void system_tick

void system_tick(void)

Maintain the system time by calling this function at a rate of 1 Hertz.

It is anticipated that this function will typically be called from within an Interrupt Service Routine, (though that is not required). It therefore includes code which makes it simple to use from within a 'Naked' ISR, avoiding the cost of saving and restoring all the cpu registers.

Such an ISR may resemble the following example...
            ISR(RTC_OVF_vect, ISR_NAKED)