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    /