ATMEGA8 datasheet(25/303 Pages) ATMEL

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ATmega8(L)

2486M–AVR–12/03

Crystal Oscillator

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

be configured for use as an On-chip Oscillator, as shown in Figure 11. Either a quartz

crystal or a ceramic resonator may be used. The CKOPT Fuse selects between two dif-

ferent Oscillator amplifier modes. When CKOPT is programmed, the Oscillator output

will oscillate 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 cannot 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 environ-

ment. Some initial guidelines for choosing capacitors for use with crystals are given in

Table 4. For ceramic resonators, the capacitor values given by the manufacturer should

be used.

Figure 11. Crystal Oscillator Connections

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

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

Table 4.

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 5.

Table 4. Crystal Oscillator Operating Modes

CKOPT

CKSEL3..1

Frequency

Range(MHz)

Recommended Range for Capacitors

C1 and C2 for Use with Crystals (pF)

101(1)

0.4 - 0.9

–

110

0.9 - 3.0

12 - 22

111

3.0 - 8.0

12 - 22

101, 110, 111

1.0

≤

12 - 22

XTAL2

XTAL1

GND

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