Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/NXP Semiconductors/LPC11Uxx/SYSCON/MAINCLKSEL#0x0
SEL=IRC_OSCILLATOR
Main clock source select
Clock source for main clock
0 (IRC_OSCILLATOR): IRC Oscillator
1 (PLL_INPUT): PLL input
2 (WATCHDOG_OSCILLATOR): Watchdog oscillator
3 (PLL_OUTPUT): PLL output
Reserved
https://github.com/cmsis-svd/cmsis-svd-data