thanks Hal,
but my cristal speed is fixed because I'm modifying a code which already runs in this situation
what about setting PCON.7? could I have a better match?

another question: connecting XTAL1 to the external clock of 3.57 and leaving XTAL2 unconnected (as in my case) also the speed of the micro is that of the external cristal? I mean, the clock at ehich the instructions are executed

Best regards,
teo