Thanks to everyone for your suggestions, very interesting and educational.
I'll give a try to this problem again to see if I can get any useful
results. As Alan B. Percy and Ben Jackson pointed out there's always going
to be some error in the output pulses. The question is, whether I can get
the error to be low or not. I think some 1 to 3% would be ok, so I'll try
the suggestions from Picdude and Scott Dattalo and see what's going to
happen. And sorry for my late reply, here in Sweden yesterday was a free day
at work.
Best regards
RA
----- Original Message -----
From: "Scott Dattalo" <scott@DATTALO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, April 30, 2003 4:30 PM
Subject: Re: [PIC]: Frequency multiplier?


> On Wed, 30 Apr 2003, Reinaldo Alvares wrote:
>
> > Thanks to all for your answers to my 50% stuff question.
> > I perhaps didn't explain myself properly or didn't understand your
answer
> > correctly.
> > Let me please rephrase my question. Let's say I want to multiply the
> > frequency of an input square wave by some factor "F".Consider a sensor
> > giving pulses from a rotating shaft, if the shaft accelerates then two
> > consecutive periods are different. Every next period will be shorter
than
> > the previous one. Now if I divide the first period "P" by "F" and output
an
> > "F" amount of pulses with periods equal to "P/F" then the PIC will still
be
> > outputting pulses while the next period is already happening. I can't
loose
> > counts since I need this for positioning, direction and speed
measurement
> > purposes. I have to watch both edges for any change in the direction of
the
> > shaft.The sensor is a quadrature encoder with only A and B outputs, no
Z. I
> > have to output two channels out of phase by ~90% to the system
processing
> > the data.The phase difference is naturally the same as the incoming
pulse. I
> > have managed to do it for a fixed or slow variable frequency.I
implemented
> > it in software on an 16F84A, I know they are outdated, but I have lots
of
> > them!. When the shaft accelerates fast then I start to miss about 3 to
10%
> > of the pulses depending on the acceleration, getting offsets in
> > position.When the shaft is slowing down then is ok because the next
period
> > will be longer. There will be space enough in time to accommodate the
> > multiplied pulses before the next ones will have to be outputted. I
might be
> > missing something here, I don't know what a phase accumulator is but
I'll
> > try to find out. It looks to me that this task is just not possible to
fix
> > on a base of period by period multiplication. I appreciate very much any
> > advise or pointer to how to solve this. And sorry for taking this
thread, I
> > didn't mean to.
>
> Okay. There are two pieces of information that are being manipulated.
> There's the frequency *and* there's the pulse width. The phase accumulator
> approach can still be made to work. The job would be *so* much easier to
> accomplish with a better endowed PIC than the lowly f84...
>
> 1) Measure the input period, i.e. time between rising edges
> 2) Measure the input pulse width, i.e. time between a rising and falling
> edge.
> 3) Scale the input period and input frequency to produce the output period
> and frequency.
>
> The hard part (with an f84) is sampling the signals fast enough. Here are
> some snippets to help:
>
> Edge detector:
>
>      movf   IOPORT,w         ;sample the incoming signal
>      xorwf  last_sample,w    ;compare to the last sample
>      andlw  IOBIT            ;
>      skpnz                   ;If they're the same then no need to
>       goto  no_change        ;process
>
>      xorwf  last_sample,f    ;Save this sample for next time
>      andwf  last_sample,W    ;Is this a rising edge?
>      skpnz
>       goto  falling_edge
> rising_edge:
>      ...
>
>      goto   no_change
>
> falling_edge:
>      ...
>
> no_change:
>
> ------------------
>
> Now to generate the scaled pulse output, I'd use phase shifted counters:
>
> http://www.dattalo.com/technical/theory/pwm.html
>
> The periods of the rising and falling edge counters are the same and equal
> the scaled frequency output. The relative phase shift corresponds to the
> desired pulse width. See
>
> http://www.dattalo.com/technical/software/pic/pwm8.asm
>
> for example of 8 parallel PWM outputs that use this technique. For a
> single channel, there several optimizations one could make.
>
> Scott
>
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