Morgan,

I think (puzzled look) I understand your method, but you have me thinking in a
diferent direction.  What if you use a PWM that you write yourself (12 or 14 or
16 bit count off of the TMR0, pre-stuff for high speed?) to generate a target
output, and use the A/D input to qualify it?  This assumes you have filtered
your home made PWM to the quality you wish.  I personally have had trouble
getting '16' bit performance out of the part.  I am curious to know what usable
performance you get.

Chris Eddy
Pioneer Microsystems, Inc.

Morgan Olsson wrote:

> PIC14000 has A/D circuitry consisting of ramp generator and a 16bit fast
> counter which capture the count when a comparator say the measured input is
> the same as the ramp.
>
> As it is possible to backward calculate the exact count for a desired
> voltage we can make a high resolution D/A if we trigger a sample/hold
> cirquit to lock the value of the ramp at the desired count.
>
> It is easy just to continuously read teh counter and send a trigger pulse
> at a desired value.  BUT the datasheet say we should not read the counter
> while it is running.
> Why?
> Anybody tried?
> Will the read make it *count* false?
> Or do we risk *readig* erroneous value?
> Probably the latter, as the counter is normally driven very fast, by the
> oscillator directly (4MHz typ), and the counter is probably not synchronous
> binary counter type (more silicon need), so the count might not have
> propagated through the whole counter when it is read by the cpu?  That is
> of course dependant on mainly clock frequency, but also the individual
> chip, temperature, supply voltage...
>
> But I believe there should be a osc frequency below which it is guaranteed
> to work OK.
>
> Ideas?
>
> We can also use the 8-bit timer to generate interrupts, but i need it for
> other stuff, and it is more complicated to get it all work.
>
> /Morgan
>
> /  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
> \  mrt@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /



Content-Type: text/x-vcard; charset=us-ascii; name="vcard.vcf"
Content-Transfer-Encoding: 7bit
Content-Description: Card for Christopher Eddy, PE
Content-Disposition: attachment; filename="vcard.vcf"

Attachment converted: wonderland:vcard.vcf 1 (TEXT/CSOm) (000063C9)