Since MDK 5.35, Xtal is greyed out as <undefined> in Options for target -> Target.
There was no problem in the version 5.33.
How do you set the value of the external oscillator?
FYI, this value is only used for the On-PC Simulation - if you are debugging on hardware this value is not used.
It looks like you are using this device in your project:
Do you have the Legacy support PACK for Arm Cortex-M devices (v 5.25) installed on your PC? See
If so, does this project work for you?
(I assume your test project hass a .uvproj, not uvprojx extension)
As a workaround, there are Virtual Simulation Registers (VTREGs) called XTAL
It initially gets populated by the value on the Project -> Options For Target -> Target tab.
When debugging, from the command window
...to see the current value.
To Change the XTAL value, type