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Archive for September 24th, 2010

Arduino Mega: Showstopper!

I planned to use an Arduino Mega for an upcoming Circuit Cellar project, but … it doesn’t work. Well, it works, but under very limited circumstances.

The problem manifests itself as a complete crash / lockup under very straightforward conditions: attempting to use the serial output will suffice. This unmodified example sketch fails: AnalogInOutSerial.

After considerable Googling, there’s the showstopper: the gcc-avr compiler fails to save-and-restore a register that gets clobbered by the object constructors. Simple code doesn’t instantiate any objects, so it works fine. The serial failure is just a symptom, which means the various workarounds suggested in the forums don’t fix the general case.

The patch offered for gcc-avr is basically four lines (a pair of save / restores on R20), but requires recompiling what seems to be the entire AVR toolchain from source. That, alas, lies far beyond my capabilities… I could probably figure out enough to recompile it, but I’m very uncertain I could accomplish that without screwing up the main gcc compiler or the setup thereof.

It is not clear to me that the many claims of “it works on this version” are correct. From the nature of the problem, the failures depend critically on addresses occupied, final layout of the program / data in Flash, and (most likely) the execution path. The “working” configurations / systems may simply not fail using the sample programs.

This is on Arch Linux, for what it’s worth, with gcc-avr 4.5.1.

If anybody can walk me through the process of rebuilding whatever must be rebuilt, preferably in a safe place, perhaps I can manually stuff the new file(s) into the proper spots(s) to replace the incorrect ones…

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