ATMEGA64-16AI datasheet(42/393 Pages) ATMEL

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ATmega64(L)

2490L–AVR–10/06

External RC Oscillator

For timing insensitive applications, the external RC configuration shown in Figure 20

can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should

be at least 22 pF. By programming the CKOPT Fuse, the user can enable an internal 36

pF capacitor between XTAL1 and GND, thereby removing the need for an external

capacitor.

Figure 20. External RC Configuration

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

quency range. The operating mode is selected by the fuses CKSEL3..0 as shown in

Table 11.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as

shown in Table 12.

Note:

1. This option should not be used when operating close to the maximum frequency of

the device.

Table 11. External RC Oscillator Operating Modes

CKSEL3..0

Frequency Range (MHz)

0101

0.1 - 0.9

0110

0.9 - 3.0

0111

3.0 - 8.0

1000

8.0 - 12.0

Table 12. Start-up Times for the External RC Oscillator Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

Recommended Usage

18 CK

–

BOD enabled

18 CK

4.1 ms

Fast rising power

18 CK

65 ms

Slowly rising power

6 CK(1)

4.1 ms

Fast rising power or BOD enabled

XTAL2

XTAL1

GND

VCC

ATMEGA64-16AI Datasheet(PDF) 42 Page - ATMEL Corporation

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