Hello simone acciarri
The internal oscillator is stable but temperature changes may vary its accuracy. We highly recommend using an external crystal for accurate applications.
You can check more detailed data at chapter 3.3 "Clock modules" in the K22 datasheet https://www.nxp.com/docs/en/data-sheet/K22P121M120SF7.pdf
Let me know if this is helpful, If you have more questions do not hesitate to ask me.
Best regards,
Omar
