Datasheet

ATmega8A [DATASHEET]

8159E–AVR–02/2013

When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces power consumption con-

siderably. This mode has a limited frequency range and it cannot be used to drive other clock buffers.

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

grammed. 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 9-3. For

ceramic resonators, the capacitor values given by the manufacturer should be used.

Figure 9-2. 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 9-3.

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 9-4.

Table 9-3. Crystal Oscillator Operating Modes

CKOPT CKSEL3:1

Frequency

Range(MHz)

Recommended Range for Capacitors C1

and C2 for Use with Crystals (pF)

1101

(1)

0.4 - 0.9 –

1 110 0.9 - 3.0 12 - 22

1 111 3.0 - 8.0 12 - 22

0 101, 110, 111 1.0  12 - 22

XTAL2

XTAL1

GND