ATtiny261A datasheet(30/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

6.2.6

Crystal Oscillator / Ceramic Resonator

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 6-4. Either a quartz crystal or a

ceramic resonator may be used.

Figure 6-4.

Crystal Oscillator Connections

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 6-11. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

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

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

range. The operating mode is selected by fuses CKSEL3:1 as shown in Table 6-11.

The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in Table

6-12.

Table 6-11.

Crystal Oscillator Operating Modes

CKSEL3:1

Frequency Range (MHz)

Recommended C1 and C2 Value (pF)

100(1)

0.4 - 0.9

–

101

0.9 - 3.0

12 - 22

110

3.0 - 8.0

12 - 22

111

8.0 -

12 - 22

Table 6-12.

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

(VCC = 5.0V)

Recommended Usage

258 CK(1)

14CK + 4 ms

Ceramic resonator,

fast rising power

258 CK(1)

14CK + 64 ms

Ceramic resonator,

slowly rising power

1K (1024) CK(2)

14CK

Ceramic resonator,

BOD enabled

XTAL2

XTAL1

GND

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