Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Infineon/XMC4700/SCU_CLK/EXTCLKCR#0x0
ECKSEL=value1
External Clock Control
External Clock Selection Value
0 (value1): fSYS clock
2 (value3): fUSB clock
3 (value4): fPLL clock divided according to ECKDIV bit field configuration
External Clock Divider Value
https://github.com/cmsis-svd/cmsis-svd-data