ATMEGA8 datasheet(27/303 Pages) ATMEL

27 / 303 page

ATmega8(L)

2486M–AVR–12/03

External RC Oscillator

For timing insensitive applications, the external RC configuration shown in Figure 12

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 12. 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 7.

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

shown in Table 8.

Note:

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

the device.

Table 7. External RC Oscillator Operating Modes

CKSEL3..0

Frequency Range (MHz)

0101

≤ 0.9

0110

0.9 - 3.0

0111

3.0 - 8.0

1000

8.0 - 12.0

Table 8. 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

ATMEGA8 Datasheet(PDF) 27 Page - ATMEL Corporation

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