ATmega128 crystal oscillator

XTAL1 and XTAL2 are the input and output of the inverting amplifier used as the on-chip oscillator, as shown in Figure 19. This oscillator can use a quartz crystal or a ceramic resonator. The fuse bit CKOPT is used to select one of the two amplifier modes. When CKOPT is programmed, the oscillator produces full amplitude oscillation at the output pin. This mode is suitable for noisy environments and when a second clock buffer needs to be driven through XTAL2. In addition, this mode has a wider frequency range. When CKOPT is left unprogrammed, the output signal amplitude of the oscillator is relatively small. Its advantage is that the power consumption is greatly reduced, but the frequency range is relatively narrow and it cannot drive other clock buffers.

For resonators, the maximum frequency is 8 MHz when CKOPT is unprogrammed and 16 MHz when CKOPT is programmed. The values ​​of C1 and C2 should be the same regardless of whether a crystal or resonator is used. The optimum values ​​depend on the crystal or resonator used, as well as on stray capacitance and electromagnetic noise in the environment. Table 8 gives some guidelines for selecting capacitors for crystals. For ceramic resonators, the values ​​provided by the manufacturer should be used. For more information on how to select capacitors and how the oscillator works, refer to the Multipurpose Oscillator Application Manual.

The oscillator can operate in three different modes, each with an optimized frequency range. The operating mode is selected by the fuse bits CKSEL3..1 as shown in Table 8.

Note: 1. This option is not available for crystals, only for ceramic resonators.

As shown in Table 9, fuses CKSEL0 and SUT1..0 are used to select the start-up time.

Note: 
1. These options can only be used when the operating frequency is not too close to the maximum frequency and the frequency stability at startup is not important for the application. Not suitable for crystals.

2. These options are designed for ceramic resonators where the frequency is sufficiently stable at startup. They can also be used for crystals when the operating frequency is not too close to the maximum frequency and the frequency stability at startup is not important for the application.