Herbert Graf wrote:
>
> > On Sun, 12 Dec 1999, Mark Willis wrote:
> > > The 16F84 PDF file, says "... For Rext
> > > values below 4 k, the oscillator operation may
> > > become unstable, or stop completely."  ...  "Thus, we
> > > recommend keeping Rext between 5 k and 100 k."
> >
> > I always got a much slower speed when using a resister over 5 k.
> >
> > A 5 k resistor with a 20pF cap should give (.00000000002 * 5000 =
> > .0000001) 10,000,000 cycles a second or 10Mhz but my results
> > seem to not be the case.
> >
> > I mainly want to get this to work so I can understand it.  It
> > seems like too simple a situation for me to get so confused
> > over.  :-)  Maybe I misread the marking on the cap? (20G)
> > Ahh thanks, I'll have to switch to resonators.. I had thought them
> > too  unstable.  I found a good source of crystals myself though.. my
> > basement and all the broken electronic junk.  :-)
>
>         I don't believe the freq of the RC oscillator is directly related to t
he
> time constant (perhaps proportionally related, maybe not even that, I don't
> know). Quite frankly, I use the RC when I only need it to work and don't
> care much about how fast the PIC runs. If one cares about the actually
> running freq I don't think RC is for you. I found that even if you do figure
> out the freq it is far from stable. Just my observations, TTYL

The only reason to use the RC osc. is when you're building a very high
volume product and saving every penney is important ... and can tolerate
the loss of frequency stability.  Ceraminc resonators are accurate and
stable enough for most applications ... and are very inexpensive, less
than a dollar, and don't require capacitors.

The PIC16F8X data sheet has several figures and tables at the beginning
of section 12 ... frequency and stability for different RC choices.

 - Nick -