> -----Original Message-----
> From: Hulatt, Jon [SMTP:jhulatt@MONSTEREUROPE.COM]
> Sent: Tuesday, September 23, 2003 3:15 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] (automotive) frequency trigger
>
> Repost, this time with a colon in the topic tag.
>
>
> 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.
>
Yes, this seems like a reasonable method to implement this, if you are happy
to use a PIC with a CCP module.  20Hz is slow enough that you could easily
measure this purely in software if you wanted to.

I have recently used a small PIC to de-restrict my Japanese import MR2 by
intercepting the signal from the speedo head to the ECU.  The PIC had to
measure the incomming pulse and generate an output pulse at the same
frequency up to a maximum limit, beyond which the output frequency remained
at this maximum.  You are welcome to the code if you wish, although it does
not use the CCP module.

> 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.
>
The simplest method would be to clamp the 12v signal using a resistor and a
zener.  Some filtering would be required to help prevent any noise problems.
The problem with an opto-isolator is that the LED within the device will
typicaly require a few mA drive current which may not be available from your
high impedance signal.  A PIC's input pin has a very high impedance (typical
CMOS inputs > 10E6 ohms), so you can use a relatively high value resistor
before the zener clamp to limit the maximum current.  The zener will also
help protect against negative noise spikes.

If noise is a big problem them consider using a comparator with hysteresis
to clean up the signal instead.

Regards

Mike




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