FUSE - Configuration and User Fuses

Fuses are part of the nonvolatile memory and hold the device configuration. The fuses can be read by the CPU or the UPDI, but can only be programmed or cleared by the UPDI. The configuration values stored in the fuses are written to their respective target registers at the end of the start-up sequence.

The fuses for peripheral configuration (FUSE) are pre-programmed but can be altered by the user. Altered values in the configuration fuse will be effective only after a Reset.
Note: When writing the fuses, all reserved bits must be written to ‘1’.