Xyvind Kaurstad [oyvind@SAFETEL.NO] wrote:
> Ok, guys.
>
> I'm having some trouble, and I can't get it figured out.
>
> I'm building a device that's sampling the RPM of an engine.
> The incoming signal is therefore in the range of max 350 Hz, and it
> is connected to RB0.
>
> I need to accurately count the time *between* pulses, to get a number
> that is inversely proportional to the RPM.
>
> The TMR0 and the external interrupt is used. (16F84)
>
> I've programmed the TMR0 to run with a prescaler of 1:2, so the TMR0
> frequency should be 1.25 MHz.
>
So that would be a 10MHz crystal then?

> I want to have a 16 bit counter, so in the interrupt routine I
> increment a register for each TMR0 interrupt. This is the high byte
> of my counter.
>
> When the external interrupt occurs, I read both the TMR0 and this
> register. Together they are transferred to another 16 bit variable
> that is used by the rest of the code. Then the TMR0 and the high byte
> register is cleared, and I exit the ISR.
>
> What troubles me is that I get a lot of jitter on my count value.
> A lot more than I would expect. The latency of the TMR0 interrupt
> should not bother me, since in the part of the ISR that handles this
> I just increment the high byte and then exits. The TMR0 itself is not
> tampered with.
>
> The latency that I get when the external interrupt occurs will cause
> a little bit of jitter, since I'm reading the TMR0, but it shouldn't
> be more than a couple of bits, especially since the TMR0 is running
> at half the instruction cycle.
>
> Any good hints?
>
> I don't have the code here, but I can post that later if necessary.
>
Ok, I did almost exactly this about 6 months back.  It was for a digital
tachometer which also used software PWM to drive a conventional meter
movement and had all sorts of bells and whistles such as max RPM memory.
All on a 16F84 with lots of room to spare.

I didn't bother with the external interupt to detect the ignition pulses, I
just used polling as the frequency was so slow.  I did use TMR0 with another
8 bit register though.  Jitter was a problem.  The reason is that you are
measuring the period between sparks with a very high precision, and engines
just do not run at constant speed.  In fact even during one revolution the
crank doesn't turn at a constant velocity.  The fewer cylinders the engine
has, the worse this effect is.

Unfortunately you need 16 bits because of the dynamic range of the counter.
Unless you use a large counter, it will either overflow at low RPM or run
out of resolution at higher RPM.  What I did was to convert to RPM, and feed
the results into a simple (digital) low pass filter.  I could get a reading
steady to within a few RPM on a 4 cylinder engine this way. You could lose
either one or two significant figures depending on your requirements.  A
least significant digit which constantly changes is very irritating, I'd
rather not see it :o)

Cheers

Mike