This keeps coming up, but:

Use "parallel resonant" crystals.  I like 18 or 20pF types.  "Series resonant"
crystals will probably oscillate but at the wrong frequency.  You use external
capacitors like the drawing in the data book, also 18 or 20 pF.  You may need a
series resistor as shown in some of the data books in HS mode.  Look at the OSC
OUT waveform with a 'scope.  If it is clipped or ringing near to or beyond the
supply rails, try a 120 ohm in series.   Check the archive for more crystal
dsicussions.


                           |  |
OSC IN ---------o----------|  |----------o
                |          |  |          |
                |                        |
              -----                      |
             -------                     |
            |       |                    |
             -------                     |
              -----                      |
                |                        |
                |          |  |          |
OSC OUT --------o----------|  |----------o
                           |  |          |
                                         |
                                         |
                                       -----
                                        ---
                                         -

- Mark Sullivan -