[Diy_efi] Crank trigger resolution

[Bernd Felsche]

Phil Lamovie tapped away at the keyboard with:
> I think you need to consider the problem from the
> desired outcome backwards.
> 
> First determine the maximum spark timing error that you
> are willing to accept.
> 
> Now the engineering decision becomes somewhat easier.
> 
> The TPU is the key. If you can't resolve the time to 1 microsecond
> then all the teeth in the world won't give you an accuracy better
> than + or - 3 degrees during hard acceleration. (street car)

Eh? 

I calculate about 28 microseconds per degree at 6000 rpm.

6000 rpm -> 100 rps -> 36000 degrees/second
Then invert. -> 27.7778 us/degree

With an 8MHz RISC microcontroller, that gives about 200 instruction
cycles per degree of crank revolution.

-- 
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