Abfahrt:
The system is running at 16.5MHz (with V-USB -- disabled it for testing just in case). I used http://www.et06.dk/atmega_timers/ to calculate timer values.
Abfahrt:
I'm trying to make the ISR run at 1s intervals.
Without actually trying it, I'm not sure this is possible. The largest clock prescaler is 16384, so with a 16.5 MHz system clock and using the overflow interrupt, the slowest the ISR could run would be
16,500,000 / 16,384 / 256 = 3.934 Hz
I might not use a web page that calculates ATmega timer values for an ATtiny.