Sorry, do you mean the Vpp for an external crystal signal?
If this is the case
When configured to use an external crystal, the oscillator circuit works in a different way than a GPIO and it depends on the configuration used to determine the voltage levels that you should see in these pins. For example, if you are not using the High Gain Oscillator bit (HGO = 0) you should see a value between EXTAL and XTAL of around 1 Volt peak to peak (Vpp). It is important to mention that the way to measure these signals is in a differential mode and it is good to know that you have an active probe with such a load capacitance because that's a typical issue that customers face when trying to measure these signals. If the HGO bit is set, then you usually see a value of VDD between these two pins, and in many cases, you have to add a serial resistor (Rs) to the oscillator circuit to avoid a crystal override.
I recommend you that you check the application note AN3208 “Crystal Oscillator Troubleshooting Guide” you can find this application note on the following link:
https://www.nxp.com/docs/en/application-note/AN3208.pdf
Regards
