> > -----Original Message-----
> > From: Michael Rigby-Jones [mailto:Michael.Rigby-Jones@BOOKHAM.COM]
> > Sent: 23 September 2003 15:38
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: ] (automotive) frequency trigger
> > 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.
> >
>
> -----Original Message-----
> From: Hulatt, Jon [SMTP:jhulatt@MONSTEREUROPE.COM]
> Sent: Tuesday, September 23, 2003 4:02 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: ] (automotive) frequency trigger
>
> How did you do this? Just polling an input pin & using a timer? Or using
> the
> interupt pin and a timer?
>
I used a timer, but only to make my main loop isochronous.  IIRC the main
loop runs in 50 cycles.  Each time around the loop an input period counter
is incremented and the input from the speed sensor is checked for a rising
edge.  If a rising edge is found, the counter value is copied to another
variable and then cleared.  The count is then checked against a limit.  If
it's lower than this limit, the value is made equal to the limit, if the
count is higher it is not changed.  A second counter produces the output
pulse, and is reloaded by the captured value from the first timer.  You
application sounds much simpler, no output pulse train to produce.

> The code could be interesting, I'd like a look please. What form was the
> signal on the MR2 ? My car is a skyline, and they are very likely to be
> very
> similar. Was is 12v square frequency/speed proportional? (like I am
> assuming/guessing) ?
>
The gearbox sensor gives 4 pulses per wheel revolution*.  The sensor itself
appears to have an open collector output, and is pulled up to 12 volts
within the speedometer.  After probing around I found an internal signal in
the speedo that was cleaned up and clamped to a 5 volt swing, so the messy
level conversion was all done for me.  In the MR2's case there is an output
from the speedometer that is also open collector, going to the engine and
power steering ECU's, where it is pulled up to 5 volts.  This is normaly a
straight copy of the input pulse train, but is now limited to give a maximum
of around 100mph (limiter kicks in somewhere between 112 and 120).

The code is written for the slightly obscure 12C672, simply because I had a
few of the surface mount variety in my drawer, which made kludging it onto
the back of the speedo (playstation modchip style) much easier.  I've
emailed the code off list to save bloating out this post.

Regards

Mike

*at least that's what I was was told, after calibrating the speedo on the
bench with a signal generator taking into account my wheel/tyre rolling
circumference, it appears to be reading a fair bit too fast.  Really need to
finish of my gameboy scope and take a look, I'll post any further findings.




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