Many AVR devices have the capability of using asynchronous clocking of the built-in timer/counter. Using this feature, the counter can be used for real-time functions. A 32.768 kHz crystal should be connected to the TOSCx pins of the AVR device.
In some AVR devices, the internal oscillator circuit used with the real-time counter provides a capacitive load of approximately 20 pF, which should be appropriate for common 32.768 kHz crystals. Refer to the device-specific data sheet for information about the capacitors. If the internal load is insufficient for the applied crystal, external capacitors can be used.