> 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?

Yes, especially if this is a one off.  If this is for volume production
you can substitute the CCP module with is little clever software in a
12F629 (I've done exactly that to detect a boat engine speed above a
pre-set threshold).

> 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.

In automotive and similar environments, I like all signals coming in thru
100Kohm resistors whenever possible.  A small capacitor afterward will
filter some spikes, and clamping diodes can easily handle the rest.  I
would further detect this signal with a significant amount of hysterisis.
If it's always a 0-12V signal, then scaling to 5V after the 100Kohm series
resistor, a little capacitive filtering, then into a schmitt trigger input
may be good enough.

The toughest issue in an automotive environment is not the signals
however, but the power.  You need to tolerate 200V spikes that last a few
milliseconds, several volts of noise continuously, and an average that
fluctuates from 9 to 15V under normal conditions.


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