odd implementation and quite annoying).
I never resolved the issue before shipping; as far as I understand, it’s impossible to fully resolve it because of Android’s stupid implementation.
Update: Ytai Ben-Tsvi (the developer behind IOIO) emailed me to say that I should have been using Android’s “Services” functionality, which I didn’t know existed. This would solve all of my state issues.
And finally, the code for the whole project is up on GitHub (which should give you the best way to see it in action).
[^antipattern]: Many consider the Singleton to be a programming anti-pattern, but I disagree for situations such as this. Yes, it does introduce global scope, but when you’re accessing hardware, or a data model (where I’ve used singletons before) it works quite well. You only want to instantiate it once, anyway. In the case of hardware it’s preferable to keep the same connection open.