Thank you for your answer.
I read carefully section 6.6.4.
My application uses exclusively a 48MHz crystal.
CLKMOD0 has effectively an external pull up resistor.
CLKMOD1 is not available (64 pins version) ; the datasheet says it has an internal pull down resistor.
Anyway, i understood that, whatever state is CLKMOD1, when CLKMOD0 state is "1", XTAL state has no consequence. So i think the pull up resistor to XTAL is not useful. I will remove it.
Regards.