LIRC and a driver

Last night I built a LIRC driver for the Weebox’s infra red detector. Full credit to the LIRC guys, this was very easy, barring one unfortunate problem (isn’t there always!)

The LIRC source comes with a serial port driver, which uses an infra-red detector plugged into the serial port directly, and uses interrupts from the DCD line to note when infra-red pulses come in. I used this as reference; with one main modification: The EP9302 can detect both rising and falling edges on its IO pins. The serial port can’t and so the serial driver sits in the interrupt routine waiting for a falling edge after it has been interrupted with a rising edge. As this can be a fair few milliseconds, this is a waste of valuable kernel time for my embedded box.

During my investigations I came across a very nasty bug which initially I thought was a compiler bug. No really, it was one of those ones where inserting a printf() fixed the problem, or indeed moving the code about very slightly or changing the optimisation settings fixed it too. In the end — pretty much as ever — it turned out to be an actual programming bug: a zero-sized array was being written to (through a fairly obfuscated set of steps). I’ve mailed the LIRC guys and hopefully they can put it right in the main line.

After finding this I was finally able to get my driver up and running, and the Xbox remote control being detected properly — hooray!

Next up I need to tidy up the mess of code I’ve made while hunting that array bug, and then I’ll get a patch for this driver published somewhere: I’ve already had one email from an interested party!

Filed under: WeeBox Project
Posted at 09:50:12 BST on 15th July 2006.

About Matt Godbolt

Matt Godbolt is a C++ developer working in Chicago in the finance industry.