Hi Roberto,

Take a look at this application note.
http://www.semiconductors.philips.com/acrobat/a.../AN448.pdf
There are some small advices for choose of crystal frequencies.
After that, look at : http://pdfserv.maxim-ic.com/arpdf/App.../app75.pdf
This belongs to HSM family, in addition to user guide.

Best regards,
Silviu