Chasing a Rollover Crash

I was reminded recently that when writing code in C, you have to take care to understand how variable are going to be used when declaring them. I was had just finished working on the code used to control the fire effects at The Crucible‘s Maker Faire 2013 booth when the system just seemed to come to a halt. That’s not quite what it was supposed to do.

The system was designed to have 3 24′ towers as the central part of the booth. On top of the towers would be accumulator based fire effects – a 24″ round sphere w/ a 2″ exhaust port, a 9″ x 24″ oblong tank w/ a massive 3″ pneumatic solenoid / exhaust and three smaller accumulators based on old fire extinguishers. The solenoids on the fire effects would all be controlled with an Arduino. The idea was that there would be no direct user interaction this year but the system would run automatically. Plug in the Arduino and away we go.

The code would run one of a number of possible sequences, pause between 30 and 90 seconds, randomly run the next sequence, pause . . . And it did that, most of the time. A couple of times after starting up the Arduino, several sequences would run and then nothing else would happen. Made me wonder if I had crashed the Arduino.

I added some Serial.print statements to the code to dump out details on what was happening internally and ran the code again. This time it ran without issue for almost 2 hours before coming to a halt. Looking at the output on the serial console showed that that pause value was -31438. Of course everything came to a halt, the system was attempting to pause negative 31,438 milliseconds! This didn’t make much sense until I reread the Arduino docs and saw that ints are 16 bit values. Of course it rolled over into a negative number.

Digging into the code I realized that I had used int’s in several places where an unsigned long was needed. Once fixed, all was right with the world and the system went on to work just fine for both days of the Maker Faire.

Perhaps I need to start writing these systems on a Raspberry Pi where I can use Python 😉

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