Crank Angle Sensor resolution - Re: Old 486 ..

[Peter Gargano]

Diehl, Jeffrey wrote:
> 
> I went to Electromotive's web page and read up on the TEC II unit.  They
> claim 1/4 degree resolution.  This seems more reasonable than the 1.1 degree
> that I can imagine.  

Remember that it's not the Crank Angle Sensor (CAS) that gives you the 
resolution, but the timer you're using to trigger from the CAS "event".
So, if you have a timer that ticks over at 4 uSec, then at 6,000 RPM
(that's 100 revs/Sec, or 10 mSec/rev) that timer will give you a resolution
of 4/10,000 x 360 degrees = 0.144 degrees.

> I was wondering how you could handle situations where
> you might want to advance the timeing, say, 1.5 degrees at 5600rpm.  

At 6,000 RPM, for 1.5 degrees with the above timer, you need an incremental count 
of 1.5/0.144 = 10 (giving 1.44 degrees, a difference of 0.06 degrees over what 
you really wanted). At lower revs the figures are even more accurate.

I mention this because, as others have pointed out, the PC may be good speed
wise, but it needs some additional hardware counter support to be truly
useful for an EFI setup. With hardware timers you can use a slow processor to
get very good resolution.

Peter
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