I figure only the inflection points need to be stored. The actual value will then be calculated on the fly. Some of the calculation can also be precomputed if needed.

A big limitation of the 683 is that it can't write it's own memory. Otherwise we could compute the table and store it in flash. We'll just have to see how things go, it is possible I'm overestimating what is possible. I'll never know if I don't try.

Looks like you've got the timing maxed out already. I'll probably have to increase the osc to 8mhz just to cover the overhead of writing it in C.


I think it should be pretty easy to use a switch or jumper. I kind of like the idea of repeatedly switching back and forth between curves to see how the engine sounds, so maybe a momentary button would work well. You'd probably only need this for testing so just tacking on a switch as needed would probably work fine. With a momentary type setup you could also just ground a pin with a wire for a sec to switch back and forth.

What is the max RPM and max advance people want to achieve?
At 10,000 RPM, 30 deg. hall position, and 27 degrees advance there's only 50 instruction cycles available before firing when running at 4mhz. Even running at 8mhz that's only 100 instructions, and that's hardly anything to work with.
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