Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/ARM/CMSDK_CM3/UART0/STATE#0x0
UART Status Register
TX Buffer Full
RX Buffer Full
TX Buffer Overun (write 1 to clear)
RX Buffer Overun (write 1 to clear)
https://github.com/cmsis-svd/cmsis-svd-data