ATmega128 datasheet(39/386 Pages) ATMEL

39 / 386 page

2467S–AVR–07/09

ATmega128

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-

figured for use as an On-chip Oscillator, as shown in Figure 19. Either a quartz crystal or a

ceramic resonator may be used. The CKOPT fuse selects between two different Oscillator

Amplifier modes. When CKOPT is programmed, the Oscillator output will oscillate will a full rail-

to-rail swing on the output. This mode is suitable when operating in a very noisy environment or

when the output from XTAL2 drives a second clock buffer. This mode has a wide frequency

range. When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces

power consumption considerably. This mode has a limited frequency range and it can not be

used to drive other clock buffers.

For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and 16 MHz with

CKOPT programmed. C1 and C2 should always be equal for both crystals and resonators. The

optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray

capacitance, and the electromagnetic noise of the environment. Some initial guidelines for

choosing capacitors for use with crystals are given in Table 8. For ceramic resonators, the

capacitor values given by the manufacturer should be used.

Figure 19. Crystal Oscillator Connections

The Oscillator can operate in three different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 8.

Note:

1. This option should not be used with crystals, only with ceramic resonators.

The CKSEL0 fuse together with the SUT1..0 fuses select the start-up times as shown in Table 9.

Table 8. Crystal Oscillator Operating Modes

CKOPT

CKSEL3..1

Frequency Range

(MHz)

Recommended Range for Capacitors

C1 and C2 for Use with Crystals

101(1)

0.4 - 0.9

–

110

0.9 - 3.0

12 pF - 22 pF

111

3.0 - 8.0

12 pF - 22 pF

101, 110, 111

1.0 -

12 pF - 22 pF

XTAL2

XTAL1

GND

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