> No, this wasn't what I was saying at all.  Go look up Nyquist.

> An analog signal updated to a new level at 100Hz can't contain
meaninful
> information past 50Hz.  But that says nothing about what the ECU
requires,
> only what is in your signal should you chose to update it 100 times
per
> second.
Ohhh, you mean like if I was tossing in a sin wave or something, I can't
send anything useful at more than 50 Hz due to sampling errors, I'll get
weird beat patterns and utter nonsense output.  Yeah, okay, I know about
that.  I wasn't even thinking about that.  I'm not outputting sine waves
or anything, just voltage levels.  A given voltage corresponds to how
much air is going through.  So, usually, at 100Hz, it's not going to
change more than a few mV, if any, between updates.

> Sounds like 200Hz bandwidth is a good spec for this purpose.  From an
> earlier post you should be able to see that the PWM output of a
30F4012 can
> easily achieve this with sufficiently low ripple and sufficiently high
> resolution.
*nods*

> > Given that I don't have access to an oscilloscope in the car, the
best I
> > can do is, unfortunately, look at my handy-dandy radio shack 10$
> > multimeter while the output is happening, and it's usually moving
around
> > by 10mV or so.

> That tells you nothing about frequency content over a few Hz due to
the
> mechanical damping of the meter movement.  Even if you had 1 volt
ripple at
> 50Hz, you'd never see it on the meter.
Exactly, hence my problem with using that tool. =/

> So why is it called an air *counter*?  The word counter implies
discrete
> events.  If so, it would be better to catch these directly by using an
input
> capture module than to convert them to analog, then back to digital
again in
> the PIC, then back to analog again to the ECU.

> > I'm trying to get rid of my stock AFM and switch to a different
system
> > of monitoring air that's a combination of other sensors.

> Why?  These systems were carefully designed.  I wouldn't mess with
them
> unless you really really know what you're doing.

Believe it or not, I know exactly what I'm doing, and in my car is a
beta setup using an ARM chip to do this, that drives around fairly well.
I'm just trying to switch to an easier/simpler setup.  The ARM setup I'm
using has a bunch of odd quirks that's making it hard to do exactly what
I want, so I want to start over with something simpler and more
applicable to what I'm trying to do.

Also, air counter is just kinda a generic name for it.  Also, airbox,
air flow meter, etc.  In general, it's counting how much air is passing
through it.  There's a flap on a spring that opens and closes depending
on how much air is forcing it open, but there's always a continuous
stream of air holding it open to some position.  It's attached to a
potentiometer that then returns an analog signal from 0-5V saying how
open it is.  So, unfortunately, not discrete events at all.

In any event, last night I ordered a simple dev kit and a PIC16F88 for
30$, which looks to do what I want.  I just need to do some more
research on exactly how to hook up the filters on the PWM output to get
what I want.

Thanks for the help, sorry I was being so vague about it,
-David

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist