>
>      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
>
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> -----Original Message-----
> From: pic microcontroller discussion list
> [EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU]On Behalf Of Tony Nixon
> Sent: Thursday, March 22, 2001 20:14
> To: PICLISTspam_OUTMITVMA.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
> > @spam@piclist-unsubscribe-requestKILLspammitvma.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

> -----Original Message-----
> From: pic microcontroller discussion list
> [EraseMEPICLISTMITVMA.MIT.EDU]On Behalf Of Olin Lathrop
> Sent: Friday, March 23, 2001 07:37
> To: RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU
> Subject: Re: [EE]: Car Tachometer
>
>
> >      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?
>
> Strobe tachs usually clip around a spark plug wire.  The clip completes a
> magnetic ferrite loop when closed, and a coil is wound around the loop.
> Again, you don't just get a single clean spike when the plug fires.  You
> will need to do come filtering and conditioning, but the information is
> there.

> a scope before deciding it can't be done.  If you have the ability to
> capture the waveform, I'd be interested in seeing it.
>
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, spamBeGoneolinSTOPspamEraseMEembedinc.com, http://www.embedinc.com
>
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>

> the first half cycle (for example) after detecting one.
>
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, .....olinspam_OUTembedinc.com, http://www.embedinc.com
>
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>

> >
> > maybe just point me in the right direction with the "extra 2 or 4 bits"
> part
>
> A single pole low pass filter is easy and fast in software.  If NEW is a new
> reading and FILT is the filter value to update with the new reading then the
> algorithm is:
>
>   FILT <-- FILT * (1 - W) + NEW * W
>
> where W is a 0 to 1 weighting fraction for the new value.  This type of
> filter has the same response as an RC low pass filter with NEW as the input
> and FILT as the output.  W determines the time constant of the filter if new
> values come in at regular intervals.

>
> > But regardless of the coil windings remember that both
> > spark plugs always fire together, the designers rely
> > on 4-stroke motor ignoring the extra spark after the
>
> My understanding is that only the spark plug that is currently under
> compression (air fuel mixture ready to be ignited) actually fires.  It
> seems unlikely to me that two plugs would fire simultaneously.  The spark
> gap tolerances (or real world values, anyway) could vary by 30% or more
> pretty easily (additionally partial plug fouling would contribute to
> different voltage requirements).  So once the first plug fires the voltage
> at the coil would drop, never reaching a high enough voltage to fire the
> other plug.
>
> These setups are often called "wasted spark", and I believe the name is
> misleading to what actually goes on in the engine.
>
> -Steve
>
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> > But regardless of the coil windings remember that both
> > spark plugs always fire together, the designers rely
> > on 4-stroke motor ignoring the extra spark after the
>
> My understanding is that only the spark plug that is currently under
> compression (air fuel mixture ready to be ignited) actually fires.  It
> seems unlikely to me that two plugs would fire simultaneously.  The spark
> gap tolerances (or real world values, anyway) could vary by 30% or more
> pretty easily (additionally partial plug fouling would contribute to
> different voltage requirements).  So once the first plug fires the voltage
> at the coil would drop, never reaching a high enough voltage to fire the
> other plug.
>
> These setups are often called "wasted spark", and I believe the name is
> misleading to what actually goes on in the engine.
>
> -Steve
>