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.

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.


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¯yvind Kaurstad, oyvind@safetel.no

Safetel AS
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