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Thread: Frequency counting
www.piclist.com/techref/microchip/time.htm?key=count
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face BY : Dan Michaels email (remove spam text)



Brusque wrote:


>    The maximum frequency this would have to count would be about 20khz.
>The strange fact is that when I suply a 100hz square wave to the circuit,
>the countings turns a lot random.
>

I'll take a closer look at your algorithm tonite, and see what I can see.
===========

>    Could you please suply some code excerpts from your frequency
>counting routines?
>

see other answer for basics.
===========

>> Using (float)casting, I guess round-off errors during division
>> isn't going to be a problem.
>
>    I'm only using it because of the need for a 32 bit counting. Maybe
>I'll try to make some kind of 32 bit integer maths with CCS.
>

No, actually, I meant you are probably better off using float, since
integer math more easily gives round-off errors and overflows, etc.
===========

>> But given your algorithm with all of its inversons/etc, do you
>> have any idea how many Timer1 ticks is the difference between
>> measuring 2480 and 2500?
>
>    That values are in ticks. The input on the PIC was a 1000hz square
>wave. Every cycle takes 1ms. The Timer1 tickles 2500 times in one 1ms
>with 10mhz clock.
>

Hmm, so then 2500-2480 = 20 amounts to 20*.4usec = 8 usec. Something
so short could be due to overhead problems. I look at it more closely
tonite [and hopefully someone else will have fixed it by then :)].

- dan michaels
http://www.oricomtech.com
==================

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