14.3.2.1.3 External Reset
The external Reset is enabled by a fuse. See the RSTPINCFG field in FUSE.SYSCFG0. Also, the internal pull-up resistor for the Reset-pin is enabled when the external Reset is enabled.
When enabled, the external Reset requests a Reset as long as the RESET pin is low. The device will stay in Reset until RESET is high again.