Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/NXP Semiconductors/LPC11Uxx/SYSCON/SYSPLLCLKSEL#0x0
SEL=IRC
System PLL clock source select
System PLL clock source
0 (IRC): IRC
1 (CRYSTAL_OSCILLATOR_): Crystal Oscillator (SYSOSC)
2 (RESERVED): Reserved
3 (RESERVED): Reserved
Reserved
https://github.com/cmsis-svd/cmsis-svd-data