Making resonators to oscillate is an analogic stuff.
You'll find resonators which will not be able to oscillate even at 5V
with PIC oscillator configuration.
The answer is the Q factor of the resonant circuit.
For lowest Q resonators the output amplitude is bellow the schmitd
trigger oscin input thresold, you may see it clearly with a 10:1 probe
on any good scope.
The key is tunning the capacitors (if there aren't inside the
resonator when you can't do anything else than change the resonator).
Decreasing the oscout capacitor value will increase the amplitude and
decrease the stability.
You have no solution for three terminal resonator other than changing
the resonator type and producer. This is happening on many PICS (I've
see it on 16F series at 628, 676, 876 etc)
Never belive entirely what datasheet said.

On 6/2/07, Peter Feucht <p.feucht@palmed-medizintechnik.de> wrote:
> Dear Piccers,
>
> Tried to operate a F628A with a 4MHz resonator at 3V yesterday - resonator
> shows 4MHz sine wave but processor never starts. Tried both XT and HS
> oscillator setup. Data sheet of 628A says Vdd=3-5,5V. After changing voltage
> regulator to 5V it works perfectly.
>
> Any idea how I can operate this configuration at 3V?
>
> Thanks for any help and best reagrds
>
> Peter
>
>
>
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