Giorgio Alboni wrote:
>
> I don't understand very well the difference between the XT and HS
> oscillations options. I use a PIC 16C84 with a 4 MHz crystal, I haven't
> current absorb problem, Which is the best choice between the two oscillator
> operation mode (I need clock stability)?
> Thanks for your time.
>
Although the Microchip databooks relate the oscillator modes (LP, XT,
HS) to frequency, this is only an indirect relationship.  In most cases
it's good enough to use LP for 32kHz, XT for 4MHz and below and HS for
greater than this.  (These figures are from memory so no flames if I'm
mistaken, please.)  When you burn the fuse, you are actually setting the
amount of feedback resistance across the cyrstal, the value of which
depends upon the crystal characteristics.  Typically (and _very_
generally), more feedback resistance gives better stability but worse
startup - vise versa for lower feedback resistance.  LP mode gives the
highest amount of feedback resistance, HS the least.  For ceramic
resonators you should use HS mode even if the frequency is less than
4MHz (I've never used a resonator but I assume the concern is startup).
I use an external oscillator so I program for LP mode for lower current
- BTW, it would be nice to have a way to totally disable the feedback.

Hope this helps.  Don't design any medical instruments based upon this
advise in case I've transposed the odd HS/XT/LP. :-)

Matt