AVR-Chat

On Wed, 2005-11-16 at 23:02 +0000, Dave Mucha wrote:
> Many PIC's have an internal oscilator.  but AFAIK, none have an 8mhz
> internal.
> But, that brings up another question.
> I was looking for a tachometer.  it seems that one needs a very
> accurate timing base to compare things too.


   Yes. Most commercial electropnjic tachometers use standard off the
shelf timing crystals.

> one range would be less than about 6,000 RPM or 100hZ and the other
> range would be less than 50,000 RPM.
> As I understand, one can count to 'n' and while counting, also count
> inputs from the sensor.   then calculate speed.
> The other way is to count input pulses to reach 'x' and then see how
> long it took and then calculate speed.
> One works better for low speeds, the other for high speed.
> but, both need to have an accurate refreance.
> So, the question is that of crystal accuracy ?

  Typical low cost oscillatro crystals such as you might find at Jamecop
or Digikey are on the order of 50 ppm.  That is approximately .05%
accurate.  For the vast majority of tachometer applications, that is FAR
more accuracy than you have any need for.  (these are the crystals that
sell for less than $2 each...slightly more expensive [say,. $5-6 each]
can be had that offer 25 ppm accuracy)

> is the interanl any better than an exteral oscilator ?

  No.  The internal oscillatros are almost always RC oscillators, which
are horrendously inaccurate, and suffer from severe thermal drift.

> better than an external crystal ?

  Again, no.   Even an inexpensive external crystal (say, less than $1)
is going to offer 100 ppm accuracy, which is .1%.  You could probably do
better than 1% accuracy with an inexpensive ceramic resonator, for about
$1.25.

Tom