Quark ... zwischen TOSC 1 und 2 kommt der Uhrenquarz.
Alle Timer können als Counter asynchon laufen, aber nur auf dem
Timer is n Schwingkreis für nen Uhrenquarz mit drinnen.

DB Seite 29:
For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is connected directly between the pins. No external capacitors are needed. TheOscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external clock source to TOSC1 is not recommended.
Die CPU-Frequenz kann auch 0 sein, im Sleepmode, der Timer kann
per Interrupt den Controller aus dem Tiefschlaf holen.


DB Seite 31:
Power-save Mode: When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. This mode is identical to Power-down, with one exception:
If Timer/Counter2 is clocked asynchronously, i.e., the AS2 bit in ASSR is set,
Timer/Counter2 will run during sleep. The device can wake up from either Timer Overflow or Output Compare event from Timer/Counter2 if the corresponding
Timer/Counter2 interrupt enable bits are set in TIMSK, and the Global Interrupt Enable bit in SREG is set.