> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Tony Nixon
> Sent: Thursday, March 22, 2001 20:14
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Car Tachometer
>
>
> Herbert Graf wrote:
> >
> >      Well we just acquired a car and I am thinking of making
> the addition of
> > a tachometer with a PIC. The reason is that it is an 88 Olds
> Delta 88 and
> > those kinds of cars trigger the spark plugs more than just during the
> > compression cycle making standard digital tachs useless, plus
> it sounds like
> > an interesting project! Has anyone does this sort of thing? I
> was thinking
> > of using a freq to voltage converter but I'd rather do it all
> digitally. I
> > was thinking about measuring the time between pulses to determine RPM
> > (because counting the number of pulses per set amount of time
> would result
> > in WAY to slow display updates and lack of resolution at lower
> RPMs). 100us
> > resolution seems good enough for my uses. What does everyone
> think of that?
> > The only thing is I would have to use the inverse of this
> measurement, how
> > easy is this to do with a PIC? I was thinking of using a lookup
> table (with
> > maybe 256 entries and some linear interpolation) but I'd rather
> do it with
> > the math if possible. ANY comments and pointers would be greatly
> > appreciated. TTYL
> >
> > --
> > http://www.piclist.com hint: To leave the PICList
> > mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
> Use this simple formula...
>
>
>           927C0h
> ------------------------  = RPM
> 100uS Loops X Cylinders
>
>
> You might be able to simplify it even more
>
> Eg. for 8 cylinders
>
>
>   124F8h
> ------------  = RPM
> 100uS Loops
>
>
> Use a 24 bit counter to count the 100uS loops.
>
> Start the count when the points open.
> Increment the counter on every 100uS loop
> The next time the points open...
>   Divide 124F8h by the count value.
>   The result is RPM.
>   Convert this to BCD and display it on 4 LEDs.
>   Debounce the points for 8 X 100uS loops.
>   Reset the counter

     Thanks alot, however I'd like to add that since the car uses an
electronic ignition system (no points) I'm planning to get my tach signal
from a fuel injector, anyone see problems with this idea? I realize that
injectors probably create a huge amount of transients on the line but I
figure I should be able to filter that out. Ideas? Thanks, TTYL

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu