Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/SiFive/FE310/PRCI/pllcfg#0x0
pllr=R1
PLL Configuration Register
0 (R1): undefined
1 (R2): undefined
2 (R3): undefined
3 (R4): undefined
1 (Q2): undefined
2 (Q4): undefined
3 (Q8): undefined
https://github.com/cmsis-svd/cmsis-svd-data