4.10.3.3 External Reset (NRESET)
According to the default settings in the RSTDISBL flag, an external reset is generated by a low level on the NRESET pin. The external reset is activated by default. See Fuse Low Byte from Related Links.
If this is not desired, the external reset function can be deactivated by the RSTDISBL fuse. Reset pulses longer than the minimum pulse width (TNRESET_MIN of 10 µs) trigger a reset even if the internal clock is not running. Shorter pulses are not ensured to generate a reset. When the applied signal on the NRESET pin reaches the reset threshold voltage, VRST, (approximately VS/2) on its positive edge, the delay counter starts the AVR after the time-out period, tTOUT, has expired.