Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/NXP Semiconductors/LPC5410x/SYSCON/SYSPLLCLKSEL#0x0
SEL=IRC_OSCILLATOR
PLL clock source select
System PLL clock source
0 (IRC_OSCILLATOR): IRC Oscillator
1 (CLKIN): CLKIN
2 (WATCHDOG_OSCILLATOR): Watchdog oscillator
3 (RTC_32_KHZ_CLOCK): RTC 32 kHz clock
Reserved
https://github.com/cmsis-svd/cmsis-svd-data