Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Atmel/ATSAM3S1C/PMC/CKGR_PLLAR#0x0
PLLA Register
Divider
PLLA Counter
PLLA Multiplier
Must Be Set to 1
https://github.com/cmsis-svd/cmsis-svd-data