10.3.2.3.4 EEPROM Write Mode
The EEPROM Write (EEWR) mode enables the EEPROM array for writing operations. Several writes can be done while the EEWR mode is enabled in the CTRLA register. When the EEWR mode is enabled, writes with the ST* instructions will be performed one byte at a time.
When writing the EEPROM, the CPU will continue executing the application. The CPU will halt if a new load/store operation starts before the EEPROM erase/write has been completed.
Erasing the EEPROM content is necessary before performing a write-to-an-address.