Sorry to restart the thread, but here were some ideas I had.
Notice that there are a lot of "Maybe"s. This is all theoretical, I have not done any research on this stuff
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The SD card library takes an ENOURMOUS amount of flash. If there is some way to only have the GPU run off it? I dont know much about TFT's.
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Perhaps you could put two 328's on it. One that does driving, one for user interface? At least it's some thing to consider. Gets rid of a lot of interrupts.
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It is a small chance, but some coders have made an arduino that has been taught BASIC. (I mean, it runs BASIC off of machine hex) If you could save your code into a .txt on the SD card and have it run off that, that would be... Amazing. Maybe you could re-rig it to run C++? It's a REALLY long shot, but if you are desperate...
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Is there some easier way to sense stuff? One that is less complicated? A simpler motor driver?