When secure region erase is enabled (in CONFIG register), a flash page can be erased by writing 0xFFFFFFFF into the first 32-bit word in a flash page.
Page erase is only applicable to the code area in the flash and does not work with UICR.
After erasing a flash page, all bits in the page are set to 1. The time it
takes to erase a page is specified by tERASEPAGE. The CPU is stalled if the CPU
executes code from the flash while the NVMC performs the erase
operation.
See Partial erase of a page in flash for information on splitting the erase time in smaller chunks.