My thinking would be different.
I think I'd time an individual pulse and calculate the speed. There was a
huge thread a couple weeks back about monitoring the speed on a dozen
shafts, much of that would be applicable here.

Also I think you'll find the signal much more robust than you think (read-
you don't need to filter for jitter and you can draw some signal)

Personally, for interfacing, I would use a resistor in series with the pic
input (maybe 10K) and a zener clamp to ground. If you're really brave, omit
the zener clamp and let the PICs input protection diodes do the work, though
I wouldn't recommend it :-).

Automotive sensors are generally low impedance, this gives them better noise
immunity in a hostile environment.

-Denny Esterline


----- Original Message -----
From: "Hulatt, Jon" <jhulatt@MONSTEREUROPE.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, September 23, 2003 8:30 AM
Subject: [EE] (automotive) frequency trigger


> Hi all,
>
> For my next project, I want to initiate some action when my car exceeds
> approximately 10mph. The "speedo" sensor is some kind of proximity switch
on
> the output end of the gearbox/diff. So, the output it a 12v ish square
wave,
> of frequency proportional to speed. My multimeter measures the frequency
at
> about 20 Hz at 10mph.
>
> So, to measure this frequency, my idea is to use the CCP, in capture mode,
> with an appropriately prescaled timer. My Capture ISR will increment a
> counter each time the CCP event fires, if the timer represents a (period)
> shorter than 40ms (ie, faster), and, when the counter gets to an
appropriate
> number, perhaps 20 ish, I'll inititiate the other code. Idea of using the
> counter is to filter out any bounces or other muck in the line.
>
> Leading to my first question- does this plan seem like a workable idea?
I've
> not much experience with a) pic and b) the automotive envorinment.
>
> Secondly, is the question of how to interface the 12v square signal with
the
> pic? I'd thought of using an optoisolator, but is there a better, or
simpler
> way of protecting the pic from a) getting 12v not 5v and b) any dirty
spike
> type things? I think I need a relatvely high impedance, because the signal
> goes into the ECU, so if I steal too much of it, I may make the car rather
> ill.
>
> Appreciate your thoughts.
>
> Jon
>
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