ATmega128 datasheet(41/386 Pages) ATMEL

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2467S–AVR–07/09

ATmega128

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. For more information on

Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator

application note.

Figure 20. External RC Configuration

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

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

(VCC = 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

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