Hi Allexandre:
The default clock CLKo out the MC13192 is 32Khz. Assumming that you are using Processor Expert, The GB60 will start using the internal oscillator, and try to change to the External Clock input. This means that you have to configure the MC13192 to generate the desired clock (if it not the 32Khz default), before the GB60 changes to the External Clock.
Now, assuming that all that is working, you cannot reset the MC13192 from the MCU, otherwise this will create a loss of the external clock, which probably reset the MCU as well. Does your routines reset the MC13192 before TX or RX?
Regards,
Alex