14.3.2.4.1 Non-Maskable Interrupts

A Non-Maskable Interrupt (NMI) will be executed regardless of the I bit setting in CPU.SREG. An NMI will never change the I bit. No other interrupt can interrupt an NMI handler. If more than one NMI is requested at the same time, the priority is static according to the interrupt vector address, where the lowest address has the highest priority.

Which interrupts are non-maskable is device-dependent and not subject to configuration. Non-maskable interrupts must be enabled before they can be used. Refer to the Interrupt Vector Mapping table of the device for available NMI sources.