Zu Caps soricht das Handbuch auf Seite 26:
To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency
Crystal Oscillator must be selected by setting the CKSEL Fuses to “1001”. The
crystal should be connected as shown in Figure 11. By programming the CKOPT Fuse,
the user can enable internal capacitors on XTAL1 and XTAL2, thereby removing the
need for external capacitors. The internal capacitors have a nominal value of 36 pF.
AVRs kann man voll statisch betreiben, also auch mit 1Hz. Für die passenden Fuses müsst ich aber auch ins Handbuch schauen.