27

2570L–AVR–08/07

ATmega325/3250/645/6450

9.3

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 9-2 on page 27. Either a quartz

crystal or a ceramic resonator may be used.

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 on page 27. For ceramic resonators, the capacitor val-

ues 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 on page

27.

Note:

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 on page 27.

Table 9-3.

Crystal Oscillator Operating Modes

CKSEL3:1

Frequency Range

(MHz)

Recommended Range for Capacitors C1

and C2 for Use with Crystals (pF)

100(Note:)

0.4 - 0.9

–

101

0.9 - 3.0

12 - 22

110

3.0 - 8.0

12 - 22

111

8.0 -

12 - 22

Table 9-4.

Start-up Times for the Crystal Oscillator Clock Selection

CKSEL0

SUT1:0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

Recommended

Usage

000

258 CK(1)

14CK + 4.1 ms

Ceramic resonator,

fast rising power

001

258 CK(1)

14CK + 65 ms

Ceramic resonator,

slowly rising power

010

1K CK(2)

14CK

Ceramic resonator,

BOD enabled

XTAL2

XTAL1

GND

C2

C1